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Proposed Rule2024-14970

Endangered and Threatened Wildlife and Plants; Proposed Listing Determinations for Ten Species of Giant Clams Under the Endangered Species Act

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Published

July 25, 2024

Issuing agencies

Commerce DepartmentNational Oceanic and Atmospheric Administration

Abstract

We, NMFS, have completed a comprehensive status review of seven species of giant clams (Hippopus hippopus, H. porcellanus, Tridacna derasa, T. gigas, T. mbalavuana, T. squamosa, and T. squamosina) in response to a petition to list these species as threatened or endangered under the Endangered Species Act (ESA). Based on the best scientific and commercial data available, including the Status Review Report, and after taking into account efforts being made to protect these species, we have determined that H. porcellanus, T. mbalavuana, and T. squamosina are in danger of extinction throughout the entirety of their respective ranges, T. derasa and T. gigas are in danger of extinction in a significant portion of their respective ranges, and H. hippopus is likely to become an endangered species within the foreseeable future throughout a significant portion of its range. Therefore, we propose to list H. porcellanus, T. mbalavuana, T. squamosina, T. derasa, and T. gigas as endangered species and H. hippopus as a threatened species under the ESA. We have determined that the fluted clam, T. squamosa, is not currently in danger of extinction throughout all or a significant portion of its range and is not likely to become so within the foreseeable future. Therefore, we find that T. squamosa does not meet the definition of a threatened or an endangered species under section 4(a) of the ESA. Further, we propose to exercise the discretionary authority of section 4(d) to extend the prohibitions of section 9 of the ESA to the proposed threatened species, H. hippopus. At this time, we do not propose to designate critical habitat for the three species proposed to be listed that occur within U.S. jurisdiction (H. hippopus, T. derasa, and T. gigas) because critical habitat for these species is not yet determinable. Using the authority of section 4(e) of the ESA, we also propose to list T. crocea, T. maxima, T. noae, and T. squamosa as threatened species due to the similarity of appearance of products derived from these species (e.g., meat, worked shell products, and pearls) to those derived from the six aforementioned species proposed to be listed based on their extinction risk. We propose a special rule to define activities that would and would not be prohibited with respect to these four species in order to mitigate the substantial enforcement challenge associated with this similarity of appearance concern. We solicit information to inform the final listing determination and to inform a future proposal for any determinable critical habitat.

Full Text

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[Federal Register Volume 89, Number 143 (Thursday, July 25, 2024)]
[Proposed Rules]
[Pages 60498-60547]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-14970]

[[Page 60497]]

Vol. 89

Thursday,

No. 143

July 25, 2024

Part II

Department of Commerce

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National Oceanic and Atmospheric Administration

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50 CFR Parts 223 and 224

Endangered and Threatened Wildlife and Plants; Proposed Listing
Determinations for Ten Species of Giant Clams Under the Endangered
Species Act; Proposed Rule

Federal Register / Vol. 89 , No. 143 / Thursday, July 25, 2024 /
Proposed Rules

[[Page 60498]]

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DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

50 CFR Parts 223 and 224

[Docket No. 240626-0177; RTID 0648-XF174]

Endangered and Threatened Wildlife and Plants; Proposed Listing
Determinations for Ten Species of Giant Clams Under the Endangered
Species Act

AGENCY: National Marine Fisheries Service (NMFS), National Oceanic and
Atmospheric Administration (NOAA), Commerce.

ACTION: Proposed rule; availability of status review; request for
comments.

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SUMMARY: We, NMFS, have completed a comprehensive status review of
seven species of giant clams (Hippopus hippopus, H. porcellanus,
Tridacna derasa, T. gigas, T. mbalavuana, T. squamosa, and T.
squamosina) in response to a petition to list these species as
threatened or endangered under the Endangered Species Act (ESA). Based
on the best scientific and commercial data available, including the
Status Review Report, and after taking into account efforts being made
to protect these species, we have determined that H. porcellanus, T.
mbalavuana, and T. squamosina are in danger of extinction throughout
the entirety of their respective ranges, T. derasa and T. gigas are in
danger of extinction in a significant portion of their respective
ranges, and H. hippopus is likely to become an endangered species
within the foreseeable future throughout a significant portion of its
range. Therefore, we propose to list H. porcellanus, T. mbalavuana, T.
squamosina, T. derasa, and T. gigas as endangered species and H.
hippopus as a threatened species under the ESA. We have determined that
the fluted clam, T. squamosa, is not currently in danger of extinction
throughout all or a significant portion of its range and is not likely
to become so within the foreseeable future. Therefore, we find that T.
squamosa does not meet the definition of a threatened or an endangered
species under section 4(a) of the ESA. Further, we propose to exercise
the discretionary authority of section 4(d) to extend the prohibitions
of section 9 of the ESA to the proposed threatened species, H.
hippopus. At this time, we do not propose to designate critical habitat
for the three species proposed to be listed that occur within U.S.
jurisdiction (H. hippopus, T. derasa, and T. gigas) because critical
habitat for these species is not yet determinable. Using the authority
of section 4(e) of the ESA, we also propose to list T. crocea, T.
maxima, T. noae, and T. squamosa as threatened species due to the
similarity of appearance of products derived from these species (e.g.,
meat, worked shell products, and pearls) to those derived from the six
aforementioned species proposed to be listed based on their extinction
risk. We propose a special rule to define activities that would and
would not be prohibited with respect to these four species in order to
mitigate the substantial enforcement challenge associated with this
similarity of appearance concern. We solicit information to inform the
final listing determination and to inform a future proposal for any
determinable critical habitat.

DATES: Comments must be received by October 23, 2024.
Public informational meetings and public hearings: In-person and
virtual public hearings on this proposed rule will be held during the
public comment period at dates, times, and locations to be announced in
a forthcoming Federal Register notice.

ADDRESSES: You may submit data, information, or written comments on
this document, identified by NOAA-NMFS-2017-0029, by either of the
following methods:
<bullet> Electronic Submissions: Submit all electronic public
comments via the Federal e-Rulemaking Portal. Go to <a href="https://www.regulations.gov">https://www.regulations.gov</a> and enter NOAA-NMFS-2017-0029 in the Search box.
Click on the ``Comment'' icon, complete the required fields, and enter
or attach your comments.
<bullet> Mail: Submit written comments to Endangered Species
Division, Office of Protected Resources (F/PR3), National Marine
Fisheries Service, 1315 East West Highway, Silver Spring, MD 20910,
USA, Attn: Giant Clams Species Listing Proposed Rule.
Instructions: Comments sent by any other method, to any other
address or individual, or received after the end of the comment period,
may not be considered by NMFS. All comments received are a part of the
public record and will generally be posted for public viewing on
<a href="https://www.regulations.gov">https://www.regulations.gov</a> without change. All personally identifying
information (e.g., name, address, etc.), confidential business
information, or otherwise sensitive information submitted voluntarily
by the sender will be publicly accessible. NMFS will accept anonymous
comments (enter ``N/A'' in the required fields if you wish to remain
anonymous).
The Status Review Report associated with this determination, its
references, and the petition can be accessed electronically at: <a href="https://www.fisheries.noaa.gov/action/proposed-rule-10-species-giant-clams-under-endangered-species-act">https://www.fisheries.noaa.gov/action/proposed-rule-10-species-giant-clams-under-endangered-species-act</a>. The peer review plan, associated charge
statement, and peer review report can be accessed electronically at:
<a href="https://www.noaa.gov/information-technology/status-review-report-of-7-giant-clam-species-petitioned-under-us-endangered-species-act-hippopus">https://www.noaa.gov/information-technology/status-review-report-of-7-giant-clam-species-petitioned-under-us-endangered-species-act-hippopus</a>.
The draft Environmental Assessment and Initial Regulatory Flexibility
Analysis associated with the proposed ESA section 4(d) regulation for
T. crocea, T. maxima, T. noae, and T. squamosa can be accessed
electronically via the Federal e-Rulemaking Portal by navigating to
<a href="https://www.regulations.gov">https://www.regulations.gov</a> and entering NOAA-NMFS-2017-0029 in the
Search box.

FOR FURTHER INFORMATION CONTACT: John Rippe, NMFS Office of Protected
Resources, (301) 427-8467, <a href="/cdn-cgi/l/email-protection#264c494e4808544f565643664849474708414950">[email&#160;protected]</a>.

SUPPLEMENTARY INFORMATION:

Background

On August 7, 2016, we received a petition from Dwayne Meadows to
list 10 species of giant clams (Cardiidae: Tridacninae) as threatened
or endangered under the ESA throughout their respective ranges. The
petitioner also requested that critical habitat be designated in waters
subject to U.S. jurisdiction concurrently with listing under the ESA.
On June 26, 2017, we published a 90-day finding (82 FR 28946)
announcing that the petition presented substantial scientific or
commercial information indicating that the petitioned action may be
warranted for 7 of the 10 species listed in the petition: Hippopus
hippopus (horse's hoof, bear paw, or strawberry clam), Hippopus
porcellanus (porcelain or China clam), Tridacna derasa (smooth giant
clam), Tridacna gigas (true giant clam), Tridacna mbalavuana (syn. T.
tevoroa; devil or tevoro clam), Tridacna squamosa (fluted or scaly
clam), and Tridacna squamosina (syn. T. costata; Red Sea giant clam),
but that the petition did not present substantial scientific or
commercial information indicating that the petitioned action may be
warranted for the other 3 species (T. crocea, T. maxima, or T. noae).
We also announced the initiation of a status review of the seven
aforementioned giant clam species, as required by

[[Page 60499]]

section 4(b)(3)(A) of the ESA, and requested information to inform the
agency's decision on whether these species warrant listing as
endangered or threatened under the ESA. We received information from
the public in response to the 90-day finding and incorporated the
information into both the Status Review Report (Rippe et al., 2023) and
this proposed rule. This information complemented our thorough review
of the best available scientific and commercial data for these species
(see Status Review below).

Listing Determinations Under the Endangered Species Act

We are responsible for determining whether species are threatened
or endangered under the ESA (16 U.S.C. 1531 et seq.). To be considered
for listing under the ESA, a group of organisms must constitute a
``species,'' which is defined in section 3 of the ESA to include any
subspecies of fish or wildlife or plants, and any distinct population
segment (DPS) of any species of vertebrate fish or wildlife which
interbreeds when mature (16 U.S.C. 1532(16)). On February 7, 1996, NMFS
and the U.S. Fish and Wildlife Service (USFWS; together, the Services)
adopted a policy describing what constitutes a DPS of a taxonomic
species (``DPS Policy,'' 61 FR 4722). The joint DPS Policy identifies
two elements that must be considered when identifying a DPS: (1) The
discreteness of the population segment in relation to the remainder of
the taxon to which it belongs; and (2) the significance of the
population segment to the remainder of the taxon to which it belongs.
Because giant clams are invertebrates they cannot be listed as DPSs,
and the DPS Policy does not apply here.
Section 3 of the ESA defines an endangered species as ``any species
which is in danger of extinction throughout all or a significant
portion of its range'' and a threatened species as one ``which is
likely to become an endangered species within the foreseeable future
throughout all or a significant portion of its range.'' Thus, we
interpret an ``endangered species'' to be one that is presently in
danger of extinction. A ``threatened species,'' on the other hand, is
not presently in danger of extinction, but is likely to become so
within the foreseeable future (that is, at a later time). In other
words, the primary statutory difference between a threatened and
endangered species is the timing of when a species is in danger of
extinction, either presently (endangered) or in the foreseeable future
(threatened).
Under section 4(a)(1) of the ESA, we must determine whether any
species is endangered or threatened as a result of any one or a
combination of any of the following factors: (A) the present or
threatened destruction, modification, or curtailment of its habitat or
range; (B) overutilization for commercial, recreational, scientific, or
educational purposes; (C) disease or predation; (D) the inadequacy of
existing regulatory mechanisms; or (E) other natural or manmade factors
affecting its continued existence (16 U.S.C. 1533(a)(1)); 50 CFR
424.11(c). We are also required to make listing determinations based
solely on the best scientific and commercial data available, after
conducting a review of the species' status and after taking into
account efforts, if any, being made by any State or foreign nation (or
subdivision thereof) to protect the species (16 U.S.C. 1533(b)(1)(A)).
On July 5, 2022, the U.S. District Court for the Northern District
of California issued an order vacating the ESA section 4 implementing
regulations that were revised or added to 50 CFR part 424 in 2019
(``2019 regulations,'' see 84 FR 45020, August 27, 2019) without making
a finding on the merits. On September 21, 2022, the U.S. Court of
Appeals for the Ninth Circuit granted a temporary stay of the district
court's July 5 order. On November 14, 2022, the Northern District of
California issued an order granting the government's request for
voluntary remand without vacating the 2019 regulations. On April 5,
2024, the Services published a final rule revising the section 4
implementing regulations (89 FR 24300). Because the 2024 revised
regulations became effective on May 6, 2024, we considered them during
the development of this proposed rule. For purposes of this
determination and in an abundance of caution, we considered whether the
analysis or its conclusions would be any different under the pre-2019
regulations. We have determined that our analysis and conclusions
presented here would not be any different.

Status Review

To determine whether each of the seven giant clam species warrants
listing under the ESA, we completed a Status Review Report, which
summarizes information on each species' taxonomy, distribution,
abundance, life history, and biology; identifies threats or stressors
affecting the status of each species; and assesses the species' current
and future extinction risk. We appointed a biologist in the Office of
Protected Resources Endangered Species Conservation Division to compile
and complete a scientific review of the best scientific and commercial
data available on the giant clam species, including information
received in response to our request for information (82 FR 28946, June
26, 2017).
The Status Review Report was subject to independent peer review
pursuant to the Office of Management and Budget Final Information
Quality Bulletin for Peer Review (M-05-03; December 16, 2004). It was
peer reviewed by four independent specialists selected from the
academic and scientific community with expertise in giant clam biology,
conservation, and management. The peer reviewers were asked to evaluate
the adequacy, appropriateness, and application of data used in the
Status Review Report, as well as the findings made in the ``Assessment
of Extinction Risk'' section of the report. All peer reviewer comments
were addressed prior to finalizing the Status Review Report and
publication of this finding.
We subsequently reviewed the Status Review Report, its cited
references, and peer review comments, and conclude that it synthesizes
the best available scientific and commercial data related to the seven
giant clam species considered here. In making our determinations, we
have applied the statutory provisions of the ESA, our regulations
regarding listing determinations, and relevant policies identified
herein.
The Status Review Report and the peer review report are available
on our website (see ADDRESSES section). Below is a summary of the
information from the Status Review Report and our analysis of the
status of the seven giant clam species.

Biological Review

Taxonomy and Species Descriptions

Giant clams are a small but conspicuous group of the planet's
largest and fastest growing marine bivalves. They fall within the order
Veneroida, family Cardiidae, and subfamily Tridacninae (Schneider,
1998). For many years, giant clams were considered to occupy their own
family (Tridacnidae) sister to Cardiidae until molecular phylogenetics
(Maruyama et al., 1998; Schneider & Foighil, 1999) and comparison of
sperm ultrastructure (Keys & Healy, 2000) supported reclassifying the
group as a subfamily within Cardiidae. This is the current, most widely
accepted classification; however, Neo et al. (2017) note that others
continue to argue that Tridacnidae should be retained as a full family
based on its highly distinct

[[Page 60500]]

morphology (Huber & Eschner, 2011; Penny & Willan, 2014).
Colloquially described as having `upside down' orientation (Penny &
Willan, 2014), giant clams lie with the hinge of their shell facing
downwards, allowing their byssus (i.e., filamentous threads) to attach
the organism to the substrate while orienting their enlarged mantle
upwards toward the sunlight (Soo & Todd, 2014). Additionally, most
giant clam species have an epifaunal lifestyle (i.e., situated on top
of the substrate) in contrast to the largely infaunal lifestyle of
their cardiid ancestors.
There are two extant genera of giant clams, Hippopus and Tridacna,
which are distinguished by several shell and mantle characteristics. In
Hippopus, a very narrow byssal orifice is bordered by interlocking
teeth, while Tridacna exhibits a well-defined byssal gape without
teeth. Additionally, when the clam is completely open, the mantle of
Tridacna extends laterally beyond the margin of the shell, whereas the
mantle of Hippopus does not (Lucas, 1988). A result of this difference
is that Hippopus species tend to gape their valves further apart than
Tridacna species, thus exposing more mantle surface area (Lucas, 1994).
There are currently 12 species of giant clams recognized in the
literature, though this number changes often as advances in molecular
phylogenetics resolve evolutionary relationships (including cryptic
speciation) that had been overlooked by traditional morphology-based
taxonomies. Joseph Rosewater's seminal work in 1965 is widely cited as
the authoritative material for early descriptions of giant clam species
and includes six current species that remain valid to date: H. hippopus
(Linnaeus, 1758), T. gigas (Linnaeus, 1758), T. derasa (R[ouml]ding,
1798), T. maxima (R[ouml]ding, 1798), T. squamosa (Lamarck, 1819), and
T. crocea (Lamarck, 1819). He later added H. porcellanus to this list
after re-examining its classification (Rosewater, 1982).
At the time of the 1965 report, T. mbalavuana had only been
formally described from fossils on Viti Levu, Fiji. However, Fijians
had long known of this species occurring in local waters as `tevoro',
or devil clam. Thus, when Lucas et al. (1991) re-discovered the species
in 1991, they described it as the new species T. tevoroa. It was not
until 2000 that T. mbalavuana and T. tevoroa were re-classified as
synonymous based on morphological similarities (Newman & Gomez, 2000).
As in the Status Review Report, we refer to this species by its
lectotype (i.e., its original classification), T. mbalavuana.
Additionally, Richter et al. (2008) described a new species, T.
costata, in 2008, but upon further analysis, it too was found to be
synonymous with a previously described species, T. squamosina, first
discovered by Rudolf Sturany (1899) during the early Austro-Hungarian
expeditions of the Red Sea (Huber & Eschner, 2011). As in the Status
Review Report, we refer to this species by its lectotype, T.
squamosina.
Based on the best available scientific and commercial data
summarized above, we find that all seven species of giant clams (H.
hippopus, H. porcellanus, T. derasa, T. gigas, T. mbalavuana, T.
squamosa, and T. squamosina) are currently considered taxonomically-
distinct species and, therefore, meet the definition of ``species''
pursuant to section 3 of the ESA. Distinguishing features of each
species are summarized below.

Hippopus Hippopus

Commonly referred to as the horse's hoof, bear paw, or strawberry
clam, H. hippopus has a heavy, thick shell that features prominent
reddish blotches in irregular concentric bands (Rosewater, 1965). The
shell interior is porcellaneous white, frequently flushed with
yellowish orange on the ventral margin (Kinch & Teitelbaum 2010;
Rosewater, 1965). Primary radial sculpture consists of 13 or 14
moderately convex rib-like folds over the surface of the valve,
extending towards the ventral slope where they become obsolete
(Rosewater, 1965). The mantle usually exhibits mottled patterns in
green, yellow-brown or grey, and the incurrent siphon lacks guard
tentacles (Neo et al., 2017). Juveniles and young, smaller adults are
usually attached to coral rubble by their byssus, whereas older
(larger, heavier) individuals are typically found unattached on the
substratum being held in place by their weight (Rosewater, 1965; Neo et
al., 2017). The largest reported shell length for H. hippopus is 50 cm,
which was documented at the Bolinao Marine Laboratory in the
Philippines (Neo et al., 2017).

Hippopus Porcellanus

Commonly referred to as the China clam, H. porcellanus grows to a
maximum size of 40 cm, but is most commonly found at shell lengths of
around 20 cm (Kinch & Teitelbaum, 2010). The shell exterior is off-
white, occasionally with scattered weak reddish blotches. The shell
interior is porcellaneous white, often flushed with orange on the
ventral margin, and the mantle ranges from a yellowish-brown to a dull
green or grey (Kinch & Teitelbaum, 2010). This species is distinguished
from its congener, H. hippopus, by its smoother and thinner valves and
presence of fringing tentacles at the incurrent siphon (Neo, Eckman, et
al., 2015).

Tridacna Derasa

T. derasa, or the smooth giant clam, is the second largest giant
clam species, with a maximum size of around 60 cm (Neo et al., 2017).
T. derasa has a heavy, plain-colored shell and can be distinguished
from other species by its low primary and secondary radial sculpture.
Primary radial sculpture consists of 7-12 broad, shallow rib-like folds
(usually 6-7 main folds), and the shells are often greatly thickened at
the umbos (i.e., the oldest, most prominent point of the shell near the
ventral margin) (Rosewater, 1965). The mantle is often characterized by
elongate patterns of brilliant greens and blues, and the incurrent
siphon is equipped with inconspicuous guard tentacles (Neo et al.,
2017).

Tridacna Gigas

T. gigas is known as the true giant clam and is the largest of all
the giant clam species, growing to a maximum shell length of 137 cm and
maximum weight in excess of 225 kg (Beckvar, 1981; Rosewater, 1965).
The shell of T. gigas is thick and heavy, equivalve (having valves of
the same size), and equilateral (symmetrical front-to-back) (Hernawan,
2012). The shell exterior is off-white, and is often covered with
marine growths (e.g., vermetids, annelid tubes, coral, etc.) (Kinch &
Teitelbaum, 2010; Rosewater, 1965). For the most part, the shell lacks
scales except near the byssal orifice where small scales may be
present. The shell interior is porcellaneous white, dull in the area
within the pallial line, and shiny above the pallial line to the dorsal
end of the shell (Rosewater, 1965). Often, the mantle is yellowish-
brown to olive-green and is a darker shade along the mantle's edge and
around the clam's siphons (Rosewater, 1965). Numerous, small, brilliant
blue-green rings are dispersed across the mantle, each enclosing one or
several hyaline organs. These rings are especially prevalent along the
lateral edges of the mantle and around the siphonal openings
(Rosewater, 1965). Smaller specimens (i.e., 150-200 mm) may be more
uniformly colored, lacking a darker shade along the edge of the mantle
and with fewer colored rings (Rosewater, 1965).
T. gigas is readily identified by many characteristics, most
notably its large

[[Page 60501]]

size. The species can also be identified by four to six unique deep
radial folds that give way to elongate, triangular projections at the
upper margins of its shells (Hernawan, 2012; Lucas, 1988), a complete
outer demibranch (the V-shaped structure of gills common to bivalves;
Rosewater, 1965), the lack of tentacles on the inhalant siphon
(Hernawan, 2012), and the lack of byssal attachment (i.e., they are
free-living; Rosewater, 1965).

Tridacna Mbalavuana

Before it was formally classified taxonomically, Fijians had long
referred to T. mbalavuana as `tevoro,' or devil clam, based on its
thin, sharply-edged valves and warty brownish grey mantle. T.
mbalavuana has been hypothesized to be a transitional species between
the Hippopus and Tridacna genera due to overlapping characteristics
(Lucas et al., 1991; Schneider & Foighil, 1999). It has Hippopus-like
features including the absence of a byssal gape, a mantle that does not
extend over the shells, and the absence of hyaline organs (Lucas et
al., 1991); however, T. mbalavuana looks most like T. derasa in
appearance (Lewis & Ledua, 1988). It can be distinguished from T.
derasa by its rugose mantle, prominent guard tentacles on the incurrent
siphon, thinner valves, and colored patches on the shell ribbing (Neo,
Eckman, et al., 2015). The shell exterior is off-white, often partly
encrusted with marine growths. It can grow to just over 50 cm long
(Lewis & Ledua, 1988; Neo, Eckman, et al., 2015) with the largest
specimen recorded at 56 cm (Lucas et al., 1991).

Tridacna Squamosa

Commonly known as the fluted or scaly giant clam due to the
characteristic leaf-like projections on its valves, T. squamosa is one
of the most widely distributed species of giant clams. The exterior of
its shell is greyish white in color, often with various hues of orange,
yellow, or pink/mauve (Rosewater, 1965). The primary radial sculpture
consists of 4-12 strongly convex, rib-like folds. The concentric
sculpture consists of ``undulate lines of growth which produce widely
spaced, broadly leaf-like, projecting scales on primary folds''
(Rosewater, 1965). The prominent scales on the shell commonly feature
different shades or colors (Kinch & Teitelbaum, 2010). The shell
interior is porcellaneous white, with an occasional hint of orange
(Kinch & Teitelbaum, 2010). Rosewater (1965) describes the mantle as
having a main ground color of greyish purple with a row of light blue
rhomboidal spots along the outer mantle margin and multicolored
irregularly-circular spots toward the center. The outer periphery of
the spots is pale yellow, inside of which is a band of dark yellow, and
the entire center is nearest to light blue. Generally, T. squamosa
reaches a maximum shell length of ~40 cm (Neo et al., 2017).

Tridacna Squamosina

T. squamosina, or the Red Sea giant clam, exhibits a strong
resemblance to T. squamosa, but can be distinguished by its
asymmetrical shells, crowded scutes, wider byssal orifice, and five to
seven deep triangular radial folds (Roa-Quiaoit, 2005; Richter et al.,
2008). Additionally, the mantle is most commonly a subdued brown
mottled pattern with a green margin that features prominent ``wart-
like'' protrusions and pale markings following the mantle contour
(Richter et al., 2008). These are the main diagnostic features
separating T. squamosina from its sympatric congeners and are
conservatively present even in small clams <10 cm shell length (Richter
et al., 2008). T. squamosina can reach at least 32 cm in shell length
(Neo, Eckman, et al., 2015)--the largest specimen recorded was found in
the southern Red Sea at Kamaran Island, off the coast of Yemen (Huber &
Eschner, 2011).

Range, Distribution, and Habitat Use

H. Hippopus
H. hippopus is widely distributed throughout the Indo-Pacific
(i.e., the tropical and subtropical waters of the Indian Ocean, the
western and central Pacific Ocean, and the seas connecting the two in
the general area of Indonesia), occurring from the Andaman and Nicobar
Islands in the west to the Republic of Kiribati in the east, and from
New Caledonia in the south to the southern islands of Japan in the
north (Neo et al., 2017).
According to Munro (1993a), H. hippopus occurs in the widest range
of habitat types of all the giant clam species. Most often, it is found
in shallow, nearshore patches of reef, sandy areas and seagrass beds
that can be exposed during low tides, but it can also be found on reefs
as deep as 10 m (S. Andr[eacute]fou[euml]t, pers. obs. cited in Neo et
al., 2017). Based on a recent survey in New Caledonia, Purcell et al.
(2020) found that H. hippopus ``strongly preferred'' lagoonal reefs.
The authors hypothesized that the species may either prefer the siltier
sediments and more turbid water of lagoon reef flats or alternatively
may have low tolerance to the wave exposure of barrier reefs.
H. Porcellanus
H. porcellanus has one of the most restricted geographic ranges of
the giant clams, centered in the Coral Triangle region. The species is
mostly known from the Sulu Archipelago and Palawan region in the
Philippines, but it has also been reported in Palau, the Milne Bay
Province (Papua New Guinea), Sabah (Malaysia), and Sulawesi and Raja
Ampat (Indonesia) (S. Wells, 1997; Neo et al., 2017).
There is very little information specifying the habitat preferences
of H. porcellanus, but according to Calumpong (1992), the species is
commonly found in shallow, nearshore sandy areas adjoining coral reefs.
Juvenile or young H. porcellanus are frequently found byssally attached
to coral heads, whereas larger mature H. porcellanus can be found on
sandy bottoms unattached to substrate (Rosewater, 1982; Kinch &
Teitelbaum, 2010).
T. Derasa
The geographic range of T. derasa primarily encompasses the Coral
Triangle region, although it extends east to Tonga and as far west as
the Cocos (Keeling) Islands in the eastern Indian Ocean (Rosewater,
1965). Adams et al. (1988) described T. derasa as having a patchy
distribution, being rare in many places throughout its range and
abundant in others. Notably, T. derasa has been one of the most widely
cultured species of giant clam and has been introduced to a number of
countries and territories throughout the central and western Pacific
Ocean. This includes the Federated States of Micronesia (FSM), Marshall
Islands, Tuvalu, Cook Islands, Samoa, and American Samoa.
T. derasa preferentially inhabits clear offshore waters distant
from areas of significant freshwater run-off (Munro, 1993a). According
to Calumpong (1992), the species appears to favor oceanic environments
(i.e., small islands and atolls) more than fringing reefs adjacent to
large island masses. The species is known to occur at depths of 4-25 m
(Lewis et al., 1988; Neo et al., 2017), and is usually found weakly
attached to the tops and sides of coral outcrops as juveniles, but may
become detached upon reaching a larger size (Adams, 1988).
T. Gigas
The natural range of T. gigas spans the shallow waters of the Indo-
Pacific and the Great Barrier Reef, from Myanmar in the west to the
Republic of Kiribati in the east, and from the Ryukyus Islands

[[Page 60502]]

of southern Japan in the north to Queensland, Australia in the south
(bin Othman et al., 2010; Neo et al., 2017). Cultured specimens of T.
gigas have been introduced in American Samoa, the Cook Islands, Hawaii,
and Samoa (Neo et al., 2017). Like most other giant clam species, T.
gigas is typically associated with coral reefs and can be found in many
habitats, including high- and low-islands, atoll lagoons, and fringing
reefs (Munro, 1993a). In a broad survey of T. gigas distribution
throughout the Great Barrier Reef, Braley (1987a) found that the
species was most common on inshore reefs potentially influenced by
seasonal fluctuations in salinity and turbidity and was rare south of
19[deg] S. The observed distribution was essentially opposite of that
for T. derasa, which was found primarily on offshore reefs and was
common in the Swain Reefs at 21-22[deg] S. These contrasting
distributions led Braley (1987b) to the conclusion that temperature may
limit the distribution of young T. gigas, while T. derasa may be more
sensitive to salinity and/or turbidity. T. gigas is typically found
between the depths of 2 to 20 m and is often found among Acropora spp.
or other hard coral communities, hard reef substrata, or on bare sand
(Braley, 1987b; Kinch & Teitelbaum 2010; Rosewater, 1965).
T. Mbalavuana
T. mbalavuana has one of the most restricted distributions of all
the giant clam species. For many years, it had only been observed in
Fiji and Tonga, but recent reports indicate that this species may be
found in low numbers outside of these two locations. According to Kinch
and Teitelbaum (2010), T. mbalavuana had been observed in the Loyalty
Islands in New Caledonia, a report later supported by Tiavouane and
Fauvelot (2016), who encountered two individuals on the northeastern
barrier reef of New Caledonia after ``exhaustive searches'' (Neo et
al., 2017). Single individuals were also reportedly observed on Lihou
Reef in the Coral Sea (Ceccarelli et al., 2009) and in the Raja Ampat
region of West Papua, Indonesia (Wakum et al., 2017), but neither of
these reports have been further corroborated.
In Fiji, individuals are most often observed along outer slopes of
leeward reefs in the eastern Lau Islands, in very clear, oceanic water
(Ledua et al., 1993). In Tonga, they are found in the northern Vava`u
and Ha`apai islands. T. mbalavuana has a deeper depth distribution than
most other giant clam species. In one study on spawning and larval
culture of T. mbalavuana, individuals were collected from waters of
Fiji and Tonga (Ledua et al., 1993). The mean depth of clams collected
in Fiji was 27.4 m, with samples collected from depths ranging from 20
to 33 m, and all specimens were found on the leeward side of reefs and
islands. Many of the clams found in Tonga were next to the edge of a
sand patch and cradled against rocky outcrops, rubble or bare rock with
steep slopes (Ledua et al., 1993).
T. Squamosa
T. squamosa is the second-most widely distributed giant clam
species, with a broad geographic range that extends from the Red Sea
and eastern Africa in the west to the Pitcairn Islands in the east, and
from the Great Barrier Reef in the south to southern Japan in the north
(bin Othman et al., 2010; Neo et al., 2017). The species has also been
introduced in Hawaii and Guam (CITES, 2004b).
T. squamosa is usually found on coral reefs or on adjacent sandy
areas (Neo et al., 2017). Juveniles are often attached to the substrate
by a ``weak but copious byssus,'' while adults can be found either
attached or free-living (Neo et al., 2017; Rosewater, 1965). T.
squamosa occurs across a broad depth range, which includes shallow reef
flats, patch reefs, and reef slopes, both inside and outside of
lagoons. Individuals have been observed as deep as 42 m in the Red Sea
(Jantzen et al., 2008). T. squamosa is typically more common on
shelving fringing reefs than reef flats (Govan et al., 1988) and seems
to prefer sheltered environments (Kinch & Teitelbaum, 2010; Munro,
1993a). Comparing the distribution of T. squamosa and T. maxima in
Mauritius, Ramah et al. (2017) found that T. squamosa were most often
attached to flat substrata, such as dead plate corals or rubble. Hardy
and Hardy (1969) reported that T. squamosa and H. hippopus occupied
much the same habitat in Palau, both being widely distributed in
relatively shallow water in the lagoon and on the barrier and fringing
reefs; although, T. squamosa was reportedly more commonly associated
with coral areas of Acropora spp. than adjacent sandy areas. In New
Caledonia, Purcell et al. (2020) interpreted the relatively high
abundance of T. squamosa on barrier reef sites compared to lagoonal
reefs as indication that the species may prefer cleaner waters, as
opposed to the siltier sediments and more turbid seawater of lagoon
reef flats. However, Lewis et al. (1988) note that the species is more
tolerant of turbid water than T. derasa. Paulay (1987) reported that
all observations of T. squamosa in the Cook Islands were from the outer
reef slope, occasionally to depths of 30 m or more.
T. Squamosina
T. squamosina is endemic to the Red Sea, with its past and present
distribution including the northeastern Gulf of Aqaba, the Sinai coast,
and eastern coast of the Red Sea down to Yemen (Huber & Eschner, 2011;
Lim et al., 2021; Richter et al., 2008; Rossbach et al., 2021). There
have also been several anecdotal accounts of the species in Mozambique;
however, later evidence of genetic divergence between specimens in the
Red Sea and Mozambique (Moreels, 2018), as well as the significant
geographic distance from its central range, suggests that the reported
sightings may be of its recently-resurrected sister species, T.
elongatissima, with which it shares a close phylogenetic history
(Fauvelot et al., 2020; Tan et al., 2021). For this reason, without
more information to verify these anecdotal sightings, we do not include
the Western Indian Ocean in the natural range of T. squamosina.
In a survey of giant clams in the Red Sea, Richter et al. (2008)
noted that live specimens of T. squamosina were found exclusively in
very shallow water habitats (e.g., reef flats, rocky and sandy-rubble
flats, seagrass beds, or under branching corals or coral heads
shallower than 2m). Thus, unlike the other two Red Sea species (T.
maxima and T. squamosa), which have broad depth distributions, T.
squamosina is restricted to the reef top and is usually weakly attached
to the substrate (Richter et al., 2008).

Diet and Feeding

During the earliest stages of larval development, giant clams
initially rely on nutrients stored in the egg yolk. Upon formation of
the velum and hollow intestines within the first 2-3 days after
fertilization, veliger larvae transition to planktivory and are able to
actively ingest flagellates (~5 [mu]m in diameter), zooxanthellae and
dissolved organic nutrients from the seawater via the mouth (Fitt et
al., 1984; Soo & Todd, 2014). Like most bivalves, giant clams retain
the ability to filter feed into adulthood by pumping water into their
mantle cavities via an inhalant siphon, filtering plankton through
ciliated gills, and passing the filtered water back out via an
excurrent siphon (Hardy & Hardy, 1969).
However, a defining characteristic of giant clams is their
mutualistic relationship with dinoflagellates of the family
Symbiodiniaceae, known commonly as zooxanthellae, which

[[Page 60503]]

provide the primary source of nutrition to adult clams. Giant clams
strictly acquire symbiotic algae from the seawater during larval
development and therefore do not inherit symbionts via parental oocytes
(Fitt & Trench, 1981; Hartmann et al., 2017). Furthermore,
zooxanthellae are housed extracellularly within a diverticular
extension of the digestive tract (Norton et al., 1992). This `tubular
system' extends throughout the upper levels of the mantle and is
arranged as a dense network of tertiary canals branching off of
secondary structures with no direct connection to the haemolymph of the
clam (Norton et al., 1992). Detailed scanning electron microscope
images have shown that zooxanthellae are often stacked in pillars
within these canals and are co-located with light-scattering iridocyte
cells that enhance photosynthesis (L. Rehm, unpub.) and protect the
algal cells from damaging UV radiation (Rossbach, Overmans, et al.,
2020; Rossbach, Subedi, et al., 2020).
Symbiosis is thought to be established during metamorphosis from
pediveliger to the juvenile clam. At this point, zooxanthellae can be
observed migrating from the stomach to the tubular system (Fitt et al.,
1986; Norton et al., 1992). Although, more recent studies have shown
that genes known to be associated with symbiosis and glycerol synthesis
are expressed in giant clam larvae, suggesting that symbiotic activity
may be initiated earlier during larval development (Mies et al., 2016;
Mies, Voolstra, et al., 2017).
Giant clams receive the majority of their metabolic carbon
requirements via symbiotic autotrophy. They provide dissolved inorganic
nutrients to support photosynthesis (e.g., NH<INF>4</INF>\+\,
NO<INF>3</INF>-, PO<INF>4</INF>\+\) via direct absorption
from the seawater and as an excretory byproduct of respiration (Hawkins
& Klumpp, 1995; Toonen et al., 2011). In return, zooxanthellae transfer
photosynthetic carbon to the host in the form of glucose, glycerol,
oligosaccharides and amino acids (Griffiths & Streamer, 1988; Ishikura
et al., 1999; Mies et al., 2016).
Under natural conditions, the contribution of autotrophy to giant
clam nutrition tends to increase with body size and has been shown to
vary between species (Klumpp & Griffiths, 1994; Klumpp & Lucas, 1994;
Hawkins & Klumpp, 1995). This may in part be related to differences in
their characteristic habitats. For example, T. derasa and T.
mbalavuana, two species which occur predominantly in clear, oceanic
environments, derive most (T. mbalavuana: 70 percent at 28 m, 105
percent at 15 m), if not all (T. derasa), of the carbon required for
growth and respiration from autotrophy (Klumpp & Lucas, 1994). Notably,
only T. mbalavuana, which is the deepest-occurring species of giant
clam, increased its photosynthetic efficiency in the lowest light
conditions (Klumpp & Lucas, 1994). H. hippopus and T. gigas exhibit a
different strategy altogether, reflecting their natural occurrence in
shallower intertidal and subtidal habitats, where there is often a
higher concentration of suspended organics in the water column. Klumpp
et al. (1992) showed that T. gigas is an efficient filter-feeder and
that heterotrophic carbon supplied significant amounts of the total
carbon necessary for its respiration and growth (65 percent in ~43 mm
individuals and 34 percent in ~167 mm individuals). In a follow-up
study, Klumpp and Griffiths (1994) similarly found that ingested carbon
provided 61 to 113 percent of total needs in 40 to 80 mm T. gigas and
36 to 44 percent in H. hippopus. Some have hypothesized that
differences in energy acquisition and expenditure may in part explain
the growth and size differences among giant clam species, and in
particular the enormous size of T. gigas. At this point, however, no
clear nutritional basis for these differences has been resolved (Klumpp
& Griffiths, 1994).
Giant clams associate with several Symbiodiniaceae genera, which
can vary by geographic location (Fitt et al., 1986). In the central Red
Sea, for example, all sampled species (T. maxima, T. squamosa, T.
squamosina) were found to exclusively harbor strains of Symbiodinium
(formerly known as clade A) (Pappas et al., 2017). In Okinawa, Japan,
T. squamosa hosted varying communities of Symbiodinium, Cladocopium
(formerly clade C), and Durusdinium (formerly clade D) (Ikeda et al.,
2017). Similarly, populations of T. squamosa, T. maxima, and T. crocea
in eastern Indonesia were found to associate with mixed communities of
these three genera (DeBoer et al., 2012). While certain symbiont genera
have been shown to confer physiological benefits to coral hosts (e.g.,
greater tolerance to thermal stress or enhanced growth rate), there is
no consistent evidence that these patterns translate directly to giant
clams (reviewed in DeBoer et al., 2012).

Growth and Reproduction

Giant clams are protandrous hermaphrodites, meaning they mature
first as males and later develop ovaries to function as both male and
female simultaneously (Wada, 1952; Rosewater, 1965). Size and age at
maturity vary by species and geographic location, but generally, giant
clams are known to reach male phase maturity at around 2-3 years of age
(Heslinga et al., 1984; Shelley, 1989) and female phase maturity as
early as 3-5 years (Heslinga et al., 1984; Isamu, 2008). In larger
species, such as T. gigas, female maturity typically occurs later at
around 8-9 years of age (Gomez & Mingoa-Licuanan, 2006). Giant clams
reproduce via broadcast spawning, in which sperm and eggs are released
into the water column where external fertilization takes place (Wada,
1954). Sperm is released first, followed by eggs after a short interval
(Munro, 1993a).
Giant clams are exceptionally fecund, with individuals producing by
many estimates tens to hundreds of millions of eggs during a single
spawning event (Lucas, 1988). This number varies by species; for
example, estimates suggest that H. porcellanus can release around 5
million eggs (Alc[aacute]zar et al., 1987), H. hippopus can release 25-
60 million eggs (Jameson, 1976; Alcala et al., 1986), and T. gigas can
release up to 500 million eggs (Crawford et al. 1986). However, despite
their high fecundity, giant clams experience very high rates of
mortality during early development (Jameson, 1976; Beckvar, 1981),
resulting in very low levels of natural recruitment (Munro, 1993a).
Reports suggest that less than 1 percent of all giant clam fertilized
eggs survive larval development and progress to the juvenile phase in
the wild (Jameson, 1976; Fitt et al., 1984; Crawford et al., 1986). As
Lucas (1994) describes, ``the extreme example is T. gigas, which being
at or near the pinnacle of fecundity, must have near the lowest level
of survival of potential recruits in the animal kingdom.''
Many have described giant clam recruitment as ``erratic'' (McKoy et
al., 1980; Adams et al., 1988; Lucas, 1994; Guest et al., 2008). For
example, Braley (1988) observed ``extremely low'' average recruitment
on the Great Barrier Reef, punctuated by a major recruitment event in
1987, which yielded the largest population of T. gigas that had been
recorded at the time. This pattern aligns with the concept of
`sweepstakes' reproduction, which is the chance matching of
reproductive activity with oceanographic conditions conducive to
spawning, fertilization, dispersal, and successful recruitment
(Hedgecock, 1994). This can lead to sporadic waves of recruitment
depending on the prevailing oceanographic conditions facilitating
fertilization and carrying a successful cohort of `sweepstakes' larvae
to a suitable settlement location. Importantly, for broadcast spawning
organisms like giant clams, which

[[Page 60504]]

primarily rely on the mixing of gametes with neighboring individuals,
this reproductive strategy can be especially sensitive to changes in
population density. In particular, low abundance and low population
density severely reduces the likelihood of such sweepstakes success by
minimizing the chance of fertilization.
There is considerable variation in the frequency and seasonality of
spawning events among giant clam species. There is no evidence of
reproductive seasonality in the central tropics, with some populations
possessing ripe gametes year-round (Heslinga et al., 1984; Munro,
1993a; Lindsay et al., 2004). At higher latitudes, spawning is most
often associated with late spring and summer months and can occur once
per year (Shelley & Southgate, 1988) or in some cases periodically over
the course of several months (Fitt & Trench, 1981; Heslinga et al.,
1984; Roa-Quiaoit, 2005). The environmental cues that initiate gamete
release are not fully understood, but there is evidence that the lunar
cycle may play a critical role. In Palau, for example, 76 percent and
24 percent of 55 observed spawning events by T. gigas occurred during
the second and fourth quarter of the lunar cycle, respectively
(Heslinga et al., 1984). Unlike many other broadcast spawning
organisms, there is little evidence that temperature is important for
the induction of spawning (Wada, 1954; Fitt & Trench, 1981).
Once one or more clams have begun to spawn, chemical cues
associated with egg release have been shown to play a role in
triggering the spawning of nearby individuals, which then release sperm
for fertilization (Munro, 1993a). While a maximum distance between
spawning individuals has not been quantified (Neo et al., 2015), in
situ observations by Braley (1984) showed that 70 percent of the
nearest spawning neighbors were within 9 m of one another, while only
13 percent were between 20-30 m of one another. Through laboratory
trials, Neo et al. (2015) found that gametes of T. squamosa remained
viable for up to 8 hours, but that viability decreased significantly
with time. Because of these factors, maintaining sufficient population
densities to facilitate fertilization among neighboring individuals is
vital to the persistence of giant clam populations.
Importantly, there is also some evidence that giant clams are able
to self-fertilize with varying fitness consequences among different
species. After observing that the end of sperm release occasionally
overlaps with the beginning of egg release in certain giant clam
species (see also Kurihara et al. (2010)), Murakoshi and Hirata (1993)
experimentally induced self-fertilization in four species of giant
clams (H. hippopus, T. crocea, T. maxima, and T. squamosa) by removing
the gonads and mixing gametes. They found that all four species are
capable of self-fertilization, but that larval development of H.
hippopus was significantly altered, and no T. maxima juveniles
metamorphosed completely to the normal pediveliger stage. Juvenile T.
crocea and T. squamosa survived up to a year post-fertilization, but
the study was not long enough to evaluate possible effects on
reproductive maturity or later-phase development. More recently, Zhang
et al. (2020) evaluated the fitness effects of self-fertilization in
three species of giant clams (T. crocea, T. derasa, and T. squamosa)
after 1 year of development. They found that there was no effect of
self-fertilization on the fertilization rate or zygotic fertility in
any species. Larval survival and growth rate was significantly reduced
in T. crocea and T. squamosa, but not T. derasa. However, while self-
fertilization may be possible in some species, numerous accounts of
spawning in culture and in situ suggest that sperm and eggs are
released successively without an overlap in timing in the vast majority
of spawning events (LaBarbera, 1975; McKoy, 1980; Wada, 1954). It is
likely that this limits the occurrence of self-fertilization in nature
and minimizes its role in giant clam productivity.
Once an egg is fertilized, the life cycle of giant clams is typical
of bivalve molluscs (Lucas, 1994; Soo & Todd, 2014). Fertilized eggs
are approximately 90-130 [mu]m in diameter (Jameson, 1976) and have a
slight negative buoyancy. They usually develop into swimming
trochophores within 12-24 hours, at which time they are able to alter
their depth distribution and begin searching for an eventual settlement
site (Ellis, 1997; Neo et al., 2015). Shell production in molluscs
begins at this early phase of development, following a thickening of
epithelial cells that will define the future shell field (Gazeau et
al., 2013). Within 36-48 hours after fertilization, larvae develop into
shelled, swimming veligers, which use a ciliated velum for locomotion
and feeding (Soo & Todd, 2014). The veligers are highly motile and
begin feeding on microalgae of up to 10 [mu]m in diameter (Munro,
1993a). Over the course of several days, the velum begins to degenerate
and a foot develops as the larvae transition into the pediveliger stage
(Soo & Todd, 2014). At this point, larvae alternate between swimming
and crawling on the substrate, using their foot for sensing and feeding
(Lucas, 1988; Soo & Todd, 2014). Pediveligers generally develop 6-14
days post-fertilization; however, Fitt and Trench (1981) noted
considerable variation in the timing of this transition, where most
took place by day 10 but others were observed up to 29 days post-
fertilization.
Larvae metamorphose into juvenile clams at an approximate size of
200 [mu]m (LaBarbera, 1975; Lucas, 1988; Soo & Todd, 2014). Juvenile
clams remain mobile and are able to crawl both horizontally and
vertically using their foot as they search for a settlement location
(Soo & Todd, 2014). Giant clam larvae tend to settle on substrates that
offer shelter in the form of grooves and crevices, highlighting the
importance of habitat rugosity during this stage of development (Soo &
Todd, 2014). Additionally, juveniles have been observed to move non-
randomly and clump towards conspecifics, which some hypothesize may be
a behavioral adaptation to enhance reproduction and predator defense
(Huang et al., 2007; Neo, 2020). Juvenile clams eventually attach
themselves to the substrate by use of byssal threads, which in some
species will remain in place throughout their lifetime. Larger species
typically lose the byssal threads after reaching adulthood and are held
in place by their size and weight (Lucas, 1988).
Growth rates vary among species, with larger species exhibiting
more rapid growth than smaller species (Munro & Heslinga, 1983; Lucas,
1988). Growth rates after settlement generally follow a sigmoid (``S''
shaped) curve, beginning slowly, then accelerating after approximately
1 year and slowing again as the animals approach sexual maturity
(Lucas, 1988; Ellis, 1997). Lucas (1994) provides examples of maximum
rates of monthly shell growth for several species as recorded under
culture conditions in the Philippines: H. hippopus--5.3 mm, T.
squamosa--4.5 mm, T. derasa--5.6 mm, and T. gigas--9.1 mm (Calumpong,
1992; Gomez & Mingoa, 1993). Shell growth continues throughout the
clam's lifespan (Lucas, 1994).
The maximum lifespan of giant clams is not known, but the oldest
reliably aged individual was a large T. gigas determined to be 63 years
old (Lucas, 1994). Similar aging studies based on the analysis of
growth rings in the shell estimated a 43 cm-long T. squamosa to be
around 22 years old (Basker, 1991), a ~20 cm-long T. maxima to be
around 28 years old (Romanek et al., 1987), and a 93 cm-long T. gigas
to be around 60 years old (Watanabe et al., 2004). Using growth and
mortality estimates, Dolorosa et al. (2014) predicted a

[[Page 60505]]

lifespan of more than 20 years for H. porcellanus.

Population Structure

Current literature indicates several consistent features of giant
clam population genetics throughout their range. The first is
significant genetic differentiation between giant clam populations of
the central Pacific region, including Kiribati, Marshall Islands,
Tuvalu and Cook Islands, and the western Pacific region, including the
Great Barrier Reef, Philippines, Solomon Islands and Fiji (Benzie &
Williams, 1995, 1997). The pattern is consistent across T. gigas and T.
maxima, although there is some variability in the inferred level of
connectivity between the Great Barrier Reef and Philippines in T.
derasa (Macaranas et al., 1992). Interestingly, the patterns of genetic
connectivity do not reflect oceanic currents as would be expected for a
passively-dispersing organism like giant clams. Hence, Benzie and
Williams (1997) hypothesize that ``other mechanisms dominate present-
day dispersal, or that [the observed patterns] reflect past
connectivity which present-day dispersal along major surface currents
has not altered over thousands of years.''
Other studies describe a relatively consistent pattern of genetic
structure within the Indo-Pacific region, often highlighting four or
five genetic clusters distinguishing populations of the Red Sea,
Western Indian Ocean, Eastern Indian Ocean, Indo-Malay Archipelago, and
Western Pacific. In every case, populations of T. squamosa and T.
maxima in the Red Sea are found to be highly divergent from all other
populations in their range (Nuryanto & Kochzius, 2009; Huelsken et al.,
2013; Hui et al., 2016; Pappas et al., 2017; Lim et al., 2018). The
same is true of Western Indian Ocean populations, though to a slightly
lesser extent (Hui et al., 2016; Lim et al., 2018). Additionally, there
is a uniform pattern of differentiation between giant clam populations
in the Indo-Malay Archipelago and those in the eastern Indian Ocean and
Java Sea (Kochzius & Nuryanto, 2008; Nuryanto & Kochzius, 2009;
Huelsken et al., 2013; Hui et al., 2016). This pattern is largely
consistent across T. squamosa, T. maxima, and T. crocea, although some
studies note variability between species with respect to certain
genetic breaks identified in the Java Sea and in Chendewasih Bay
(Nuryanto & Kochzius, 2009; Huelsken et al., 2013). Population genetic
data from T. maxima and T. crocea (species which are not subject to
this rulemaking) suggest that there may also be genetic breaks between
the western Pacific islands and Indo-Malay Archipelago (Nuryanto &
Kochzius, 2009; Huelsken et al., 2013; Hui et al., 2016). However,
similar data are not available for any of the seven species considered
here.
On a smaller scale, giant clam populations within the northern and
central Great Barrier Reef exhibit high genetic connectivity (Benzie &
Williams, 1992, 1995, 1997). Evans and Jerry (2006) found tenuous
evidence of isolation-by-distance in this region, which would suggest
that populations may be connected by the prevailing southward flow of
the East Australian Current. In contrast, Kittiwattanawong et al.
(2001) found that T. squamosa in the Andaman Sea are genetically
distinct from those in the Gulf of Thailand, likely due to the physical
barrier of the Malay Peninsula minimizing dispersal between these
populations.

Current and Historical Distribution and Population Abundance

There are no current or historical estimates of global abundance
for any of the seven giant clam species considered here. Therefore, we
rely on the best available scientific and commercial data, including
formal and informal survey data, qualitative descriptions of abundance
or population trends, and anecdotal reports from specific sites, to
evaluate the status of each species in each country, territory, or
region throughout its range.
Much of the information used to determine the status of each
species is derived from Table 4 of Neo et al. (2017), which we have
supplemented or revised based on more recent survey data or reports. We
have also adjusted the criteria used to define each qualitative
abundance category, which Neo et al. (2017) had previously defined as
follows: Abundant: >100 individuals (ind) ha-1, Frequent: 1-
10 ind ha-1, Rare: <0.1 ind ha-1. In doing so, we
considered the reproductive ecology of giant clams, and in particular,
the observations of Braley (1984) regarding the distance between
nearest-spawning T. gigas during a natural spawning event. Braley
(1984) measured that 70 percent of nearest-spawning individuals were
within 9 m of one another, while only 13 percent were between 20-30 m
of one another, suggesting that spawning synchrony decreases with
distance. As broadcast spawning organisms, giant clams rely on
sufficient population density in order to facilitate successful
external fertilization of their gametes. Based on the distances above,
we determined the minimum population density in a 1-hectare (10,000
m\2\) square grid in which individuals could be evenly spaced at 9 and
30 m apart. Respectively, these distances represent populations that we
consider to be ``Abundant,'' where we expect relatively high
reproductive success, and ``Frequent,'' where we expect lower but
moderate reproductive success. A ``Rare'' population in which
individuals are spaced farther than 30 m apart on average is likely to
have infrequent, sporadic reproductive success. This approach led to
the following criteria: Abundant: >100 ind ha-1 (9-m
distance), Frequent: 10-100 ind ha-1 (30-m distance), and
Rare: <10 ind ha-1 (>30-m distance).
Importantly, precise quantitative assessments of abundance are not
possible in most instances, as many regions lack current or
comprehensive survey data (see the accompanying Status Review Report
for all reported estimates of population density from specific
surveys). Thus, where survey data are limited to only a few sites or
where recent survey data are not available, we also take into account
other available information, including qualitative descriptions of
abundance or population trends, to reach a determination on the likely
status of the species throughout each country, territory, or region in
its entirety. In other words, although survey data from a single site
may indicate a relatively abundant population, if the species is
considered absent from all other areas, the species may be considered
``frequent'' or ``rare'' on average in that location. This methodology
generally follows the approach used by Neo et al. (2017).
Additionally, it is important to note that, in the interest of
simplicity, these qualitative abundance categories are based on an
assumption of uniform spacing between individuals. However, a number of
studies report that giant clams often occur in a clumped distribution,
where individuals are concentrated in a number of small, distantly-
separated groups. In these cases, the abundance categories may
underestimate the productivity of the respective population. In other
words, if survey data indicate that a species occurs in some location
at low abundance on average, reproductive success is more likely if the
individuals are clustered in a few small groups, minimizing the
distance between neighboring individuals, than if they are spread
uniformly across the seafloor.
In table 1 below, we summarize the status of each species in each
of the locations where it has been observed. Full narrative
descriptions of the data

[[Page 60506]]

and scientific studies that informed the following abundance
assessments can be found in the accompanying Status Review Report
(Rippe et al., 2023).

Table 1--Summary of the Population Status for Each of the Seven Giant Clam Species in All Countries, Territories, and Regions Where They Have Been
Observed (Adapted From Neo et al., 2017 and Supplemented With More Recent Information Where Available)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Location HH \1\ HP \1\ TD \1\ TG \1\ TMB \1\ TS \1\ TSI \1\
--------------------------------------------------------------------------------------------------------------------------------------------------------
Red Sea:
Djibouti................. ................ ................ ................ ................ ............... +.............. ...............
Egypt.................... ................ ................ ................ ................ ............... ++............. +
Israel................... ................ ................ ................ ................ ............... dd............. ...............
Jordan................... ................ ................ ................ ................ ............... ++............. +
Saudi Arabia............. ................ ................ ................ ................ ............... +++............ +
Somalia.................. ................ ................ ................ ................ ............... +.............. ...............
Sudan.................... ................ ................ ................ ................ ............... ++............. ...............
Yemen.................... ................ ................ ................ ................ ............... ++............. dd
Southeast Africa:
Cargados Carajos ................ ................ ................ ................ ............... +.............. ...............
Archipelago.
Comoros.................. ................ ................ ................ ................ ............... ++............. ...............
Kenya.................... ................ ................ ................ ................ ............... +.............. ...............
Madagascar............... ................ ................ ................ ................ ............... ++............. ...............
Mauritius................ ................ ................ ................ ................ ............... +.............. ...............
Mayotte.................. ................ ................ ................ ................ ............... dd............. ...............
Mozambique............... ................ ................ ................ ................ ............... +.............. dd
La R[eacute]union........ ................ ................ ................ ................ ............... dd............. ...............
Seychelles............... ................ ................ ................ ................ ............... +.............. ...............
South Africa............. ................ ................ ................ ................ ............... dd............. ...............
Tanzania................. ................ ................ ................ ................ ............... +.............. ...............
Indian Ocean:
India.................... +............... ................ ................ +............... ............... +.............. ...............
Australia (NW Islands)... ++.............. ................ ++.............. +............... ............... +.............. ...............
Christmas Island......... ................ ................ +............... -............... ............... +.............. ...............
Cocos (Keeling) Islands.. ................ ................ +............... -............... ............... -.............. ...............
Chagos................... ................ ................ ................ ................ ............... dd............. ...............
Maldives................. ................ ................ ................ ................ ............... +.............. ...............
Sri Lanka................ ................ ................ ................ ................ ............... dd............. ...............
East Asia:
Japan.................... +............... ................ ................ +............... ............... +.............. ...............
Taiwan................... -............... ................ -............... -............... ............... +.............. ...............
China.................... ................ ................ ................ -............... ............... +.............. ...............
South China Sea.......... +............... ................ +............... +............... ............... ++............. ...............
South Asia:
Indonesia................ +............... +............... +............... +............... ............... +++............ ...............
Malaysia................. +............... +............... +............... +............... ............... +++............ ...............
Myanmar (Burma).......... dd.............. ................ ................ dd.............. ............... dd............. ...............
Cambodia................. ................ ................ ................ dd.............. ............... ++............. ...............
Brunei................... ................ ................ ................ ................ ............... dd............. ...............
Philippines.............. +............... +............... +............... +............... ............... ++............. ...............
Singapore................ -............... ................ ................ -............... ............... +.............. ...............
Thailand................. ................ ................ ................ -............... ............... +.............. ...............
Vietnam.................. ................ ................ ................ dd.............. ............... ++............. ...............
East Timor............... ................ ................ ................ dd.............. ............... ............... ...............
Pacific Ocean:
Australia (Great Barrier ++.............. ................ ++.............. ++.............. dd............. ++............. ...............
Reef).
Fiji..................... REIN............ ................ +............... REIN............ +.............. ++............. ...............
New Caledonia............ +............... ................ +............... -............... +.............. +.............. ...............
Papua New Guinea......... +............... +............... +............... +............... ............... +.............. ...............
Solomon Islands.......... +............... ................ +............... +............... ............... +++............ ...............
Vanuatu.................. ++.............. ................ REIN............ REIN............ ............... +.............. ...............
FSM...................... +............... ................ INT............. REIN............ ............... +.............. ...............
Guam..................... REIN............ ................ REIN............ REIN............ ............... +.............. ...............
Republic of Kiribati..... +............... ................ ................ +............... ............... +.............. ...............
Marshall Islands......... ++.............. ................ INT............. +............... ............... ++............. ...............
CNMI..................... REIN............ ................ REIN............ REIN............ ............... -.............. ...............
Palau.................... ++.............. +............... ++.............. +............... ............... ++............. ...............
American Samoa........... REIN............ ................ INT............. INT............. ............... +.............. ...............
Cook Islands............. ................ ................ INT............. INT............. ............... +.............. ...............
French Polynesia......... ................ ................ ................ ................ ............... +.............. ...............
Pitcairn Islands......... ................ ................ ................ ................ ............... ++............. ...............
Niue..................... ................ ................ ................ ................ ............... +.............. ...............
Samoa.................... REIN............ ................ INT............. INT............. ............... +.............. ...............
Tokelau.................. ................ ................ ................ ................ ............... +.............. ...............
Tonga.................... REIN............ ................ +............... REIN............ +.............. +.............. ...............

[[Page 60507]]

Tuvalu................... dd.............. ................ INT............. -............... ............... +.............. ...............
United States (Hawaii)... ................ ................ ................ INT............. ............... INT............ ...............
United States (Johnston ................ ................ ................ dd.............. ............... ............... ...............
Atoll).
United States (Kingman ................ ................ ................ ................ ............... +.............. ...............
Reef).
United States (Wake ................ ................ ................ dd.............. ............... dd............. ...............
Atoll).
Wallis and Futuna Islands ................ ................ ................ ................ ............... +++............ ...............
--------------------------------------------------------------------------------------------------------------------------------------------------------
Note: Species abundance categories are as follows. +++: Abundant (>100 ind ha-1), ++: Frequent (10-100 ind ha-1), +: Rare (<10 ind ha-1), -: Locally
extinct, INT: Introduced to non-native location; REIN: Reintroduced (cultured specimens) to locations where the species had previously been
extirpated; dd: Data Deficient (i.e., reports of species presence are not confirmed). Empty cells indicate locations where a species has not been
observed.
\1\ Species names are abbreviated as follows: HH: H. hippopus, HP: H. porcellanus, TD: T. derasa, TG: T. gigas, TMB: T. mbalavuana, TS: T. squamosa,
TSI: T. squamosina.

Extinction Risk Analysis

Methods

In determining the extinction risk of each species, it is important
to consider both the demographic risks facing the species, as well as
current and potential threats that may affect the species' status. To
this end, the status review synthesized the best available scientific
and commercial data regarding the five threat categories listed in
section 4(a)(1) of the ESA. These are: (1) the present or threatened
destruction, modification, or curtailment of its habitat or range; (2)
overutilization for commercial, recreational, scientific, or
educational purposes; (3) disease or predation; (4) inadequacy of
existing regulatory mechanisms; or (5) other natural or manmade factors
affecting its continued existence. Second, we conducted a demographic
risk analysis following the Viable Population (VP) approach derived
from McElhany et al. (2000), which addresses four biological
descriptors of species status: abundance, productivity (i.e.,
population growth rate), spatial distribution, and diversity. The VP
approach reflects concepts that are well-founded in conservation
biology and considers demographic factors that individually and
collectively provide strong indicators of extinction risk. It is
designed to both capture the biological symptoms of past threats that
have contributed to the species' current status and provide insight
into how the species may respond to present and future threats.
With respect to each threat and each demographic risk factor, we
assigned a qualitative score from 1 to 5 representing its estimated
contribution to the species' extinction risk (``very low,'' ``low,''
``moderate,'' ``high,'' or ``very high'' risk). Detailed definitions of
these risk levels can be found in the accompanying Status Review
Report. We also assigned a confidence rating from 0 to 3, reflecting
the quantity and quality of information used to assign the score, as
follows: 0 = No confidence (i.e., no available information); 1 = Low
confidence (i.e., very limited available information); 2 = Medium
confidence (i.e., some reliable information available, but reasonable
inference and extrapolation is required); 3 = High confidence (i.e.,
reliable information with little or no extrapolation or inference
required).
Lastly, all information from the threats assessment and demographic
risk analysis was synthesized to estimate the overall risk of
extinction for each species. For this analysis, we used three reference
levels of extinction risk (``low,'' ``moderate,'' and ``high''), which
are consistent with those used in prior ESA status reviews. ``Low''
risk indicates a species that is not at a moderate or high level of
extinction risk (see ``Moderate'' and ``High'' risk below). A species
may be at a low risk of extinction if it is not facing threats that
result in declining trends in abundance, productivity, spatial
structure, or diversity. A species at low risk of extinction is likely
to show stable or increasing trends in abundance and productivity with
connected, diverse populations. ``Moderate'' risk indicates a species
that is on a trajectory that puts it at a high level of extinction risk
in the foreseeable future (see ``High'' risk below). A species may be
at moderate risk of extinction due to projected threats or declining
trends in abundance, productivity, spatial structure, or diversity.
``High'' risk indicates a species that is at or near a level of
abundance, productivity, spatial structure, and/or diversity that
places its continued persistence in question. The demographics of a
species at such a high level of risk may be highly uncertain and
strongly influenced by stochastic or depensatory processes. Similarly,
a species may be at high risk of extinction if it faces clear and
present threats (e.g., confinement to a small geographic area; imminent
destruction, modification, or curtailment of its habitat; or disease
epidemic) that are likely to create present and substantial demographic
risks.
Importantly, these extinction risk categories are not meant to be a
direct translation of the final listing determination for the species,
as listing determinations must also consider ongoing conservation
efforts of any State, foreign nation, or political subdivision thereof
(16 U.S.C. 1533(b)(1)(A)) to determine whether the species meets the
ESA's definition of an ``endangered species'' or ``threatened
species.'' Rather, the extinction risk assessment in the Status Review
Report represents the scientific conclusion about the overall risk of
extinction faced by the species under present conditions and in the
foreseeable future based on an evaluation of the species' demographic
risks and assessment of threats.

Defining the ``Foreseeable Future''

The appropriate time horizon for evaluating whether a species is
more likely than not to be at a high level of risk in the ``foreseeable
future'' varies on a case-by-case basis. For example, the time horizon
may reflect certain life history characteristics (e.g., long generation
time or late age-at-maturity) and the time scale over which identified
threats are likely to impact the biological status of the species. In
other words, the foreseeable future represents the period of time over
which we can reasonably determine that both future threats and the
species' response to

[[Page 60508]]

those threats are likely. See generally 50 CFR 424.11(d). It does not
necessarily need to be limited to the period that the species' status
can be quantitatively modeled or predicted within predetermined limits
of statistical confidence. Reliable projections may be qualitative in
nature.
With these criteria in mind, we determined that the ``foreseeable
future'' for the following extinction risk analyses spans approximately
~50-60 years. Based on what is known about the life history traits of
giant clams, with longevity estimated to be at least 50 years (up to 60
years for T. gigas), maturity ranges from 3 to 9 years, and exceedingly
low recruitment, it would likely take at least this amount of time
(i.e., multiple generations) for the effects of any management actions
to be realized and reflected in population abundance indices.
Similarly, the impact of present threats to the species would be
realized in the form of noticeable population declines within this
timeframe, as has been demonstrated in the available literature. As the
primary operative threats to giant clams are overutilization for
subsistence and commercial harvest, this timeframe would allow for
reliable predictions regarding the impact of current levels of harvest-
related mortality on the biological status of all the species.
One important exception to this timeframe is in regard to the
future impacts and threats related to climate change. Based on the
current standard for climate projections, under which most available
models are extended to the end of the century, we use the same
timeframe (i.e., present day-2100) to define the ``foreseeable future''
in assessing the likely future threat of climate-related habitat
degradation and climate-related impacts to giant clam fitness.

Threats Assessment

Below, we describe the natural and anthropogenic threats to each of
the seven giant clam species within the framework of the five threat
categories outlined in section 4(a)(1) of the ESA. Because a number of
species occupy overlapping ranges and often co-occur in similar
habitats, certain threats may apply to more than one species. In each
section, we highlight the severity of the threat to each of the species
affected and provide additional species-specific information where
appropriate. Additional details may be found in the Status Review
Report (Rippe et al., 2023).

The Present or Threatened Destruction, Modification, or Curtailment of
Its Habitat or Range

As is mentioned in the species descriptions above, giant clams are
often closely associated with coral reefs, inhabiting all types of
shallow-water reef ecosystems (i.e., fringing, barrier and atoll
reefs), as well as various reef-adjacent habitats. However, there is no
conclusive evidence that giant clams directly rely on live, pristine
corals for their survival. Certain species are habitat generalists
(e.g., T. squamosa, T. gigas)--they are often observed among live
corals but can also be found in other habitats, which are not pristine
coral reef (e.g., sand, rock, dead coral rubble, seagrass beds,
macroalgae zones). Others are more specialized--T. mbalavuana is found
exclusively at depth on reef slopes, T. derasa is found predominantly
in offshore coral reef areas, while H. hippopus, H. porcellanus and T.
squamosina tend to prefer sandy areas, shallow lagoon flats and
seagrass beds adjacent to coral reefs.
Available research on larval settlement preference offers some
clues as to what may be driving the association with coral reefs.
Several studies show that T. squamosa larvae prefer to settle on
substrates of relatively high rugosity and are drawn to crustose
coralline algae (CCA), but actively avoid settling on live coral
(Courtois de Vicose, 2000; Calumpong et al., 2003; Neo et al., 2009).
Additionally, the small giant clam (T. maxima) has shown an ability to
discriminate between ``favorable'' and ``unfavorable'' habitats,
preferring to settle near the effluent of conspecifics and near the
effluent of live coral and CCA, rather than cyanobacteria and sponges
(Dumas et al., 2014). However, this information is limited to only one
of the seven species being analyzed in connection with this proposed
rule, and there are no such data for species that are predominantly
found in sand flats and seagrass beds, where rugosity is especially low
and settlement cues might differ.
Based on the known features of giant clam biology and larval
development, Lucas et al. (1989) hypothesized that the proximity of
giant clams to coral reefs is, to some extent, a result of two
environmental requirements, which are maximized in shallow reef
habitats: (1) high light conditions to support the photosynthetic
nutrition that giant clams derive from their algal symbionts, and (2)
substrate rugosity to provide cryptic settlement locations for
vulnerable recruits and juveniles. While we cannot conclude that these
factors are equally important to all species of giant clams, it is
within the context of these two habitat requirements that we discuss
the following threats to coral reef ecosystems and their potential
impacts to giant clams.
Climate Change Impacts to Coral Reefs
Reef-building corals typically occur in waters that range between
25 [deg]C-30 [deg]C and are highly sensitive to temperature excursions
outside of this range (Brainard et al., 2011). Prolonged exposure to
high temperature anomalies can lead to coral bleaching, where the coral
host expels its symbiotic zooxanthellae, leaving the tissue translucent
and revealing its white skeleton underneath. Bleaching-associated
mortality is quite variable and can depend on the duration and
intensity of elevated temperatures, geographic location, bleaching
history, species present, and other factors (Pandolfi et al., 2011;
Putnam & Edmunds, 2011; van Hooidonk & Huber, 2012). Mild to moderate
bleaching does not always lead to death; however, repeated and
prolonged bleaching can cause widespread coral mortality on regional or
global scales. Extreme summer temperature anomalies associated with
strong El Ni[ntilde]o events have led to three recognized global
bleaching events in 1997-98, 2009-10 and 2014-17 (Hughes, Kerry, et
al., 2017; Lough et al., 2018; Eakin et al., 2019). The latest (2014-
17) was the longest and most severe global bleaching event in recorded
history. It affected every major coral reef region and led to the
mortality of one third of the Great Barrier Reef in Australia (Couch et
al., 2017; Hughes, Kerry, et al., 2017; Hughes, Kerry, et al., 2018).
In addition, many other regional-scale bleaching events over the last
several decades have caused widespread coral mortality in reef
communities throughout the Indo-Pacific (Brainard et al., 2011; Hughes,
Anderson, et al., 2018).
While coral bleaching patterns can be complex, there is a general
consensus that rising global ocean temperatures have led to more
frequent and severe coral bleaching and mortality events (Hughes,
Anderson, et al., 2018; Lough et al., 2018). Without drastic action to
curb greenhouse gas emissions, this trend is projected to continue
throughout this century (van Hooidonk et al., 2016). Additionally,
several studies have shown that warming can significantly increase
coral susceptibility to disease (Bruno et al., 2007; Sokolow, 2009;
Brainard et al., 2011; Howells et al., 2020). The combination of these
warming-related impacts has already caused dramatic

[[Page 60509]]

declines in many coral species and changes to the composition and
structure of coral reefs around the world (Brainard et al., 2011;
Hughes, Barnes, et al., 2017; Hughes, Kerry, et al., 2018). During the
major 2016 coral bleaching event on the Great Barrier Reef, for
example, the fast-growing, structurally complex tabular and branching
species suffered disproportionately (>75 percent mortality on heavily
bleached reefs), shifting reef communities towards taxa with simpler
morphological characteristics and slower growth rates (Hughes, Kerry,
et al., 2018). Other studies similarly suggest that coral reef
ecosystems, rather than disappear entirely as a result of warming, will
likely persist, but with unpredictable changes to their community
composition and ecological function (Pandolfi et al., 2011; Hughes et
al., 2012).
Coral reefs are also facing increasing risk from ocean
acidification, the process by which atmospheric carbon dioxide
(CO<INF>2</INF>) is absorbed into the surface ocean, resulting in
reduced seawater pH and reduced availability of carbonate ions. Due to
anthropogenic CO<INF>2</INF> emissions, average surface ocean pH (total
scale, pHt) has already decreased by more than 0.1 pHt units below the
pre-industrial average of 8.17, and is expected to fall up to an
additional 0.42 pHt units by 2100 under the worst-case emissions
scenario from the Intergovernmental Panel on Climate Change (IPCC) (RCP
8.5) (P[ouml]rtner et al., 2014).
Such reductions in ocean pH could lead to drastic changes to the
net calcification balance in many coral reef ecosystems. Numerous
laboratory and mesocosm experiments have demonstrated a correlation
between lower pH (or elevated partial pressure of CO<INF>2</INF>,
pCO<INF>2</INF>) and decreased coral calcification rates (Anthony et
al., 2008; Ries et al., 2009; Anthony et al., 2011; Gazeau et al.,
2013; Albright et al., 2018). Brainard et al. (2011) provide a table
summarizing the existing literature on the topic (table 3.2.2 of the
report), and for every species studied, net calcification rate either
declines, or in very few, there is no significant effect. In a pair of
controlled mesocosm experiments, net community calcification of a small
enclosed coral reef was found to increase under enhanced alkalinity and
decrease after the addition of CO<INF>2</INF> (Albright et al., 2016;
Albright et al., 2018), indicating that current levels of acidification
are already impairing ecosystem-level calcification and will likely
exacerbate this effect in the future. Coupled with dwindling coral
cover due to warming-associated bleaching and mortality, continued
acidification could transition many reef systems from net overall
accretion to net erosion within this century (Eyre et al., 2018;
Cornwall et al., 2021).
Others anticipate that ocean acidification will also weaken the
structural integrity of coral reefs, both by promoting the efficiency
of bioeroding organisms and by reducing reef cementation (i.e.,
secondary processes of carbonate precipitation that bind the reef
framework). Observations from coral reefs of the eastern Pacific, which
occur in naturally low-pH upwelling zones reveal some of the highest
rates of bioerosion documented globally, as well as poorly cemented,
fragile, and unstable reef frameworks (Glynn, 1988; Eakin, 1996, 2001;
Manzello et al., 2008). Crustose coralline algae (CCA) contribute
significantly to reef cementation by consolidating loose rubble and
sealing porous dead coral skeletons (Adey, 1998; Littler & Littler,
2013). There is major concern that CCA may be among the most sensitive
taxa to declines in seawater pH, because they build their skeletons
with magnesium-rich calcite, a highly soluble form of carbonate
(Andersson et al., 2008). Although some argue that the risk to CCA may
be over-estimated, as certain aspects of their skeletal structure and
biology have proven resilient to projected future conditions (Nash et
al., 2013; Nash et al., 2015; Nash et al., 2016). At this point, the
potential impacts of ocean acidification on CCA are not fully resolved.
Given the documented and projected impacts of ocean warming and
acidification on coral reef ecosystems, we assessed the direct
implications of these impacts on the extinction risk of the seven giant
clam species. In our previous status review for 82 species of corals,
Brainard et al. (2011) concluded that ``the combined direct and
indirect effects of rising temperature, including increased incidence
of disease, and ocean acidification [. . .] are likely to represent the
greatest risks of extinction to all or most of the candidate coral
species over the next century.'' They assessed the threat of continued
ocean warming to be ``highly certain'' and graded the threat as
``high'' for most regions where the candidate corals are known to
occur. Based on this assessment, we find it likely that live coral
cover in general will continue to decline due to more frequent and
severe bleaching events, and that ecosystem-scale calcification rates
will decline as a result. Critically for giant clams, the negative
impacts of warming are most pronounced in the fast-growing branching
and tabular coral species, which are the primary contributors to the
three-dimensional complexity of reef habitats. Thus, continued loss of
live coral cover and of these coral species in particular will likely
severely reduce the rugosity of future reef ecosystems. There is also
evidence that ocean acidification will further inhibit calcification
rates of living corals and weaken the structural integrity of the reef
framework, although the magnitude of these effects is not clear. As
with ocean warming, the primary implication of these effects for giant
clams will be reduced habitat rugosity.
Nevertheless, there are two important layers of uncertainty
associated with these predictions, and especially their potential
impacts to giant clam habitat. First, with respect to ocean
acidification, carbonate chemistry is notoriously difficult to model
precisely in open systems, as it relies on many physical and biological
factors, including seawater temperature, proximity to land-based runoff
and CO<INF>2</INF> seeps, proximity to sources of oceanic
CO<INF>2</INF>, salinity, nutrients, as well as ecosystem-level
photosynthesis and respiration rates. The last factor, in particular,
means that in many cases, daily fluctuations in pH or carbonate
chemistry can significantly outweigh projected long-term changes to the
average (Manzello et al., 2012; Johnson et al., 2019). Secondly, as
mentioned above, there is very little research establishing the degree
to which giant clams rely on coral reef rugosity and thus might be
impacted by any reduction thereof. The few larval choice experiments to
date suggest that T. squamosa prefers rough to smooth surfaces and is
attracted to CCA. However, most giant clam species can be found in an
array of habitat types, and some even seem to prefer areas of low
rugosity, such as sand flats and seagrass beds (e.g., H. hippopus, H.
porcellanus, and T. squamosina). No studies have quantified how or if
giant clams might be affected under varying levels of coral reef
complexity.
If giant clams are sensitive to reductions in net ecosystem
calcification and reef rugosity, the projected climate change-related
impacts to coral reefs would likely pose a significant threat to T.
derasa, T. gigas, T. mbalavuana, and T. squamosa within the foreseeable
future, as these species are known to inhabit coral reef environments.
We would expect decreased larval recruitment and juvenile survival
across broad portions of their range. These early life stages are
already known to suffer exceptionally

[[Page 60510]]

high mortality rates naturally, and any further reduction in
productivity would greatly threaten the viability of remaining giant
clam populations.
However, without more information on the direct association between
substrate rugosity and giant clam survival and productivity, it is
difficult to estimate with any confidence the degree to which reef
rugosity must decline to threaten the persistence of these species.
Likewise, given the lingering uncertainty in the dynamics and effects
of ocean acidification, it is not possible to estimate a timespan over
which such a risk can be expected. Thus, while it is likely that
continued ocean warming and acidification will drastically alter coral
reef communities and reduce the rugosity of many reef habitats, we
concluded that the potential effect on the quality or suitability of
giant clam habitat cannot be confidently assessed.
Coastal Development
The physical degradation of nearshore habitats due to coastal
development poses an additional threat to giant clams throughout much
of their range. Sedimentation associated with the construction and
maintenance of coastal infrastructure can reduce the amount of suitable
substrate available for larval settlement. There is extensive evidence
for such an effect in corals--increased sediment load has been shown to
deter larval recruitment (Babcock & Davies, 1991), reduce settlement
success and survival (Hodgson, 1990; Babcock & Smith, 2002), and
decrease the effectiveness of CCA to induce settlement (Ricardo et al.,
2017). We could not find any research directly investigating this
effect in giant clams; however, similarities in the biology and
behavior of giant clam larvae would suggest that comparable results can
reasonably be expected. Like coral larvae, giant clam larvae prefer
rough settlement surfaces and are likely deterred by unconsolidated,
fine-grained silt that is typical of anthropogenic sedimentation.
Moreover, CCA provide a similarly important settlement cue for giant
clams (Courtois de Vicose, 2000; Neo et al., 2009; Neo et al., 2015),
and a reduction in effectiveness would likely decrease larval
recruitment and settlement success.
Importantly, compared to habitat degradation due to climate change,
coastal development poses a more localized threat to giant clam
populations in specific regions. In the Red Sea, for example, Roa-
Quiaoit (2005) notes intense modification to the Jordanian coastline
over ``four decades of rampant development of ports, industrial and
tourism areas, as well as extreme events such as oil spills.'' Surveys
of giant clam density in the area revealed an inverse relationship
between the population density of T. squamosa and metrics of human
impact and coastal use. The author argues that the observed 12-fold
reduction of giant clam density in Jordan over three decades is in
major part due to this intense habitat modification. Similar examples
of anthropogenic impacts to the coastal environment have also been
documented in many areas of the Indo-Pacific region, although this is
often discussed in relation to the health of coral reef ecosystems. In
Singapore, approximately 60 percent coral reef area was lost during the
20th century due to land reclamation and associated sedimentation
(Chou, 2006; Guest et al., 2008). On three specific Singapore reefs--
Tanjong Teritip, Pulau Seringat, and Terumbu Bayan--Neo and Todd (2012)
note that giant clams were once found, but the areas have since been
reclaimed (covered over) in their entirety. In addition, more than 20
percent of coral reefs in Indonesia, 35 percent of reefs in Malaysia,
25 percent of reefs in Papua New Guinea, and 60 percent of reefs in the
Philippines are threatened by the impacts of coastal development,
including runoff from construction and waste from coastal communities
(Burke et al., 2012).
In addition to undergoing intense coastal development activities
over the past several decades, many of these areas are not well
regulated with respect to coastal runoff and often do not prioritize
sustainable management of the coastal environment (e.g., Gladstone et
al., 1999; O. A. Lee, 2010). In contrast, the Great Barrier Reef in
Australia and island nations of the central and western Pacific, two
other important areas of giant clam distribution, likely do not suffer
the same effects of coastal development. Australia strictly enforces an
integrated management plan to protect the Great Barrier Reef from the
effects of coastal land use change via numerous national and State
regulations, and the relatively small populations of most Pacific
island nations minimize the impact of coastal development on
surrounding waters.
Because T. mbalavuana and T. derasa reside preferentially in
offshore coral reef areas, we conclude that habitat degradation of the
nearshore environment related to coastal development likely does not
pose a significant threat to these two species. With respect to H.
hippopus, T. gigas, and T. squamosa, considering the relatively
localized impacts of coastal development (e.g., near heavily urbanized
areas) compared to the size of the species' ranges, we conclude that
the threat of habitat destruction, modification, or curtailment related
to nearshore impacts of coastal development likely poses a low risk to
H. hippopus and T. gigas, and a very low risk to T. squamosa.
Specifically, we find the risk to be lower for T. squamosa due to the
species' expansive geographic range as well as its current abundance
and distribution, compared to H. hippopus and T. gigas.
Because the restricted range of H. porcellanus is centered in a
region of intense urban development (i.e., within the densely populated
Indo-Malay Archipelago), we conclude that habitat destruction and
modification of the nearshore environment poses a moderate risk to the
species. In other words, it likely contributes significantly to the
species' long-term extinction risk, but given the localized nature of
these impacts, does not in itself constitute a danger of extinction in
the near future. H. porcellanus is also faced with an acute threat of
habitat destruction in the northern portion of its range, where
fishermen primarily from Tanmen, China have been razing shallow reef
areas of the South China Sea in a search for giant clam shells (see
Tanmen Destructive Shell Harvesting below). The damage from these
operations is extensive and has likely eliminated any H. porcellanus
that may have previously occurred in the islands of the South China
Sea.
With respect to T. squamosina, we considered reports indicating
specific areas of the Red Sea coastline which have been targeted for
development of tourist activities and infrastructure, including
Hurghada and the Gulf of Aqaba coastline from Sharm el-Sheikh to
Nuweiba (Egypt), Eilat (Israel), and Aqaba (Jordan). These areas are
significant, as they directly overlap with the majority of recent T.
squamosina observations. As is mentioned above, Roa-Quiaoit (2005)
estimated that 70 percent of the Jordanian coastline has been developed
into ports, industrial centers, and tourism areas over the past several
decades. Additionally, near Hurghada, Mekawy and Madkour (2012)
observed dredging activities associated with a newly-constructed harbor
and offshore trash disposal from boats. The authors also described
industrial and tourist activities in several other areas along the
coast of mainland Egypt (e.g., oil drilling in El-Esh, dense industrial
and tourism-related development near Safaga Harbor, high human activity
in Quesir), which they argue have likely been the principal factors
driving the

[[Page 60511]]

declining abundance of giant clams (primarily T. maxima) in these
areas. Similarly, Hassan et al. (2002) reported ``major decreases in
giant clam populations between 1997 and 2002, with many small clams
seen in 1997 not surviving through to 2002.'' The authors attributed
this population loss directly to sedimentation from major construction
activities in South Sinai. While these studies address impacts to giant
clams broadly, it is likely that T. squamosina experiences a similar
threat in these areas. Lastly, Pappas et al. (2017) suggest that
coastal development may, in combination with overutilization, explain
the apparent absence of T. squamosina in the central Red Sea, but do
not provide any data to support this claim.
Thus, while we do not have any data specifically linking habitat
destruction, modification, or curtailment with the abundance of T.
squamosina, based on the species' distribution in nearshore habitats,
documented evidence of the impact of coastal development on giant clam
abundance generally, and ongoing regional development goals, we
conclude that this threat poses a high risk to T. squamosina. In other
words, we find that it contributes significantly to the species' long-
term extinction risk and is likely to contribute to its short-term
extinction risk in the near future.
Tanmen Destructive Shell Harvesting
Despite a relatively small geographic scope, giant clam shell
harvesting in the South China Sea has caused severe destruction of
shallow water habitats. In the last decade, the small fishing village
of Tanmen in China's Hainan province became a regional epicenter for
giant clam shell handicraft and trade (Hongzhou, 2016; Larson, 2016;
Lyons et al., 2018). From 2012 to 2015, the number of retailers of
giant clam shell handicraft increased from 15 to more than 460, the
number of shell carving workshops increased from a dozen to more than
100, and by the end of this period, it was estimated that this industry
supported the livelihood of nearly 100,000 Tanmen residents (Hongzhou,
2016; Bale, 2017; Wildlife Justice Commission, 2021).
As the industry grew, many Tanmen fishermen increasingly abandoned
the traditional fishing industry and shifted focus to giant clam shells
as their primary livelihood. With local stocks of giant clams having
been depleted by a long history of overharvesting, many fleets resorted
to destructive methods of digging out large portions of coral reef
using their boat propellers to access the shells of long-dead clams
that had been buried under the reef substrate (Wildlife Justice
Commission, 2021). As reported by V. R. Lee (2016), harvesting boats
are anchored with a long rope or chain against which the propeller
holds tension as it carves an arc-shaped scar in the reef (see also
Wingfield-Hayes, 2015). The majority of this activity has occurred the
South China Sea, and an analysis of satellite imagery revealed
extensive damage in the Spratly Islands and Paracels, with an estimated
160 km\2\ of coral reef in these areas completely destroyed by the
combination of clam dredging and island-building activities (McManus,
2017).
In response to international pressures and following a 2016
arbitral tribunal ruling that China was aware of and responsible for
``severe harm to the coral reef environment'' in the South China Sea
due in part to these activities (Permanent Court of Arbitration, 2016),
steps were taken to halt destructive clam shell harvesting operations.
China began to enforce anti-corruption measures aimed at undermining
demand for the expensive jewelry and statues carved from giant clam
shells (Bale, 2017), and in January 2017 the Hainan Province People's
Congress passed new regulations that effectively banned the commercial
trade of all giant clam species in Hainan (Wildlife Justice Commission,
2021). However, while giant clam shell harvesting operations were found
to decline significantly between 2016 and 2018, the Wildlife Justice
Commission (2021) reports several lines of evidence to suggest that
``illegal giant clam shell trade persists in China in a covert manner
with one clear supply area'' (Hainan Province), and that a new influx
of clam harvesting boats have returned since 2018. Thus, while the
extensive damage to the habitat in this region would likely take
several decades or more to undo if the ecosystems were allowed to
recover, the ongoing threat of illegal harvesting is likely to prevent
any substantial habitat recovery in the foreseeable future.
This threat of habitat loss is relevant to the species that are
known to occur in this region and that are typically found in reef flat
environments where the harvesting operations primarily occur. This
includes T. gigas, T. squamosa, H. hippopus, and most critically H.
porcellanus, which has a highly restricted range centered in the
Sulawesi region of Indonesia but that extends northward into the
Philippines and portions of the South China Sea (Wells, 1997; bin
Othman et al., 2010; Neo et al., 2017). As is mentioned above, the
damage from these operations has likely eliminated any H. porcellanus
that may have previously occurred in the islands of the South China
Sea.

Overutilization for Commercial, Recreational, Scientific, or
Educational Purposes

The pervasive harvest of giant clams for subsistence and domestic
sale, and several periods of short-lived but intensive commercial
harvest have severely depleted giant clam populations throughout their
respective ranges. Once the center of giant clam diversity in the
region, the Philippines saw commercial exploitation of giant clams for
the international shell trade decimate populations of H. hippopus, H.
porcellanus, T. gigas, and T. squamosa. Similar trends have been
observed throughout Southeast Asia (i.e., Indonesia, Singapore,
Thailand, Cambodia, Vietnam, and in the South China Sea), where each of
these species except T. squamosa is now considered rare or locally
extinct (Neo et al., 2017). Likewise, illegal harvest of giant clams
for the international clam meat trade, primarily by Taiwanese fishermen
or to supply Taiwanese demand, severely reduced giant clam populations
throughout the western and central Pacific. As a result, as in
Southeast Asia, nearly all of the species (excluding T. squamosa) are
now considered rare or extinct throughout most of their Pacific range
(Wells, 1997; Neo et al., 2017). Although international demand
(primarily for the aquarium trade) is increasingly met by the growing
field of giant clam mariculture, wild-sourced clams are still observed
in international trade, and the potential for laundering wild clams
with mariculture-produced specimens cannot be discounted (Sant, 1995).
Ongoing harvest for subsistence or domestic market supply, as well
as persistent poaching, continues to limit substantial population
recovery of giant clams throughout much of their range. As broadcast-
spawning organisms with little to no mobility, giant clams are reliant
on sufficient population density to facilitate gamete fertilization.
Thus, even if small populations of giant clams have survived the years
of exploitation, in many cases individuals may be too dispersed to
successfully reproduce. Furthermore, the largest individuals were often
targeted for the meat and shell trade, leading to altered size
structures in remnant giant clam populations. Juveniles and smaller
adults are known to be more susceptible to predators and to exhibit
lower reproductive output, which will likely continue to limit
population recovery in the near future. It is for these reasons that we
consider overutilization to be the most significant threat to all seven
giant clam species. Below, we

[[Page 60512]]

summarize the threats posed by overutilization related to subsistence
fisheries, domestic markets, international trade, and illegal poaching,
highlighting specific details related to each affected species.
Subsistence Fisheries
Giant clams have long been, and continue to be, an important
component of traditional livelihoods and culture throughout their
geographic range (Craig et al., 2011). As described by Lindsay et al.
(2004), ``there are few locations within the Pacific where tridacnids
are not gathered on a daily basis and found in local markets'' (Munro,
1993a). Archaeological evidence from shell middens (piles of discarded
shells), which can be found across the Indo-Pacific from as far back as
2000 years ago (Swadling, 1977), as well as anecdotal accounts and
local fishing practices all point to the importance of giant clam in
Indo-Pacific diets (Neo & Loh, 2014). The shells of giant clams are
also frequently carved for use as tools, containers, and ornaments
(Copland & Lucas, 1988; Lucas, 1994).
Because H. hippopus is unattached to the substrate and occupies
nearshore habitats that are relatively accessible to humans, it is an
easy target for reef gleaners (i.e., fishers that collect organisms by
hand from nearshore sand and reef flats). Consequently, it has been a
popular species for local harvest and consumption throughout its range.
Many years of subsistence harvest have driven widespread population
declines and local extirpations from many Pacific island nations and
territories, including American Samoa, CNMI, and Guam.
In Fiji, for example, Seeto et al. (2012) discovered H. hippopus
fossils in shell middens from two Lapita-era settlements (1100-550
B.C.), and found that shell size increased with midden depth,
suggesting that human consumption contributed to population reductions
and to its eventual extirpation. Surveys from Palau in the 1970s
indicated that H. hippopus populations declined drastically as a direct
result of overharvest (Bryan & McConnell, 1975). In Singapore, H.
hippopus was considered rare historically (S. K. Lee, 1966; Dawson &
Philipson, 1989), but consistent harvest pressure is thought to have
prevented the species from establishing a sustainable population in the
area and ultimately led to its extirpation (Neo & Todd, 2012).
Additionally, H. hippopus continues to be exploited for consumption by
coastal communities in Indonesia (Naguit et al., 2012), Malaysia (Neo &
Todd, 2012), New Caledonia (Purcell et al., 2020), the Andaman and
Nicobar Islands (Nandan et al., 2016), Papua New Guinea (Kinch, 2003),
and virtually every other country where it occurs, except for Australia
(Wells, 1997).
H. hippopus has also been extirpated from American Samoa, CNMI, and
Guam due to a long history of harvest for subsistence consumption and
for sale in local markets (Munro and Heslinga, 1983; Sant, 1995; Wells,
1997; Green and Craig, 1999; Pinca et al., 2010). According to Score
(2017), giant clams have a ``special significance'' in American Samoa
culture and are often used as offerings during family and community
gatherings when available. Moreover, Cunningham (1992) describes the
cultural significance of giant clams to the Chamorro people, who live
throughout the Mariana Islands, including CNMI and Guam. The common use
of H. hippopus as a source of food and to make tools likely led to its
extirpation in these locations (Wells, 1997).
Similar to H. hippopus, the tendency of H. porcellanus to occupy
shallow nearshore areas make the species highly vulnerable to
harvesting (Dolorosa et al., 2014). Heavy exploitation from both
subsistence and commercial harvest has led to severe population
declines throughout its range (Dolorosa et al., 2014; Neo et al.,
2017). Villanoy et al. (1988) determined that H. porcellanus was
overexploited in the Philippines as early as the 1980s, and more
recently, Rubec et al. (2001) reported that H. porcellanus has been
depleted to such an extent that it is no longer commercially viable for
harvest in the Philippines. Ultimately, while subsistence harvest was
widespread, heavy fishing pressure on giant clam stocks in the
Philippines for the commercial shell trade has been the primary cause
of population decline, and has led to local extinctions throughout the
region (see International Trade in Giant Clam Shells and Shell-Craft
below).
Because of their large size and fast growth rates, T. derasa and T.
gigas have historically been two of the most widely exploited giant
clam species for the consumption of their meat. Reports from throughout
their ranges indicate that both species are harvested for subsistence
consumption in nearly every location where they occur, with the major
exception being the Great Barrier Reef and northwestern (NW) islands of
Australia. There are certain Pacific island communities that attribute
unique significance to T. gigas as a cultural symbol and place high
value on the species as a food item for special occasions (Hviding,
1993). The shell of T. gigas is also valued as a traditional resource
among many coastal communities for use as basins or as personal or
religious decorations (Juinio et al., 1987; Hviding, 1993; Lucas,
1994). Both T. derasa and T. gigas are reported to have been extirpated
from CNMI and Guam as a result of longstanding subsistence harvest
(Wells, 1997; Pinca et al., 2010).
Based on the best available scientific and commercial data, it is
likely that past and current subsistence harvest has played a
significant role in the low abundance of T. mbalavuana throughout its
range. S. Lee et al. (2018) attributes its absence from areas outside
of the eastern Lau group in Fiji to a combination of ecological factors
and ``serial overfishing.'' Additionally, Lewis and Ledua (1988)
reported that in Fiji, T. mbalavuana is occasionally harvested
unintentionally with T. derasa, due to the similarity in appearance
between the two species. In Tonga, T. mbalavuana has traditionally been
harvested for subsistence consumption and to supply domestic markets
(Ledua et al., 1993), and although its occurrence in deeper areas may
have offered some protection from harvest historically, the advancement
of SCUBA and hookah gear has facilitated greater access to previously
inaccessible stocks (Lewis & Ledua, 1988; Lucas et al., 1991; Neo et
al., 2017). Interviews with a number of traditional fishermen indicated
that the abundance of T. mbalavuana in Tonga had declined considerably
during their lifetimes (Ledua et al., 1993). Harvest of giant clams for
subsistence consumption and domestic markets is ongoing and largely
unregulated in Fiji and Tonga.
Compared to the more common T. maxima and T. crocea (that are not
themselves subject to this rulemaking), which often co-occur with T.
squamosa, T. squamosa is typically larger and easier to physically
remove from the reef, which makes it highly susceptible to harvest,
particularly in shallow nearshore areas. For this reason, T. squamosa
is an important resource in subsistence fisheries in nearly every
location across its range, and in several locations, it is the
preferred giant clam species for meat consumption (Neo et al., 2017).
Few exceptions include Australia, where giant clam harvest is strictly
prohibited, and remote areas where the distance from human settlements
and infrastructure limits accessibility. However, in most locations
where the species occurs, longstanding subsistence harvest has
reportedly driven widespread population declines (Neo et al., 2017).

[[Page 60513]]

There are several studies that provide some insight as to the
impact of past and current harvest on the abundance of the T.
squamosina in the Red Sea. Paleolithic artifacts indicate that modern
humans have been exploiting mollusks in the Red Sea for at least
125,000 years (Richter et al., 2008). During this time, Richter et al.
(2008) found that giant clam communities in the Red Sea have changed
dramatically from before the last interglacial period (122,000 to
125,000 years ago), when T. squamosina constituted approximately 80
percent of the shell remains, to T. squamosina comprising less than 5
percent of shells in freshly discarded shell middens. While the authors
acknowledge that variable recruitment rates and mortality among the
three Red Sea giant clam species may be attributed to natural
disturbances, a concurrent decline in the size of giant clam shells
strongly suggests that overutilization has played a significant role
(Richter et al., 2008). In general, giant clam stocks in the Red Sea
(including T. maxima, T. squamosa, and T. squamosina) have declined to
less than 5 percent of their historical abundance in the 1980s and
1990s, largely due to artisanal reef-top gathering for meat and shells
(Richter et al., 2008).
As with H. hippopus and H. porcellanus, the distribution of T.
squamosina in shallow, nearshore habitats makes it particularly
accessible to reef-top gatherers and exacerbates the threat of
overutilization. Bodoy (1984) reported that giant clams had been
subject to ``heavy exploitation in the vicinity of Jeddah, Saudi
Arabia, and they [were] often collected on the reef flat, both for food
and for decorative purposes.'' Additionally, two firsthand accounts
from Gladstone (2000, 2002) described the harvest of ``a significant
number of clams'' (primarily T. maxima, which is not subject to this
rulemaking) from the Kharij As Sailah and Kharij Al Qabr areas of the
Farasan Islands, noting that ``clams were easily harvested in the
shallow reef flats.'' Overall, the best available scientific and
commercial data suggest that giant clams have been harvested
extensively in the Red Sea for many years, and given their traditional
importance in the diets of coastal communities, harvest is likely
ongoing in most areas of the Red Sea.
Domestic Markets (Meat and Shells)
In areas where giant clams were historically abundant, commercial
fisheries often developed alongside subsistence harvesting to supply
the local demand for giant clam meat and shells. In Fiji, T. squamosa
and T. derasa were harvested by small-scale commercial operations and
sold in 11 municipal markets or other direct sales outlets (Lewis et
al., 1988). From 1979-1987, annual sale of giant clam meat in the
domestic market ranged between 6 and 42 tons (Adams, 1988; Lewis et
al., 1988; Wells, 1997). With respect to both species, Lewis et al.
(1988) reported that the commercial harvest had driven once abundant
populations to low densities, particularly near major urban centers.
Local markets also exist in a number of other Pacific countries and
territories, although data on giant clam meat are often not reported at
the species level. This is because of the difficulty in identifying the
species once the meat is harvested since the shells are often left in
the water, or because giant clam meat may have been mixed together or
recorded collectively with other shellfish products when it was landed.
Wells (1997) reported varying prices for giant clam meat from markets
in American Samoa, the Solomon Islands (amounting to about 1 tonne of
giant clam meat sold per year), the Marshall Islands (H. hippopus and
T. squamosa), Niue, Vanuatu, Samoa, and FSM, where in 1990, 3.66 tonnes
of giant clam meat were sold in the main markets of Chuuk. Data
collected over a 10-week period in Tonga suggested that annual landings
of giant clam meat for the domestic market might be 639-1,346 kg
(Tacconi & Tisdell, 1992). Wells (1997) noted that in Jepara,
Indonesia, giant clam meat was often sold dried, suggesting that the
lack of fresh meat may be due to local overutilization of stocks. In
Myanmar, clam meat was often marketed fresh for local consumption
(Munro, 1989).
Additional reports indicate that domestic markets have continued in
many of these localities into at least the early 2000s. In 1998-1999,
nearly six tonnes of giant clam products were sold at a single market
in Samoa (Skelton et al., 2000). Giant clam meat was still reported to
be sold openly at markets in Malaysia as of 2003 (Shau-Hwai & Yasin,
2003). Until bag limits were established in 2009, the declared
commercial catch of giant clams in New Caledonia varied between 1.5 and
9 tonnes per year. This included T. derasa, T. squamosa, and H.
hippopus, and the authors indicate that it is often the adductor muscle
that is sold in stalls of local markets. In the decade since the bag
limits were put in place, commercial catch has fallen below 2 tonnes
per year (Purcell et al., 2020). Kinch and Teitelbaum (2010) report
that a high demand for giant clams to supply the local market in Tonga
``has resulted in the over-exploitation of giant clam stocks in some
areas.'' In Papua New Guinea, Kinch (2003) attributes sparse
populations of giant clams to commercial harvest, particularly that of
Brooker Islanders. From January to September 1999, the author recorded
the total sales of giant clam adductor muscle from Brooker Islanders to
a local fishing company, which included 551 kg (or 1,970 clams) of
specimens under 400 g and 146 kg (or 170 clams) greater than 400 g.
Notably, nearly one-third of the T. gigas individuals included in these
sales were not full-grown adults, which likely had an effect on the
future productivity of those populations. Similarly, harvesting of
giant clams for sale and subsistence use in Vanuatu has led to severely
reduced populations that are ``now considered close to collapse in many
locations despite the presence of suitable habitats for juveniles and
adults'' (Dumas et al., 2012).
Domestic markets for giant clam shells are often related to the
tourism industry. In the Andaman and Nicobar Islands of India, Nandan
et al. (2016) report that giant clams, including T. squamosa and H.
hippopus, are fished for the tourism-based ornamental shell industry.
Additionally, in Thailand, giant clams shells are usually first sold to
local traders in Phuket, and then sold to tourists as ornamental shells
or various shell crafts (e.g., ashtrays, soap trays, lamps)
(Chantrapornsyl et al., 1996). Shells have also been a popular souvenir
for tourists visiting beach and resort areas of the Philippines and
Indonesia (Tisdell, 1994). At the Pangandarin and Pasir Putah beach
resorts in Java, Indonesia, as many as 39 and 35 giant clam shells,
respectively, were available for sale in 2013, despite a prohibition on
the harvest and sale of giant clams (except under ``exceptional
circumstances'') under Indonesian law since 1987 (Nijman et al., 2015).
Prior to this prohibition, a major industry based on the use of
giant clam shells for production of floor tiles (a.k.a, `teraso' tiles)
led to the extensive harvest of giant clams in Indonesian waters. While
much of the shell material was dead shells of T. derasa and T. gigas
buried in reef flats, living specimens were known to be taken when
found (Lucas, 1994). As described by Lucas (1994), there were tile
production centers at Jakarta, Semarang, Bali, Manado, and likely
Suabaya in the early 1980s, and clam shell trade routes had developed
throughout the Indonesian islands to supply the industry. The best
estimates of giant clam shell import to the Semarang tile production
center from the nearby Karimun Jawa islands varied between about 20 and
200 tonnes per month over the period 1978-1983 (Brown & Muskanofola,
1985). At the

[[Page 60514]]

Jakarta production center, the clam shell trade was estimated to reach
at least 600 tonnes per month in 1982 (Usher, 1984 cited in Lucas,
1994). This industry is no longer active in Indonesia as a result of
the 1987 prohibition; however, it is likely that such intense demand
contributed significantly to the depletion and current rarity of T.
derasa and T. gigas in Indonesian waters and limited any potential for
their recovery. Moreover, despite regulatory protection, all species of
giant clams remain heavily exploited in Indonesia for their meat and
shells, and some for the live aquarium trade (Neo et al., 2017). As a
result of this overutilization, the larger giant clam species are now
thought to occur in only a few locations archipelago-wide (Hernawan,
2010).
International Trade of Giant Clam Meat and Poaching
While giant clam meat is consumed throughout the Indo-Pacific
region, Taiwan has consistently had the largest market and demand for
giant clams. Some of the earliest references indicate that giant clams
around Taiwan were depleted many decades ago (Pearson, 1977; Tisdell &
Chen, 1994). As local stocks were rapidly exhausted, Taiwanese vessels
began to range farther from their home ports, and from the 1960s to the
mid-1980s, a surge of Taiwanese fishing vessels began illegally
entering the waters of other Pacific nations in search of giant clam
adductor muscle, particularly from the larger species, T. gigas and T.
derasa (Munro, 1993a; Kinch & Teitelbaum, 2010). Occasionally, these
vessels operated under agreements with local communities in exchange
for resources (Adams, 1988), but in the vast majority of cases, giant
clams were harvested illegally and to an unsustainable degree (Lucas,
1994; Kinch, 2002). The clam poachers progressively worked their way
through the Pacific, typically concentrating their efforts on
uninhabited islands and reefs where giant clam stocks had been
virtually untouched and where local surveillance was limited. Reports
of Taiwanese poaching include areas of the Philippines, FSM, Indonesia,
Papua New Guinea, the Solomon Islands, Australia (the Great Barrier
Reef), Palau, Fiji, Kiribati, and the Marshall Islands (Dawson &
Philipson, 1989; Sant, 1995).
Data on the landings of giant clam meat in Taiwan are generally
unavailable due both to their illegal nature and because in the
records, landings were combined with meat of other marine molluscs and
collectively referred to as `ganbei' or `compoy' (Lucas, 1994; Tisdell
& Chen, 1994). Tisdell and Chen (1994) report that imports of ganbei
ranged from 9 tons in 1977 to 621 tons in 1988. Other estimates of
giant clam adductor muscle landings in the 1960s and 1970s range
between 100 and 400 tons per year (Carlton, 1984; Dawson & Philipson,
1989). Dawson and Philipson (1989) estimated that during the peak of
the Taiwanese fishery for giant clams, harvest did not likely exceed
100 tons of adductor muscle per year, though Munro (1989) regarded this
to be an underestimate. Accounting for the potential harvest of the
smaller species, T. derasa and H. hippopus, which have an adductor
muscle about one-third the weight of T. gigas, those landings
correspond to 300,000 to 450,000 clams per year. According to Dawson
(1986), ``it seems certain [. . .] that the total illegal harvest of
giant clams over the twenty-odd years that such activities have
occurred in the region can safely be measured in the millions.''
Poaching by long-range Taiwanese vessels peaked in the mid-1970s
and gradually declined during the 1980s as the extension of exclusive
economic zones, improved surveillance of reef areas, boat seizures, and
depleted stocks made the fishery less profitable (Lucas, 1994). In
addition, growing pressure from many Indo-Pacific nations forced the
Taiwanese government to take stricter actions against giant clam
harvesters (Dawson, 1986). The last five `compoy' (i.e., clam and other
shellfish) fishing licenses were rescinded by the Taiwanese government
in 1982, mainly due to pressure from the Australian government, and
beginning in 1986, the Taiwanese government began rejecting all
requests for approval of Taiwanese involvement in any clam fishing
activities, regardless of whether foreign agreement or license
documents were provided. There is evidence, however, that some poaching
activities continued in remote locations. From 1982 to 1987, at least
four Taiwanese vessels were apprehended on outlying reefs of the
Solomon Islands, in each case carrying clam meat from tens of thousands
of giant clams (Govan et al., 1988). The authors note that the small
size of the adductor muscles recovered indicates that large clams had
likely already been harvested from the reef at an earlier date.
Even as Taiwanese poaching operations declined, the demand for
giant clam meat in Taiwan persisted, incentivizing the development of
legal commercial fisheries for export throughout the Indo-Pacific
(Lewis et al., 1988; Basker, 1991; Lucas, 1994). It was estimated that
imports of adductor muscle to Taiwan from these newly formed fisheries
totaled approximately 30-40 tons in 1987 and 1988 (Tisdell & Chen,
1994). The fisheries, however, rapidly depleted local stocks and were
in most cases short-lived, typically being shut down by local
authorities in the span of a few years. In the Maldives, for example,
commercial harvest of giant clams began in June 1990 and continued
until early in 1991. Two buyers were operating and collectively
harvested over 90,000 individuals; one buyer exported 9.8 tons to a
Taiwanese buyer (Basker, 1991). Concerned over the high exploitation
rate, the Ministry of Fisheries and Agriculture conducted an assessment
of the giant clam stocks and fishery, and the resulting report
recommended closing off high density areas to further fishing and other
restrictions (Basker, 1991). The commercial fishery was subsequently
closed, and collection of giant clams remains prohibited in the
Maldives. Likewise, a commercial fishery in Papua New Guinea reportedly
removed at least 85 tons of adductor muscle over a 5-year period,
equivalent to over 750 tons total flesh weight, until it was closed due
to depleted stocks (Munro, 1993a).
Adams (1988) described one example of the impact of extreme
commercial harvesting pressure in Fiji when a ship named `Vaea'
intensively harvested giant clam stocks in 1985. Teams of two
harvesters on Hookah gear reportedly caught 50-250 clams per day. At
one site, harvesters had taken approximately 80 percent of the standing
stock of T. derasa, or nearly 15,000 individuals, from an area of 25.9
square miles down to a depth of 20 meters. Adams (1988) estimated that
harvesting rates averaged 70 percent of the total living stock at each
reef, less for scattered populations and more for denser ones. From
1984 to 1987, T. derasa catch rates in Fiji varied between 20 and 40
tons of flesh per year, half of which was exported (Adams, 1988). The
Fijian fishery as a whole (including municipal markets, wholesale and
retail outlets, and exports) landed over 149 tons during this period,
with the largest annual harvest reaching 49.5 tons in 1984, the year in
which exports began (Lewis et al., 1988).
By the early 1990s, pervasive stock depletions across the Indo-
Pacific severely limited Taiwanese imports of giant clam meat (Tisdell
and Chen, 1994). In the years since, many countries in the region have
banned commercial export of giant clams, some have imposed size and/or
bag limits, and many have become signatories to the Convention on
International Trade in Endangered Species of Wild Flora and Fauna
(CITES). The regulatory

[[Page 60515]]

implications of CITES participation are discussed more thoroughly below
in the section on Inadequacy of Existing Regulatory Mechanisms, but one
of its requirements is that Parties must submit an annual report of
their trade in CITES-listed species, including the number and type of
permits and certificates granted, the countries involved, and the
quantities and types of specimens traded. All species of giant clams
have been listed under appendix II of CITES since 1985, and we can
therefore rely to some extent on trade statistics from the CITES
reporting database to characterize more recent patterns in the
international market for giant clams.
In most cases, countries have limited their reporting to the family
or genus level, and outside of a few instances of trade reported for T.
derasa, T. gigas, and T. squamosa, no other species were identified
specifically. Additionally, of all the transactions reported from 1983
to 2020, 50.4 percent and 39.5 percent were en route to New Zealand and
the United States, respectively, while Japan, Singapore, and Australia
comprised the remaining 10.1 percent of imports. Law Enforcement
Management Information System (LEMIS) trade data provided by USFWS for
the period 2016-2020 indicate that nearly all of the imports of giant
clam meat over the past 5 years were classified to be of `Personal'
nature, likely representing shipments intended for families or friends
of Pacific islanders (Shang et al., 1994). Prior to 2000, there are
several years in which countries reported significant export of meat
from giant clams that had been born or bred in captivity. This includes
3615 kg and 472 kg of T. gigas and T. derasa meat, respectively,
exported from Solomon Islands in the 1990s, 1695 kg of T. derasa meat
exported from Palau in 1990-1991, and 65 kg of T. gigas meat exported
from Australia.
A number of other countries have reported significant export of
giant clam meat (species unknown) since the late 1990s, primarily to
New Zealand and the United States. Nearly all of these exports are of
wild-caught specimens, many of which have been seized or confiscated at
the border due to improper or missing CITES export permits. The major
exporters of giant clam meat in the last two decades include the Cook
Islands, Kiribati, Marshall Islands, FSM, and Tonga. At the higher end,
Tonga has exported an average of 1210 kg giant clam meat per year since
2005, and at the lower end, the FSM has averaged 58 kg per year during
the same period.
Importantly, a number of the key countries in the trade of giant
clam meat are not CITES contracting parties (e.g., Cook Islands,
Kiribati, Marshall Islands, FSM) or have only become so relatively
recently (e.g., Palau in 2004, Solomon Islands in 2007, Tonga in 2016).
Thus, any trade reported for these countries is based on values
reported by the CITES party involved, and any trade among two non-
contracting nations is not included in these estimates. Additionally,
the USFWS Office of Law Enforcement in Honolulu, Hawaii has reported
that approximately 450 lbs (200 kg) of giant clam meat per year is
refused (i.e., seized, confiscated, or re-exported) from Tonga, FSM,
and the Marshall Islands (K. Swindle, USFWS, pers. comm., December,
2017). This is likely a significant underestimate of the total amount
of giant clam meat that comes into the United States (as a whole)
illegally, as many shipments outside of those that pass through
Honolulu likely make it past enforcement inadvertently (K. Swindle,
USFWS, pers. comm., December, 2017). For these reasons, the CITES data
should be viewed as incomplete, and the reported quantities are likely
an underestimate of the total trade in giant clam meat.
International Trade in Giant Clam Shells and Shell-Craft
Giant clam shells have been used for a variety of decorative and
utilitarian purposes, including as beads, vases, lamps, ashtrays, and
wash basins. H. hippopus and T. squamosa are considered the most
popular giant clam species for the shell trade (Shang et al., 1994)
because of their unique physical characteristics (e.g., attractive
colors, bowl-like shape, etc.), although nearly all of the species have
been harvested depending on the intended use, cultural preference, or
geographic availability.
The Philippines has historically operated as the largest exporter
of giant clam shells and shell-craft, accounting for over 95 percent of
the global exports of giant clam shell products from 1983 to 2020.
During the peak of the shell trade from 1979 to 1992, total exports
from the Philippines surpassed 4.2 million kg (Juinio et al., 1987;
Wells, 1997). While all species of giant clam that occur in the
Philippines have been exploited, the two Hippopus spp. and T. squamosa
were the most frequently used for ornamental purposes and handicrafts,
and T. gigas was most frequently used for basins (Lucas, 1994). Juinio
et al. (1987) noted that T. derasa may have also been harvested but was
often not distinguished by shell dealers as a separate species; rather,
it was known as a ``heavier variety'' of T. gigas or H. porcellanus.
Export records from the Philippines Bureau of Fisheries and Aquatic
Resources indicate an initial peak in 1979, when 1,003 tonnes of giant
clam shells were exported, corresponding to 895,000 shell pairs.
Exports then declined to a minimum of 63 tonnes (or 67,000 shell pairs)
in 1982, which was thought to reflect saturation of the international
demand. Juinio et al. (1987) reported that the demand for giant clam
shells could be met from existing stock piles (except those of H.
porcellanus, which was still considered to be highly marketable).
However, exports began to increase again in the late 1980s and peaked
in 1991 with nearly 1.2 million shells, over 460,000 carvings, and over
1,186 tonnes of shells (equivalent to about 825,000 shell pairs)
exported in a single year (Wells, 1997). This occurred despite the
government of the Philippines instituting a ban on the export of giant
clams (except T. crocea, not subject to this rulemaking) in 1990. In
the following year, exports declined to 374,000 shells and 70,000
carvings, likely due to the issuance of CITES Notification No. 663 (16
January 1992) urging all CITES Parties to refuse trade permits for
Tridacninae products from the Philippines, in accordance with
Philippine legislation (Wells, 1997). In the three decades since 1992,
reported exports of giant clam shells from the Philippines have been
considerably lower (but not absent), totaling only 8,528 shells and
6,359 carvings (CITES Trade Database, accessed 22 Mar 2022).
Ultimately, widespread subsistence harvest in conjunction with the
heavy fishing pressure on giant clams to supply the commercial shell
trade decimated the populations of several giant clam species (e.g., H.
hippopus, H. porcellanus, T. gigas, and T. squamosa), with local
extinctions widespread throughout the Philippines (Juinio et al.,
1987). Wells (1997) reported that exports until 1992 were dominated by
H. hippopus, T. squamosa, and H. porcellanus, with H. hippopus
comprising 53 percent of shell exports and 94 percent of carvings. Even
the few remaining locations thought to be the species' last strongholds
in Philippine waters (e.g., in the Sulu Archipelago and Southern
Palawan) were overharvested by the mid-1980s (Villanoy et al., 1988).
Presently, five of the seven giant species considered here (H.
hippopus, H. porcellanus, T. derasa, T. gigas, and T. squamosa) can
still be found in the Philippines and they are all protected by
Philippine law. Native T. gigas populations are restricted to small
portions of Tubbataha Reefs Natural Park in very low abundances; T.
derasa,

[[Page 60516]]

H. hippopus, and H. porcellanus are considered rare, and T. squamosa is
considered frequent (Neo et al., 2017).
The United States, Japan, Australia and various European countries
have historically been the largest importers of shells and shell-craft
from the Philippines (Juinio et al., 1987; Wells, 1997). The United
States alone has accounted for over 50 percent of shells and over 60
percent of shell carvings imported between 1983 and 2020. More
recently, however, dwindling giant clam populations as well as greater
regulatory protections in many countries have limited the shell trade
among the traditional major importers of the 1980s. Instead, the
majority of international trade has shifted increasingly to illegal
means. From 2016 to 2020, the global trade in giant clam shells based
on CITES reports totaled 65,129 shells and 221 shells carvings
(primarily T. gigas), of which over 92 percent originated in Indonesia
and over 97 percent was imported by China. This has occurred despite a
prohibition on the harvest and export of giant clams under Indonesian
law since 1987. While not at the same scale as the Philippines,
Indonesia has participated in the trade of giant clam shells and shell
products since the 1980s. Once giant clams were listed as protected
species in 1987, Tisdell (1992) suggested that unrecorded exports of
giant clam shells continued to occur from Indonesia to the Philippines.
Likewise, several reports in the years since indicate that enforcement
of the harvest and export ban remains grossly insufficient and, as is
suggested by the CITES reports, substantial export of giant clam shells
from Indonesia is ongoing (Allen & McKenna, 2001; Nijman et al., 2015;
Harahap et al., 2018).
Presently, the largest market for giant clam shells is in the city
of Tanmen, in the southern Chinese Province of Hainan. As discussed
previously, a major shell-crafting industry developed in this region
during the 2000s. During the peak of the Tanmen shell-crafting industry
in 2013-2014, there were an estimated 150 processing workshops
supplying 900 craft shops with giant clam shell products in the
province (Wildlife Justice Commission, 2021). The annual sales revenue
of giant clam shell handicrafts in 2014 was estimated to be $75 million
USD (Lyons et al., 2018). In January 2017, the Hainan Province People's
Congress passed new regulations banning the commercial trade of giant
clams in Hainan. However, investigations conducted 2 years later by the
Wildlife Justice Commission (2021) found that there were still more
than 100 craft shops in Tanmen, although fewer than 20 percent were
still in business. Giant clam shell products were also being sold
openly in hundreds of stores in other parts of the Hainan Province,
such as Haikou, Sanya, Guangdong and Fujian provinces, and could be
ordered on social media platforms, such as WeChat, for delivery to
other locations (Wildlife Justice Commission, 2021). This has been
corroborated by first-hand news reporting from Scarborough Shoal in
April 2019, which documented ongoing shell harvesting by fishing boats
flying the Chinese flag (ABS-CBN News, 2019). The ABS-CBN film crew
captured many large piles of extracted giant clam shells around the
harvesting area, some even extending above the water surface.
This industry primarily targets the shells of deceased clams
embedded in the reef substrate; however, live clams are also taken
whenever found. Large shells in particular are of the highest value,
putting the remaining T. gigas populations in the area at the greatest
risk. According to Lyons et al. (2018), ``the more valuable [T. gigas]
pieces come with a certificate of origin, specifying, for example, that
it comes from Scarborough Shoal, Spratlys, or Paracels and,
occasionally, even the specific reef concerned.'' This suggests that T.
gigas shells are considered to have different grades or qualities
depending on where in the South China Sea they were harvested. As a
result of this intense market demand in combination with the
destructive shell harvesting methods described above, Gomez (2015)
noted that T. gigas is now ``virtually extinct'' in the center of the
South China Sea, including the Paracels, the Macclesfield Banks, and
the Spratlys.
International Trade of Live Giant Clams for Aquaria
The largest current market for giant clams is that of live
specimens for the aquarium trade and, to a lesser extent, to supply
broodstock for mariculture operations. It can be difficult to
distinguish the purpose of live specimen transactions from CITES
reports alone, but Wells (1997) concluded ``that the aquarium trade is
now the main market for both wild-collected and mariculture clams.'' In
the 25 years since that report, the market for giant clams as aquarium
specimens has continued to grow, with giant clams now representing one
of the most desired groups of invertebrates in the aquarium industry
(Wabnitz et al., 2003; Teitelbaum & Friedman, 2008; Mies, Dor, et al.,
2017). They are a sought-after commodity and have been described as a
``must have'' item by collectors and aquarium hobbyists (Lindsay et
al., 2004). The smaller, more brightly colored species (i.e., T. maxima
and T. crocea, species not subject to this rulemaking) are by far the
most popular in the marine ornamental trade, but T. squamosa, T. gigas,
T. derasa, and H. hippopus are also traded in smaller numbers (Lindsay
et al., 2004; Kinch & Teitelbaum, 2010).
CITES records indicate that the primary source countries for the
seven species considered here include Australia, Palau, Vietnam,
Solomon Islands, and Marshall Islands, among others. Notably, the vast
majority of giant clams exported from Australia, Palau and Marshall
Islands have been bred/born in captivity and thus pose less risk to
wild populations; however, much of the export volume from Vietnam,
Solomon Islands, Tonga, and more recently, Cambodia, are of wild-
sourced specimens.
Of the seven species considered here, T. derasa and T. squamosa
have been the most popular in the trade of live specimens, according to
CITES reports. Comparing the two, exports of T. derasa have been higher
from Pacific island nations, such as Palau, Solomon Islands, Marshall
Islands, Tonga, and FSM. Nearly all recent trade of this species is of
captive-bred/born individuals, with wild harvest in these countries
contributing minimally, if at all, by 2010. T. squamosa, by comparison,
has been harvested more often by countries in Southeast Asia, such as
Vietnam, Cambodia and Indonesia, and many of the recent exports from
Vietnam and Cambodia are of wild-sourced individuals. Exports from
Vietnam peaked in the 2000s and have declined over the last decade,
while exports from Cambodia have increased more recently, reaching
nearly 10,000 T. squamosa specimens in 2019. Neo et al. (2017) notes
that the decline in exports from Vietnam is related to trade
restrictions implemented in response to concerns and regulations
sourcing wild specimens, and it is possible that some giant clams from
Vietnam have been re-routed for export through Cambodia. In fact,
according to CITES reports, over 99 percent of the recorded T. squamosa
exports from Cambodia were imported by Vietnam, implying a close trade
connection between the two nations. Neither H. hippopus nor T. gigas
have been harvested consistently for the aquarium trade, although with
respect to T. gigas, Craig et al. (2011) attributed this to a lack of
available supply rather than a decline in demand. Because of declining
populations throughout much of its range, the majority T. gigas

[[Page 60517]]

specimens for the aquarium trade in the late 2000s were being sourced
from just a few small island nations, primarily Tonga (Craig et al.,
2011). However, according to CITES records, trade of T. gigas from
Tonga has not occurred since 2011. T. gigas is not considered to be
native to Tonga, but had reportedly been introduced there as part of
stock enhancement and aquaculture programs (Munro, 1993a; Wells, 1997).
According to a CITES assessment in 2004, the introduced populations of
T. gigas had by that point died out, so it is not clear where the
exported specimens originated (CITES, 2004a).
The United States has consistently been one of the top import
markets for live giant clams, along with Canada, several countries in
Europe, Japan and Hong Kong (Wabnitz et al., 2003; Craig et al., 2011).
In 2002, 70 percent of the giant clams exported for the aquarium trade
went to the United States (Mingoa-Licuanan & Gomez, 2002 cited in Craig
et al., 2011). According to CITES reports from 1983-2020, the United
States has accounted for 24.2 percent of the total recorded imports of
H. hippopus, 53 percent of imports of T. derasa, 56 percent of imports
of T. gigas, 38.4 percent of imports of T. squamosa, and 12.8 percent
of imports of Tridacninae specimens that were not identified to the
species level. Throughout the full record since 1983, 50.6 percent of
the imports to the United States were recorded as captive-bred/born
specimens, while 44.7 percent were recorded as wild-sourced; however,
according to LEMIS data for the period 2016-2020, wild-sourced
specimens now represent only 4 percent of imports, with captive-bred/
born specimens accounting for the remaining 96 percent.
Summary of Risks to Specific Species Due to Overutilization for
Commercial Purposes
After considering the best available scientific and commercial data
presented above and in the Status Review Report, we reached several
different conclusions regarding the threat of overutilization for
various commercial purposes to the seven giant clam species considered
here. We summarize these conclusions of the risks for this threat
category for each species below.

H. hippopus

A long history of subsistence harvest punctuated by two decades of
intense commercial exploitation for the shell and shell-craft industry
have led to severe declines of H. hippopus populations throughout its
range. As is mentioned above, H. hippopus has been one of the most
popular giant clam species in the international shell trade because of
its size and physical characteristics (e.g., attractive colors, bowl-
like shape) (Shang et al., 1994). The Philippines operated as the
largest exporter of giant clam shells in the 1970s and 1980s, with H.
hippopus being the most frequently traded species during this time.
According to CITES annual report data, over 277,000 kg, 341,000 shell
pairs, 2 million ``shells'' (without associated units), and 1.7 million
shell carvings of H. hippopus were exported from the Philippines from
1985 to 1993. This period of intense harvest left H. hippopus severely
depleted throughout the Philippines and much of Southeast Asia, where
it remains at very low abundance except in a few isolated areas.
While most countries have imposed prohibitions on the commercial
exploitation of giant clams and CITES records indicate that recent
international trade of H. hippopus is minimal, subsistence harvest
continues to pose a threat to the species in most populated areas where
it occurs. Without more thorough monitoring from many of these
locations, it is difficult to determine if this ongoing harvest is
causing further population declines, but at the very least, it is
likely preventing any substantial rebound of depleted populations
throughout its range. An important exception is Australia, where
anecdotal reports suggest that strictly enforced harvest bans have been
largely successful in preventing overutilization and protecting
reportedly healthy stocks of this species. For these reasons, and
considering the documented effects of past harvest for the
international shell trade on species abundance, we conclude that
overutilization of H. hippopus contributes significantly to the
species' long-term risk of extinction.

H. porcellanus

As is mentioned above, heavy fishing pressure on H. porcellanus in
the Philippines for the commercial shell trade has been the primary
cause of population decline, and has led to local extinction of the
species throughout the region (Juinio et al., 1987). Villanoy et al.
(1988) documented the export volume of giant clam shells from one major
shell dealer in the Zamboanga region of the Philippines, San Luis Shell
Industries. From 1978 to 1985, approximately 413,230 pairs of shells
were exported by this company, of which about 37 percent (or nearly
153,000) were H. porcellanus. Based on comparisons to data provided by
Juinio et al. (1987), the authors estimate that this shell dealer
accounted for approximately 18.5 percent of the estimated total export
volume of giant clam shells from the Zamboanga region during this
period, suggesting that the total harvest of H. porcellanus during this
period was likely much higher. According to CITES annual reports, from
1985 to 1992, the Philippines exported an additional 576,298 H.
porcellanus shells, 145,926 shell pairs, 179,043.5 kg of shell
material, 293,110 shell carvings, and 38,138 kg of shell carvings. All
were either reported to be wild-caught or did not include the source of
harvest. No other nation reported export volumes close to this
magnitude during this time. Malaysia reported the export of 500 kg of
shell material in 1985, and Indonesia reported the export of 100 kg of
shell material in 1986, but there are no other CITES reports relating
to H. porcellanus from these two countries. CITES reports also indicate
that 16 H. porcellanus were exported as live specimens from the
Philippines to Norway and Germany in 1992 and 1997, respectively; there
have been no exports of live H. porcellanus specimens since.
Additionally, export of 35 live specimens from the Solomon Islands to
Germany and the United States was reported in 1997, but this is likely
a reporting error, as this species has not been observed in the Solomon
Islands.
In Indonesia, H. porcellanus is extremely rare. It was
historically, and still is reportedly, exploited for its meat and
shells when it is found (Pasaribu, 1988; Neo et al., 2017).
Consequently, the species is now thought to occur in only a few
locations in Indonesia (Hernawan, 2010; Wakum et al., 2017). Likewise,
H. porcellanus abundance is also declining in Malaysia, in part due to
ongoing harvest of meat and shells (Neo et al., 2017). As they are
considered rare and are restricted to Sabah and Pulau Bidong on the
east coast of Peninsular Malaysia, continued harvest likely threatens
the persistence of these populations. Additionally, international
poaching continues to pose a threat, as authorities from both Malaysia
and the Philippines reported an increase in the number of fishing boats
illegally harvesting giant clams as recently as 2010-2015 (Neo et al.,
2017).
Overall, it is clear that intense historical commercial demand for
H. porcellanus led to severe population declines and the current low
abundance of the species throughout its range. Furthermore, ongoing
subsistence harvest and poaching of giant clams throughout the South
Asia region continue to threaten the few

[[Page 60518]]

populations of H. porcellanus that remain. Accordingly, we conclude
that overutilization is contributing significantly to the long-term
extinction risk of H. porcellanus and is likely to contribute to short-
term extinction risk in the near future.

T. derasa and T. gigas

Due to the similarities of the threat to T. derasa and T. gigas, we
present the conclusions for these two species together. Overall, the
best available scientific and commercial data indicate that both T.
derasa and T. gigas have been widely exploited for many years for their
meat, shells, and as popular aquarium specimens. Many consider T. gigas
to be the most heavily exploited among all giant clams (Craig et al.,
2011; Mies, Scozzafave, et al., 2017; Neo et al., 2017), noting its
extensive harvest for its meat and shells in nearly every location
where it has occurred. Similarly, T. derasa is also highly valued as a
food source throughout the entirety of its range. For over two decades,
both species were subject to an intense commercial demand for the meat
of their adductor muscle, primarily from consumers in Taiwan.
Widespread harvest and poaching to supply this commercial market caused
severe, documented population losses throughout the majority of the
species' ranges. The commercial demand for giant clam meat began to
decline by the end of the 1980s due to the low abundance of remaining
populations in conjunction with stricter harvest regulations and
improved enforcement. However, due to their traditional importance as a
food source in many cultures, subsistence harvest of T. derasa and T.
gigas continues in most locations throughout their respective ranges,
which may lead to further population decline and likely prevents any
substantial recovery of depleted populations.
Furthermore, recent CITES records and available reports indicate
that T. gigas shells continue to be traded in high volumes from
Indonesia to China despite a prohibition on the harvest and export of
giant clams that has been in place under Indonesian law since 1987
(Allen & McKenna, 2001; Nijman et al., 2015; Harahap et al., 2018).
The Great Barrier Reef and outlying islands of NW Australia are,
for the most part, an exception to the range-wide trends for these
species. Northern areas of the Great Barrier Reef were subjected to
widespread poaching of T. derasa and T. gigas in the 1970s and 1980s,
but improved surveillance of Australian fishing grounds and stronger
enforcement of harvest bans reduced the poaching pressure considerably.
As a result, harvest of the two species in Australian waters since the
1980s has likely been minimal. Recent quantitative estimates of
abundance are scarce, but based on past surveys and the strong
protective measures in place, most experts consider the Great Barrier
Reef to have relatively large, stable populations of giant clams,
including T. derasa and T. gigas (Neo et al., 2017; Wells, 1997).
Overall, we consider the severe impact of past harvest on species
abundance range-wide alongside reports of ongoing subsistence and
commercial use in most locations except Australia. Based on this
information, we conclude that overutilization of T. derasa and T. gigas
contributes significantly to the species' long-term extinction risk.
However, because the threat is minimal in Australia, which represents a
substantial proportion of suitable habitat within these species'
respective ranges, and where populations are reportedly healthy, this
factor likely does not constitute a danger of extinction to the two
species in the near future.

T. mbalavuana

As is discussed above, harvest of giant clams for subsistence
consumption and domestic markets is ongoing and largely unregulated in
Fiji and Tonga. Thus, given the highly restricted range and general
scarcity of T. mbalavuana, we conclude that the threat of
overutilization for commercial purposes contributes significantly to
the species' long-term extinction risk and is likely to contribute to
the short-term risk of extinction in the near future.

T. squamosa

T. squamosa has been harvested extensively for both subsistence and
commercial purposes for several decades, which has led to documented
population declines in many areas of its range (Neo et al., 2017).
While most countries have imposed prohibitions on the commercial
exploitation of giant clams, the demand for T. squamosa in the
ornamental aquarium market continues to pose a threat to wild
populations in Cambodia and Vietnam. Additionally, subsistence harvest
is ongoing in most populated areas where the species occurs. Without
more thorough monitoring from many of these locations, it is difficult
to determine if this ongoing harvest is causing further population
declines, but at the very least, it is likely preventing any
substantial rebound of depleted populations throughout its range. As
with other species, an important exception is Australia, where
anecdotal reports suggest that strictly enforced harvest bans have been
largely successful in preventing overutilization and protecting
reportedly healthy stocks of giant clams. For these reasons, and
considering the documented effects of past harvest on species
abundance, we conclude that overutilization of T. squamosa contributes
significantly to the species' long-term risk of extinction, but does
not in itself constitute a danger of extinction in the near future.

T. squamosina

The best available scientific and commercial data suggest that
giant clams (including T. squamosina) have been harvested extensively
in the Red Sea for many years. Given their traditional importance in
the diets of coastal communities, harvest is likely ongoing in most
areas of the Red Sea. In combination with the natural accessibility of
T. squamosina in shallow nearshore areas, this past and ongoing harvest
pressure has likely contributed significantly to the exceptionally low
abundance of this species throughout the region. We are aware of 30
documented observations of T. squamosina since its re-discovery in
2008. This includes 17 specimens from the Gulf of Aqaba and northern
Red Sea (Roa-Quiaoit, 2005; Richter et al., 2008; Huber & Eschner,
2011; Fauvelot et al., 2020), seven individuals from the Farasan
Islands in southern Saudi Arabia (Fauvelot et al., 2020; K.K. Lim et
al., 2021), and six individuals from an unnamed site in the southern
Red Sea (Rossbach et al., 2021). As an indication of its exceptionally
low abundance at present, Rossbach et al. (2021) surveyed 58 sites
along the entire eastern coast of the Red Sea, from the Gulf of Aqaba
down to southern Saudi Arabia, and observed six T. squamosina at only
one survey site in the southern Red Sea. Similarly, Pappas et al.
(2017) did not encounter any T. squamosina at nine survey sites in the
central Red Sea. With so few T. squamosina remaining, we conclude that
this factor is likely to contribute to short-term extinction risk in
the near future.

Disease or Predation

There are a number of infectious diseases and parasites that have
been reported in giant clams, most often either bacterial or protozoan
in origin (Braley, 1992; Mies, Scozzafave, et al., 2017). Bacterial
infections are most often caused by Rickettsia sp., which infect the
ctenidia (gill-like respiratory organ) and the digestive lining of the
clam (Norton et al., 1993; Mies, Scozzafave, et al., 2017). Protozoan

[[Page 60519]]

infections are often caused by either Marteilia sp. or Perkinsus spp.
Giant clams with Marteilia infections show no external symptoms, but
the infection will eventually cause superficial lesions on the kidney
(Mies, Scozzafave, et al., 2017).
Perkinosis, also known as pinched mantle syndrome, is caused by
Perkinsus spp. Giant clams typically do not exhibit any symptoms of the
infection until they become immunosuppressed due to some other
environmental stress. At that point, the protozoan population is able
to proliferate, and in some cases causes mortality of the host clam.
Once the clam dies, trophozoites of Perkinsus spp. become waterborne
and can infect nearby individuals (Mies, Scozzafave, et al., 2017). A
significant rate of infection by Perkinsus spp. was previously observed
at several sites on the Great Barrier Reef, with 38 of 104 sampled
individuals (including T. gigas and H. hippopus) being infected (Goggin
& Lester, 1987). Additionally, several Perkinsus infections were
observed in association with a mass mortality of giant clams at Lizard
Island in Australia in 1985; however, the cause of the death was never
determined and the infections may have been coincidental (Alder &
Braley, 1989).
Giant clams are also affected by external parasites, including
snails, sponges, and algae. Pyramidellid snails are particularly
invasive, exploiting the clams by inserting their proboscises (i.e.,
feeding appendage) into the clam tissue and consuming the hemolymph
within the siphonal mantle (Braley, 1992). On rare occasions, the
snails may prove fatal to juvenile clams, but they are unlikely to
cause mortality in adult clams (Mies, Scozzafave, et al., 2017). Other
external parasites (i.e., sponges and algae) are typically more of a
nuisance to giant clams rather than fatal infestations. For instance,
boring sponges (e.g., Cliona) may drill holes into the clam's shells,
and algae (e.g., Gracilaria sp.) may overcrowd the shell and prevent
the mantle from extending, but neither of these parasites typically
cause mortality (Mies, Scozzafave, et al., 2017).
When disease is present, giant clams exhibit physical symptoms that
are usually quite obvious, including a retracted mantle (typically the
initial symptom), a gaping incurrent siphon (indicative of more
advanced disease), and discarding of the byssal gland (Mies,
Scozzafave, et al., 2017). While some diseases may respond to
antibiotics, concentrations and dosages for giant clams have not been
well studied. Overall, the prevalence and severity of disease likely
vary across the extensive range of giant clams, but there is no
information to indicate that disease is an operative threat to giant
clams to the extent that it is significantly increasing the extinction
risk of the species addressed here.
Much of what is known regarding predation of giant clams has been
learned from the ocean nursery phase of mariculture activities, when
juveniles are outplanted to their natural environment (Govan, 1992).
Giant clams are widely exploited as a food source on coral reefs, with
75 known predators that employ a variety of attack methods (see table 3
in Neo, Eckman, et al. (2015) for a comprehensive list). These
predators are largely benthic organisms, including balistid fishes,
octopods, xanthid crabs, and muricid gastropods (Govan, 1992). The
fishes (e.g., wrasse, triggerfish, and pufferfish) prey on both
juvenile and adult giant clams by biting the mantle edge, the exposed
byssus, or extended foot. Other predators (e.g., crabs, snails, and
mantis shrimp) have been observed chipping, drilling holes into, and/or
crushing the shells of smaller individuals (see review in Neo et al.
2015). Heslinga et al. (1984) observed several instances of predation
firsthand in association with giant clam culturing operations in Palau.
Large muricid snails (Chicoreus ramosus) were found to attack, kill,
and eat T. squamosa specimens up to at least 300 mm shell length, and a
single hermit crab was able to crush 26 T. gigas juveniles (20-30 mm)
when inadvertently left in the culture tank. The authors also noted
circumstantial evidence of predation by Octopus spp. in Palau based on
the characteristically chipped shells of giant clams often observed
outside of octopus dens.
Giant clams employ a suite of defense mechanisms, both
morphological and behavioral, to resist predatory attacks (Soo & Todd,
2014). For example, their large body size, small byssal orifice, and
strong shells create physical barriers to predation. In addition, T.
squamosa is equipped with hard, scaly projections on its shell known as
scutes that have been shown to provide protection from crushing
predators (Han et al., 2008). Giant clams also exhibit behavioral
defense mechanisms, such as aggregation, camouflage, rapid mantle
withdrawal (Todd et al., 2009) and squirting water from siphons (Neo &
Todd, 2010). While the ability of giant clams to endure intense
predation pressure and acclimate to repeated disturbance can have
implications on their survival, these attributes have not been studied
extensively (Soo & Todd 2014). Similar to disease, we find no evidence
to indicate that predation presents a significant threat to the
extinction risk of the giant clam species addressed here.

The Inadequacy of Existing Regulatory Mechanisms

Giant clams are protected from overutilization to varying degrees
by a patchwork of regulatory mechanisms implemented by the many
countries, territories, and Tribal entities within their range. These
local-scale measures are also supplemented by CITES international trade
regulation, and in some areas, by multi-national initiatives aimed at
supporting sustainable regional giant clam fisheries. We address each
of these regulatory mechanisms in the following section and also
include a brief discussion of international climate change regulations
in the context of their potential effects on the extinction risk of
giant clams. More detailed information on these management measures can
be found in the accompanying Status Review Report (Rippe et al., 2023).
Local Regulations
There is national legislation in place in more than 30 countries
and territories specifically related to the conservation of giant
clams. Many also provide indirect protection via marine parks and
preserves or ecosystem-level management plans. In general, management
of giant clam populations has been most effective in Australia, where
early harvest prohibitions and strict enforcement have been largely
successful in stabilizing giant clam population declines and limiting
illegal poaching (Wells et al., 1983; Dawson, 1986; Lucas, 1994). Many
Pacific island nations have also implemented strict measures to
mitigate fishing pressure on giant clams. These include total bans on
commercial harvest and export of giant clams (e.g., Fiji, Papua New
Guinea, Solomon Islands, Vanuatu, FSM, Guam, Republic of Kiribati and
Palau), minimum size limits for harvest (e.g., French Polynesia, Niue,
Samoa, American Samoa, Guam, and Tonga), harvest quotas or bag limits
(e.g., New Caledonia, the Cook Islands, and Guam), and gear
restrictions on the use of SCUBA or certain fishing equipment
(Andr[eacute]fou[euml]t et al., 2013; Kinch & Teitelbaum, 2010; Neo et
al., 2017). We are not aware of any local regulations in place
restricting the harvest of giant clams in CNMI, although the harvest of
all coral reef-associated organisms in Guam and CNMI is managed under
the 2009 Fishery Ecosystem Management Plan for the Mariana Archipelago.

[[Page 60520]]

In many Pacific islands, national legislation is also supplemented
or enforced by way of customary fishing rights and marine tenure
systems. This is the case in parts of Fiji, Samoa, Solomon Islands,
Cook Islands, Papua New Guinea, and Vanuatu, where indigenous village
groups hold fishing rights and regulate access to adjacent reef and
lagoon areas (Govan et al., 1988; Fairbairn, 1992a, 1992b, 1992c;
Wells, 1997; Foale & Manele, 2004; Chambers, 2007; UNEP-WCMC, 2012).
The rights of each Tribal group over its recognized fishing area
include the right to carry out and regulate subsistence fishing
activities. In certain circumstances, a local village or villages may
impose temporary area closures to reduce harvesting pressure and allow
giant clam stocks to recover (Foale & Manele, 2004; Chambers, 2007).
The effectiveness of these measures to address overutilization,
however, is variable, and with limited capacity for long-term
monitoring programs in the region, it can be difficult to properly
assess. In general, anecdotal reports indicate that giant clam
populations throughout the Indo-Pacific region continue to face severe
stress (Neo et al., 2017).
In the Philippines, for example, numerous reports following the
giant clam export ban in 1990 suggested problems with enforcement,
particularly within Badjao communities. The Badjao people live a
predominantly seaborne lifestyle and are spread across the coastal
areas of the southern Philippines, Indonesia, and Malaysia, with a
total population estimated to be around one million (Government of the
Philippines National Statistics Office, 2013; Rincon, 2018). Many in
these communities were encouraged by buyers to collect and stockpile
giant clam shells in the hope that the ban on giant clam export would
eventually be lifted (Salamanca & Pajaro, 1996; Wells, 1997). Middlemen
would reportedly advance money and provisions to fishermen on the
condition that the shells be sold to them exclusively. The Badjaos
would then harvest clams, consume or discard the meat and stockpile the
shells (Salamanca & Pajaro, 1996). The non-compliance was exacerbated
by varying interpretations of the law by Philippine authorities, who
issued numerous CITES export permits in 1991-1992 under the presumption
that the law excluded `pre-ban stock' (Wells, 1997). The ban was
ultimately never lifted, and CITES reports indicate that the legal
export of giant clams has ended in the Philippines. However, a recent
report by the Wildlife Justice Commission (2021) found that authorities
have continued to find stockpiles of giant clam shells throughout the
country. Authorities have made 14 seizures from 2016 to 2021, including
of a 132,000-ton stockpile in the southern Philippines in October 2019
and several stockpiles in the Palawan area, one of the centers of giant
clam abundance in the region. It is unclear how many of the shells were
collected prior to the ban in 1990 versus how many were collected
illegally in the years since, but it suggests that the market for giant
clam shells remains active more than 30 years after the ban was
instituted. In an interview with ABS-CBN News (2021), Teodoro Jose
Matta, executive director of Palawan Council for Sustainable
Development, claimed that the clams are being smuggled to Southeast
Asia and Europe and attributed the activities to a criminal syndicate
operating across the Philippines, not just in Palawan. To our
knowledge, these claims have not been corroborated by authorities.
Similar confusion over giant clam harvesting regulations has
impeded the effectiveness of regulations to address overutilization in
Papua New Guinea. An initial ban on the purchase and export of wild-
caught giant clams was put in place in 1988 by the Department of
Environment and Conservation (DEC) (Kinch, 2002; UNEP-WCMC, 2011). It
was lifted in 1995 following the development of a management plan for
sustainable harvest; however, Kinch (2002) noted that although the
Milne Bay Province Giant Clam Fishery Management Plan had been drawn up
by the National Fisheries Authority (NFA)--the CITES Scientific
Authority for Papua New Guinea--it was never officially adopted ``owing
to confusion between the NFA and the DEC over responsibility for the
enforcement of the plan and because of opposition from commercial and
political interests.'' The ban was reinstated in 2000 following reports
that a local fishing company was exporting wild-caught specimens as
captive-bred. Kinch (2002) suggested that further ``conflict and
confusion between the fisheries and environmental legislation'' ensued
and recommended that it be addressed to ensure success of the
regulation. Unfortunately, the last known monitoring survey in Papua
New Guinea was conducted in 1996 in the Engineer and Conflict Island
Groups. Based on survey findings, it was estimated that the overall
density of giant clams (all local species) had declined by over 82
percent since the early 1980s, while the density of T. gigas had
declined by over 98 percent (Ledua et al., 1996). Without more recent
data, we cannot determine whether the regulatory actions have had any
effect on this trajectory.
Furthermore, despite various levels of harvest and export
prohibitions among many of the Pacific island nations, Kinch and
Teitelbaum (2010) highlight a number of common challenges to ensuring
sustainable giant clam management in these communities. This includes a
lack of capacity for conducting stock assessments, promoting giant clam
mariculture, enforcing harvesting regulations, and monitoring and
actively managing giant clam harvest. The list also includes a lack of
education and awareness among community members about sustainable giant
clam harvest, uncoordinated legislative structure, and a lack of
international collaboration to promote a sustainable and scalable
market for captive-bred giant clams. According to the assessment by
Kinch and Teitelbaum (2010), each of the countries experiences these
challenges to a different degree, but overall it highlights the
difficulties in effectively managing giant clam populations for smaller
island nations that may lack enforcement resources or expertise. This
is compounded, in many cases, by the traditional importance of giant
clams as a coastal resource, which may limit the willingness among
indigenous communities to adopt the recommended practices (Neo et al.,
2017).
In addition to the two examples above, there are a number of other
reports highlighting the inadequacy of local regulations to address the
threat of overutilization throughout Indo-Pacific region. In Malaysia,
and particularly in Borneo, illegal collection of giant clams was
reported to occur despite a national prohibition on the collection of
giant clams (Ibrahim & Ilias, 2006). In the Solomon Islands, commercial
harvest and export was banned in 1998, but CITES records indicate that
export of wild-sourced clams and shells from the Solomon Islands has
continued to occur throughout the 2000s and as recently as 2015. Yusuf
and Moore (2020) note that despite being fully protected under
Indonesian law and widespread public awareness of associated harvest
prohibitions, giant clams are still harvested regularly in the Sulawesi
region of Indonesia, including mass collections for traditional
festivals. When asked about enforcement of legal protections, locals
explained that surveillance in certain areas was generally absent (or
at best sporadic and ineffective), and throughout the region was
``minimal, often perceived as misdirected and/or unfair, and mostly

[[Page 60521]]

ineffective.'' Due in part to the ineffectiveness of the existing
regulations, Yusuf and Moore (2020) have documented progressive
declines in giant clam populations from 1999 to 2002, 2007, and 2015,
with ``some larger species (T. gigas, T. derasa, T. squamosa, and H.
porcellanus) no longer found at many sites.'' Low abundance of T.
squamosa, T. derasa, T. gigas, and H. hippopus has also been observed
in the Anambas Islands of Indonesia, where Harahap et al. (2018) report
ongoing harvesting and habitat destruction. In Mauritius, giant clams
are protected under the Fisheries and Marine Resources Act of 2007, but
a recent study shows continued population declines even within marine
protected areas (Ramah et al., 2018). There are few studies
highlighting success of local regulations, but Rossbach et al. (2021)
report based on interviews with local fishermen that giant clams are no
longer targeted in Saudi Arabia since a harvest prohibition was imposed
in the early 2000s. Although we note that giant clams were listed as
``Taxa of High Conservation Priority'' in Saudi Arabia's First National
Report to the Convention on Biological Diversity in 2004 (AbuZinada et
al., 2004), we could not find any national regulations associated with
this designation.
The general lack of long-term monitoring data makes it difficult to
evaluate the effectiveness of local regulatory mechanisms to address
threats from overutilization for commercial purposes beyond relying on
anecdotal reports. In many areas, for example, harvest prohibitions
have been instituted within the last decade or two, but there have been
few, if any, follow-up surveys conducted in the time since. However,
using what survey data are available, we can infer that existing
regulations have been inadequate to protect any of the seven giant clam
species from overutilization. Despite widespread commercial export
bans, the capacity for enforcing existing regulations is often limited,
existing regulations do not restrict continued subsistence harvest in
many locations, and illegal harvest and trade of giant clams
(particularly for the shell trade) continues to occur (Kinch &
Teitelbaum, 2010; Yusuf & Moore, 2020; Wildlife Justice Commission,
2021). For these reasons, we conclude that the inadequacy of local
harvest regulations to address overutilization associated with
subsistence fisheries and illegal harvest in all locations outside of
Australia contributes significantly to the long-term extinction risk of
H. hippopus, T. derasa, T. gigas, and T. squamosa. Moreover,
considering the exceptionally low abundance and restricted ranges of H.
porcellanus and T. mbalavuana, we conclude that the inadequacy of local
harvest regulations to address overutilization associated with
subsistence fisheries likely also poses a short-term risk of extinction
for these species in the near future.
With respect to T. squamosina, we also considered the likely effect
of marine protected areas (MPAs), which are the principal regulatory
mechanism relevant to the protection of giant clams from
overutilization in the Red Sea. Based on the known distribution of T.
squamosina, there are three MPAs that are most relevant to the species:
Ras Mohammed National Park in South Sinai, Aqaba Marine Park in Jordan,
and the Farasan Islands Protected Area in southern Saudi Arabia. These
are three areas where T. squamosina has previously been observed, and
remaining populations likely benefit from the prohibitions against
hunting or collecting wildlife within the boundaries of the MPAs.
According to Gladstone (2000), a prohibition on the collection of giant
clams in the Farasan Islands appeared to be effective, with harvest-
related mortality falling to 1.7 percent, compared to an estimated
11.1-47.8 percent mortality rate prior to the regulation. Ras Mohammed
National Park is also regarded as effective in the protection of 345
km\2\ of marine area, which includes important fringing reef habitats
in the southern portion of the Gulf of Aqaba.
Collectively, however, these three protected areas encompass only a
small fraction (5,756 km\2\) of the coastal marine area in the Red Sea.
Throughout most of the region, harvest of giant clams remains largely
unregulated. As is described above, historical harvest of giant clams
has likely led to the exceptionally low abundance of T. squamosina in
the Red Sea, and there are reports that harvest is ongoing in most
locations. Thus, given the lack of national regulations pertaining to
the harvest of giant clams in the Red Sea, we find that an inadequacy
of existing regulatory mechanisms to address the threat of
overutilization contributes significantly to the long-term extinction
risk for T. squamosina. However, because several MPAs have been
established in key areas where the species has been recently observed,
we conclude that this factor does not in itself constitute a danger of
extinction in the near future.
Regulations for International Trade
Giant clams are listed under appendix II of CITES, which consists
of species that ``are not necessarily now threatened with extinction,
but may become so unless trade is closely monitored.'' This designation
does not necessarily limit trade of the species, but instead requires
that any species in trade has been legally acquired and a finding that
trade is not detrimental to the survival of the species by the
exporting Party's Scientific Authority. CITES regulates all
international trade in giant clams (including living, dead, and
captive-bred specimens) and requires the issuance of export permits and
re-export certificates. For each listing, a Party may take a
reservation to that listing, meaning the Party will not be bound by the
provisions of the Convention relating to trade in that species. While
the reservation is in effect, the Party is treated as a non-Party
regarding trade in the particular species. Currently, Palau has
reservations on all of the giant clam listings. Parties with
reservations or other non-Parties that trade with a CITES Party are
required to have documentation comparable to CITES permits. It is up to
the Party State receiving the export whether to accept this
documentation in lieu of CITES permits.
Effective enforcement of CITES is largely dependent on whether the
countries involved are signatories to the Treaty, as well as the
accuracy of trade data supplied by the Parties (Wells, 1997). Of the 60
countries and territories where the seven giant clam species considered
here naturally occur, 52 are signatories to the Treaty. This includes
the United States and all of its Pacific island territories. A number
of countries that have historically played a significant role in the
trade of giant clam products are not CITES contracting parties (e.g.,
Cook Islands, Kiribati, Marshall Islands, FSM) or have only become so
relatively recently (e.g., Palau in 2004, Solomon Islands in 2007,
Maldives in 2012, Tonga in 2016). However, all CITES Parties trading in
CITES listed species with countries that are not members of CITES, or
with CITES Parties that have taken a reservation on the species, must
still seek comparable documentation from the competent authorities of
the reserving Party or the non-member country, which substantially
conforms with the usual requirements of CITES for trade in the species.
Importantly, even in instances where exporting countries are Parties to
CITES, the trade data must be interpreted cautiously for reasons that
may include frequent

[[Page 60522]]

discrepancies in recorded import and export quantities, inconsistencies
in the terms or units used to describe the trade, occasional omissions
of seized or confiscated specimens, erroneous data entry, and delays or
failure to submit trade statistics to the Secretariat (UNEP-WCMC, 2012;
CITES, 2013; Neo et al., 2017).
Overall, the threat of inadequate regulations related to the
international trade of giant clam products is relevant only to the
species that are traded in significant quantities. This does not
include T. mbalavuana or T. squamosina, as we could not find any
information to indicate that there has ever been an international
commercial export market for these species. With respect to H.
hippopus, T. derasa, and T. squamosa, CITES annual report data indicate
that the large majority of recent international trade of these species
is of culture-raised specimens and products. Since 2010, only 2,756 H.
hippopus shells and 7,302 live H. hippopus specimens have been recorded
in trade. Approximately 51.2 percent of traded shells during this
period were of wild-caught origin, primarily from the Solomon Islands
in 2014, while 34.1 percent were reportedly culture-raised. Of the live
specimens, only 2.6 percent were wild-caught, while 96.2 percent were
reportedly culture-raised.
Similarly, since 2010, 154,245 of the 158,319 live T. derasa
specimens recorded in trade were culture-raised (97.4 percent), while
only 3,514 were reportedly wild-caught (2.2 percent). A smaller
proportion of shells and shell products recorded in trade since 2010
were of cultured T. derasa, but the total trade volume is significantly
lower. In total, 3,775 of the 11,100 T. derasa shells and shell
products were of culture-raised specimens (34 percent), while 7,312
were wild caught (65.9 percent).
The primary market for T. squamosa in international trade is of
live clams for the ornamental aquarium industry, and it appears that
most major exporters have transitioned their supply to cultured
specimens. The major exceptions are Cambodia and Vietnam, which
together have exported over 50,000 wild-caught T. squamosa since 2010.
The government of Vietnam instituted a quota system to regulate the
commercial harvest of wild giant clams after concerns were raised in
the early 2010s about the level of exploitation. However, the
subsequent rise in the export of live T. squamosa from Cambodia to
Vietnam suggests that this regulation simply diverted the harvest to
neighboring waters. While this harvest pressure likely threatens the
persistence of T. squamosa populations in Cambodia in the long term,
available reports suggest that the species is still frequent in both
countries.
Based on these data, we conclude CITES regulations have been
effective at transitioning much of the international supply of H.
hippopus, T. derasa, and T. squamosa products away from wild harvest
and towards mariculture operations and therefore, minimizing the risks
to these three species from overutilization associated with
international trade. In other words, it is unlikely that this factor
contributes significantly to the extinction risk for these species.
With respect to H. porcellanus, only five shells have been recorded
in international trade since 2010--two exported from Malaysia to the
Netherlands in 2013, and three exported from the Philippines and seized
in the United States in 2011 and 2016. However, it is likely that the
low trade levels are as much a reflection of the species' low abundance
as they are of the effectiveness of international regulation.
Regardless, although commercial trade of this species significantly
reduced its abundance in the past, there is little evidence to suggest
that international trade is a threat currently operating on this
species, and given the available information to suggest otherwise, the
regulations appear to be adequate to address that threat.
With respect to T. gigas, unlike H. hippopus and T. derasa, CITES
records indicate that the majority of the reported trade since 2010 is
of wild-caught specimens, suggesting that mariculture has not played a
significant role in diverting harvest away from wild populations. As
recently as 2018, Indonesia exported 59,000 wild-harvested T. gigas
shells to China despite the reportedly low abundance of T. gigas
throughout the region and despite both nations being CITES contracting
Parties. While most countries and territories within the range of T.
gigas are regulated under the provisions of CITES, the associated
protections were clearly not adequate to prevent widespread population
loss and local extirpations of the species from many of the same
locations (Neo et al., 2017). Thus, we conclude that inadequate
regulation of international trade to address the threat of
overutilization contributes significantly to the long-term extinction
risk of T. gigas.
Regulations on Climate Change
In the final rule to list 20 reef-building corals under the ESA (79
FR 53851), we assessed the adequacy of existing regulatory mechanisms
to reduce global greenhouse gas (GHG) emissions and thereby prevent
widespread impacts to corals and coral reefs. We concluded that
existing regulatory mechanisms were insufficient to effectively address
this threat. Since the publication of that final rule in 2014, 197
countries and the European Union (EU) adopted the Paris Agreement on
climate change, which set a goal of limiting the global temperature
increase to below 2 [deg]C and optimally keeping it to 1.5 [deg]C.
Since the Agreement was entered into force on November 4, 2016, 191
countries and the EU have ratified or acceded to its provisions, and
each Party has made pledges to decrease GHG emissions to achieve its
goals (UNFCC, 2018). The United States, which currently accounts for
one-fifth of the world's emissions, pledged to cut its emissions by 26-
28% percent. However, according to the 2023 Synthesis Report for the
IPCC's Sixth Assessment Report, there remains a ``substantial emissions
gap'' between the projected emissions trajectory associated with the
climate actions currently proposed by the Parties to the Paris
Agreement and the trajectories associated with mitigation pathways that
limit warming to 1.5 [deg]C or 2 [deg]C by 2100 (IPCC 2023). The IPCC
reported with high confidence that current limited progress towards GHG
emissions reduction make it likely that warming exceeds 1.5 [deg]C by
2100 and make it considerably harder to limit warming to less than 2
[deg]C. In addition, the IPCC projected with medium confidence that the
current emissions trajectory without strengthening of policies will
lead to an estimated global temperature increase of 3.2 [deg]C by 2100,
with a range of 2.2 [deg]C to 3.5 [deg]C (IPCC, 2023).
At this rate, unless average emissions reduction goals are
significantly strengthened, van Hooidonk et al. (2016) project that
over 75 percent of reefs will experience annual recurrence of severe
bleaching events before 2070. In a similar analysis, Hoegh-Guldberg et
al. (2007) investigated four emissions reduction pathways that are used
by the Intergovernmental Panel on Climate Change and found that only
the most aggressive scenario would allow the current downward trend in
coral reefs to stabilize. The study predicts that even moderate
emission reductions will still lead to the loss of more than 50 percent
of coral reefs by 2040-2050. Thus, regardless of whether the goals of
the Paris Agreement are met, impacts to coral reefs are expected to be
widespread and severe. However, as is

[[Page 60523]]

discussed above, while there is clear evidence that coral reefs will
undergo substantial changes as a result of ocean warming and
acidification, it is unclear whether and to what degree the changes in
coral reef composition and ecological function will threaten the
survival and productivity of giant clams. Furthermore, as is discussed
below in Other Natural or Man-Made Factors, there is substantial
evidence to suggest that giant clams may experience significant
physiological changes under projected ocean warming scenarios. The
precise magnitude of these impacts is unknown, but any significant
changes in metabolic demand, reproductive success, and the possibility
of bleaching due to warming summer temperatures, will likely increase
the risk of extinction. For this reason, we find with respect to all
seven species that the inadequacy of regulations to address climate
change may, in combination with the aforementioned impacts, contribute
significantly to the long-term or near future risk of extinction, but
is unlikely a significant threat on its own.
Inadequacy of Regulations in the South China Sea
As is discussed above, H. hippopus, H. porcellanus, T. gigas, and
T. squamosa also face the threat of habitat destruction in portions of
the South China Sea where fishermen, primarily from the Hainan Province
of China, have been razing shallow reef areas in a search for giant
clam shells (see Present or Threatened Destruction, Modification, or
Curtailment of Its Habitat or Range). In an effort to curtail this
destructive activity, the Hainan Province People's Congress passed
regulations in January 2017 to prohibit the commercial trade of all
giant clam species in the province. However, a recent report from the
Wildlife Justice Commission (2021) suggests that the illegal harvest
and trade of giant clam shells continues to occur in the region, with
new harvesting boats returning to the Hainan Province since 2018. For
this reason, we conclude that the inadequacy of existing regulations to
address the threat of habitat destruction in the South China Sea due to
giant clam shell harvesting operations contributes significantly to the
long-term extinction risk of H. hippopus, T. gigas, and T. squamosa. In
addition, due to the exceptionally low abundance and highly restricted
range of H. porcellanus, which includes the southern portion of the
South China Sea, the combination of these threats likely also
contributes to the near future extinction risk for H. porcellanus.

Other Natural or Man-Made Factors

There are several other natural or manmade factors that impact
giant clams, such as ocean warming and acidification, coastal pollution
and sedimentation, and stochastic mortality events. Below, we summarize
each of these factors, and where sufficient information is available,
evaluate the severity of the associated threat to each of the seven
giant clam species.
Ocean Warming
As is mentioned above, giant clams associate symbiotically with a
diverse group of dinoflagellates of the family Symbiodiniaceae which
reside within a network of narrow tubules that branch off the primary
digestive tract and spread throughout the upper layers of the mantle
(Norton et al., 1992). Giant clams provide dissolved inorganic
nutrients to the zooxanthellae via direct absorption from the seawater
or as an excretory byproduct of respiration, and in return, receive
photosynthetic carbon in the form of glucose, glycerol,
oligosaccharides and amino acids, comprising the majority of their
metabolic carbon requirements (Klumpp et al., 1992; Hawkins & Klumpp,
1995). Exposure to stressful environmental conditions, however, can
cause dysfunction in the symbiosis and, in extreme cases, can lead to a
bleaching response wherein the zooxanthellae is expelled from the
mantle tissue. When they bleach, giant clams lose a critical source of
nutrition and experience drastic changes to their physiology, including
decreased glucose and pH in the hemolymph, an increased concentration
of inorganic carbon (e.g., CO<INF>2</INF> and
HCO<INF>3</INF>-), and a reduced capacity for ammonium
assimilation (Leggat et al., 2003).
Elevated temperatures, in particular, are known to induce bleaching
in giant clams. Widespread bleaching of giant clams was observed in the
central Great Barrier Reef, Australia in 1997-1998, when elevated water
temperatures in conjunction with low salinity caused 8,000 of 9,000
surveyed T. gigas to experience varying levels of bleaching (Leggat,
pers. comm., cited in Buck et al., 2002; Leggat et al., 2003). Some
individuals suffered a complete loss of symbionts, while others were
only affected in the central part or at the margins of the mantle
tissue (Grice, 1999). A follow-up experiment designed to replicate the
environmental conditions during this event demonstrated that elevated
temperatures combined with high solar irradiance

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