On the status of giant clams, relics of Tethys (Mollusca

On the status of giant clams, relics of Tethys (Mollusca

Proceedings 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000, Vol. 2.
On the status of giant clams, relics of Tethys (Mollusca: Bivalvia:
Tridacninae)
W. A. Newman1 and E. D. Gomez2
ABSTRACT
The giant clams, subfamily Tridacninae, include two extant genera, Hippopus and Tridacna, represented by three and
twelve extinct and two and seven extant species, respectively. The extant species are presently restricted to the IndoWest Pacific, with the center of diversity in the Indo-Malayan region. However, fossil evidence not only shows the
family once had a Tethyan distribution but a greater genetic diversity during the Tertiary than it does today. Reliction
included total extinction in the tropical Atlantic and some extinction and range reduction in the Indo-West Pacific as
recently as the Holocene. The suggestion of Schneider and Ó Foighil (1999), that Tridacna tevoroa Lucas et al.
(1991) should have a Neogene fossil record, making it a paleo- rather than a neo-endemic, is fulfilled since it proves
to be a junior synonym with T. mbalavuana Ladd (1934) from the Upper Tertiary of Fiji. Tridacna rosewateri
Sirenko and Scarlato (1991), while similar to T. squamosa, may be a distinct species endemic to the Mascarene
Plateau. In response to increasing human populations, concomitant resource exploitation and environmental
deterioration in the Indo-West Pacific, some species of this relic subfamily have become depleted or even locally
extinct, so that current management practices include rearing and re-introductions as well as regulatory measures.
Keywords Giant clams, Hippopus, Tridacna, Persikima,
Chametrachea, Endemism, Reliction, Human impact,
Conservation
Introduction
If there were no fossil record, it would appear the
tridacnines evolved in the Indo-Malayan region and the
species radiated to varying degrees throughout the
Indo-West Pacific. However, from the fossil record
(Table 1), we know this is not the case. Thus, while
knowledge of the recent history of the species is very
important, understanding the broader historical aspects
can add significantly to our appreciation of their present
demise.
The distribution of extinct and extant Tridacninae
(Rosewater 1965, Cox et al. 1969, Lucas 1988 and herein)
shows this subfamily underwent a marked decline during
the breakup of Tethys. Like a number of other relic
family-group taxa (cf. Houbrick 1984a-c for some
extreme examples), it is now contributing to the
remarkably high marine endemism of the Southwest
Pacific and Australia (Newman 1991). This is in contrast
to many other reef organisms at the family-group level,
such as many reef corals (Veron 2000), and coral
barnacles (Ross and Newman, this symposium) which
have undergone dramatic radiations during the same
period.
Table 1. General spatial and temporal distribution of the Tridacninae (data from Rosewater 1965, 1982, Cox et al.
1969, Lucas et al. 1990, Sirenko and Scarlato 1991, Schneider 1998 and herein); † = extinct, + = extant, - = no record,
IWP= Indo-West Pacific, WI = West Indies, & E/E = ratio of extinct to extant species.
†Goniocardium
†Avicularius
†Byssocardium
Hippopus
Tridacna(Tridacna)
T. (Chametrachea)
M. Eoc.-U. Eoc.
M. Eoc.-L. Oligo.
M. Eoc.-L. Mio.
Mio.-Recent
L. Mio.-Recent
Mio.-Recent
The Tridacninae appeared in the Paleogene when a
number of genera were to be found in Western Tethys
(Table 1). However, most of these became extinct before
the Neogene, apparently due to climatic change
concomitant with the breakup of the Tethys Sea. These
changes included restriction of the tropics by cooling of
1
2
IWP
+
+
+
WI
†
†
-
Europe
†
†
†
†
†
E/E
3/2
4/3
8/4
high latitudes beginning in the Oligocene, and warming of
the tropics particularly in the Miocene, followed by the
perturbations of the Pleistocene (Shackleton 1984,
Valentine 1984, Newman 1986, Stanley 1986, Paulay
1996).
The Scripps Institution of Oceanography, La Jolla, CA 92993-0202,USA. email: [email protected]
The Marine Science Institute, University of the Philippines, Quezon City, The Philippines
Of the 15 extinct species recorded for the only
surviving genera, Hippopus and Tridacna, nearly half
went extinct in the Neogene and the survivors were
further restricted, at least peripherally, up into the
Holocene.
Since the process is apparently being
intensified by human activities, largely due to exploitation
for meat and shell, some management policies have been
proposed (Gomez and Alcala 1988).
But before
discussing these, some taxonomic considerations need
attention.
Taxonomic considerations
We follow Schneider and Ó Foighil (1999) in
considering the subgenus Persikima Iredale, 1937 a junior
synonym of Tridacna s.s., and the taxonomic status of the
recently described species, T. mbalavuana and T.
rosewateri, is discussed below.
1) Tridacna (Tridacna) mbalavuana Ladd, 1934: We
borrowed the fossil type material of this Fijian species
from the Bernice P. Bishop Museum, compared it to the
valves of extant T. tevoroa Lucas et al. 1991 of
comparable size from Tonga, and they are virtually
identical.
Therefore we consider the two species
synonymous (see Appendix for synonymy). While Ladd
(1934) correctly determined the anterior-posterior axis of
T. mbalavuana, whereby the byssal gape is anterior
(Stasek 1965, Lucas et al. 1991), Rosewater (1965:380)
reversed it, and Norton and Jones (1992, key, dichotomy
5a) did likewise.
Lewis and Ledua (1988) and Lucas et al. (1990, 1991)
recognized close affinities between T. mbalavuana, and
the only other species then assigned to Persikima, T.
derasa, Lucas et al. (1990) used the radial sculpture in the
former as one of the several characters distinguishing
them. However, the genetic analysis of Schneider and Ó
Foighil (1999) did not provide a genetic basis for
distinguishing Persikima and Tridacna s.s., and therefore
they made Persikima a subjective junior synonym of
Tridacna s.s. While we follow the latter authors in
placing T. mbalavuana in the subgenus Tridacna, how
this will be accepted remains to be seen.
2) Tridacna (Chametrachea) rosewateri Sirenko &
Scarlato, 1991: These authors compare their new species
to the other two species known from the region,
wide-ranging T. squamosa and T. maxima which are in
the same subgenus (Chametrachea). They note T.
rosewateri differs from the former in having 1) a thinner
shell, 2) larger byssal orifice, 3) scales more densely
arranged on the primary ribs, and 4) larger interdigitating
occludent projections. Its soft parts are unknown, as are
its burrowing capabilities, but since the authors can
readily distinguish its shells from those of T. squamosa,
they concluded it is a distinct species.
Sirenko and Scarlato (1991:7) give a table of shell
measurements for Tridacna rosewateri, as well as for
some specimens of T. squamosa and T. maxima, the only
other tridacnines known from the western Indian Ocean.
Their table includes ratios for various measurements
(L/W, W/I.p, V.m/B.o, and V.m/H.l). Such ratios
normalize the data between species as well as individuals
and thus afford a ready means of comparison. To
facilitate such comparisons we calculated their means
(Table 2). The mean V.m/H.l ratios for these three
populations, being closest, are the least informative.
Those for T. rosewateri are intermediate between the
other two, there is a slight overlap between those for
rosewateri and squamosa, and the difference between
them is small, so their significance is low. Nonetheless,
as we shall see, this is the only one of the four sets of
means in which rosewateri is more similar to squamosa
than it is to maxima!
Table 2. Mean ratios for ratios on shell measurements for
three species of Tridacna (Chametrachea), calculated
from ratios of Sirenko and Scarlato (1991:7). B.o =
length of byssal orifice, H.l = length of hinge line, I.p =
height of interdigital projections, L = length of shell, W =
width without scales, V.m = length of ventral margin.
W/I. V.m/ V.m/
L/W
p
B.o
H.l
T.rosewateri (n=9)
3.0
1.5
2.3
1.2
T.squamosa (n=4)
2.4
3.1
3.8
1.0
T. maxima (n=8)
2.4
3.3
1.8
1.5
In the second set (V.m/B.o, Table 2) there is a slight
overlap between T. rosewateri and T. maxima, and both
differ appreciably from squamosa. Thus T. rosewateri
appears more similar to T. maxima than T. squamosa by
this character, but this alone is of no great weight.
In the remaining two sets (L/W and W/I.p), the means
for rosewateri and maxima are not only virtually identical
but are quite distinct from that for T. squamosa. By these
two ratios as well as the previous one, rosewateri is more
similar to maxima than it is to squamosa. By these ratios
then, in addition to the characters given by Sirenko and
Scarlato (1991), T. rosewateri appears to be distinct from
T. squamosa. Therefore it seems to represent a good
species. Nonetheless, a genetic comparison between
rosewateri and squamosa could be instructive, and there
is a comparable problem with a population attributed to
squamosa near the eastern extend of its range that also
appears worthy of attention (see Reliction below).
Temporal and spatial distributions
The principal sources on spatial and temporal
distributions for tridacnines are Rosewater (1965, 1982),
Cox et al. (1969), Copland and Lucas (1988), and Lucas
et al. (1991). Despite there being relatively few,
conspicuous species, range limits and patchiness need to
be better documented if we as well as posterity are to
recognize changes that are likely to continue to occur.
Therefore these published records are supplemented here
with data from more specialized papers and with personal
communications from a number of workers (see also
Appendix & Acknowledgments).
These data are
summarized in Appendix where we arbitrarily list the
living species in three groups according to geographic
range rather than by their taxonomic status or relative
size; 1) two long-range species (Tridacna maxima and T.
squamosa, 2) four medium-range species (Hippopus
hippopus, T. gigas, T. derasa, and T. crocea), and 3) three
short-range species (H. porcellanus, T. mbalavuana, and
T. rosewateri).
The Tridacninae was originally Tethyan (Table 1).
All three extinct genera attributed to the subfamily
appeared in the Eocene, were apparently limited to the
tropical Atlantic, and all but one apparently died out
before the Miocene. An Upper Cretaceous record for a
Tridacna from Madagascar (Rosewater 1965, Cox et al.
1969) seemed doubtful, and it has since been concluded
that it is more likely Neogene (Schneider 1998:352). We
concur with this conclusion, as we do with Taviani's
(1994) rejection of the Pliocene record for Tridacna in
Italy. Of the two extant genera, Hippopus, known from
the Early Miocene of the West Indies and the Marianas
Islands, is considered morphologically more primitive
(less derived) than Tridacna (Upper Miocene). This is
consistent with the fossil record and recent morphological
and molecular phylogenetic analyses (Schneider 1998,
Schneider and Ó Foighil 1999). Schneider and Ó Foighil
(1999, Fig. 3) combine their inferred phylogeny and the
stratigraphic record of extant species in an instructive
figure.
Relic hypothesis
"Relic" populations are survivors of ancient radiations
having no living ancestors, in contrast to "relicts" or
populations isolated from parent populations. In light of
the geographical distribution of the fossils, there is no
question the Tridacninae is a relic subfamily.
Furthermore, T. mbalavuana shares some characters, such
as the absence of hyaline organs, a pedal gape and mantle
projections, with the more generalized tridacnines,
Hippopus, and therefore appears relatively primitive (less
derived) (Lucas et al. 1991). However, without a fossil
record, it could be argued it had lost advanced features;
e.g., was regressive. Therefore the fossil record for T.
mbalavuana not only corroborates the relic hypothesis of
Lewis and Ledua (1988), but the genetic evidence which
suggested it should be at least early Neogene in age
(Schneider and Ó Foighil 1999).
As noted in the introduction, similar relic patterns are
known for a number of formerly Tethyan groups that now
have their centers of distribution in the West Pacific,
particularly the Southwest Pacific (Fleming 1979,
Newman 1991). In addition, it is noteworthy that T.
mbalavuana is found at the eastern limit of its larger
subcongeners, T. derasa and T. gigas (the latter not being
known to occur east of Fiji; Lewis and Ledua 1988).
Tridacna mbalavuana is most similar to T. derasa, but
these two species for the most part segregate by depth, the
former below and the latter above the 20 m line (Lucas et
al. 1991). Of all the species in the region, only T. maxima
and squamosa, which range all the way to the Red Sea,
are presently known to range further east. Thus T.
mbalavuana is not only the most primitive (least derived)
of the living Tridacna (Lucas et al. 1991), it is apparently
making its last stand so to speak in a quadruple refuge [1)
the southern hemisphere, 2) insular outposts, 3) eastern
limit of three other tridacnine species (H. hippopus, T.
gigas and T. derasa), and 4) inhabiting depths largely
unexploited by them and the more wide-ranging species
(T. maxima and T. squamosa)].
Reliction
"Relict" populations, in contrast to a "relic"
populations (see above), are those separate by some
vicariant event, and for the Tridacninae, the disjunction in
the distribution of T. gigas between western Carolines and
the northern Marshalls is an obvious example (see
Rosewater 1965, Plate 279). However, a more notable
one became evident with the discovery of T. squamosa in
the Pitcairn Islands (Paulay 1989).
Table 3. Families, genera, numbers of species, and representative species of reef corals displaying a disjunction
between the western and eastern portions of the Pacific Plate comparable to that seen in Tridacna squamosa (data from
Veron 2000).
Family
Acroporidae
Pocilloporidae
Siderastreidae
Agariciiade
Fungiidae
Mussidae
Faviidae
Poritidae
Totals
Genera
Montipora
Acropora
Astreopora
Pocillopora
Psammnocora
Coscinaraea
Pavona
Leptoseris
Cycloseris
Lobophyllia
Favia
Goniastrea
Montastrea
Plesiastrea
Leptastrea
Porites
No. Species
5
8
1
1
1
1
1
1
1
1
3
1
1
1
2
2
16
Example
M. grisea
A. listeri
A. myriophthalma
P. damicornis
P. profundacella
C. columna
P. maldivensis
L. mycetoseroides
C. vaughani
L. hemprichii
F. stelligera
G. australensis
M. curta
P. versipora
L. pruinosa
P. lobata
Volume: page (in Veron 2000)
1:94
1:334
1:442
2:26
2:149
2:160
2:192
2:213
2:244
3:44
3:102
3:170
3:216
3:226
3:237
3:284
31
Discovery of the Pitcairns population revealed a
disjunction in the known distribution of this species, from
the Marshall and Cook Islands to the Pitcairns, a hiatus
that includes the Lines Islands and French Polynesia
where T. squamosa must be either absent or extremely
rare (Appendix).
Instructively, this disjunction is not limited to T.
squamosa (see Newman 1986 for examples).
Furthermore, Veron (2000:432) notes such disjunctions
under "Units within species." At least 31 species
representing 8 families and 16 genera of reef corals have
disjunct distributions in this region (Table 3). Granted,
some of these are rare where known to occur, but others
are abundant or common, and still others not included in
the table are found in, as well as on either side of the
disjunction. So, it is not likely to be simply a sampling
artifact. While the cause of this distributional hiatus
remains to be determined, it probably has a bearing on our
understanding of the high endemism found to the east of
the Pacific plate (Newman and Foster 1987, Foster and
Newman 1987) as well as on our appreciation of the
demise of the tridacnines.
Demise of the Tridacninae
In addition to climatic deterioration and the physical
breakup of Tethys accompanying it (Newell 1970), there
have been marked changes in the physiography of reefs,
especially during and following the Pleistocene (Purdy
1974). It has been suggested some tridacnine distributions
reflect past rather than present current patterns due to the
elevation of islands during low stands of the sea (Benzie
and Williams 1997), the latter having been considered
important in latitudinal distributions (Newman 1986).
Furthermore, sea level reached its present extent a few
thousand years ago, following nearly 10,000 years of
relatively rapid rise, lagoons and back-reefs began to fill
and low islands began to build more quickly relative to
sea level. These changes will no doubt continue to alter
the habitats available to shallow-water organism such as
tridacnines.
Climatic and habitat changes of the Cenozoic led to
the restriction of tropical reefs (cf. Newell 1970, Stehli
and Wells 1971), and while physiologically similar by
virtue of their symbiotic zooxanthellae, tridacnines were
even more restricted. Even though some reef corals are
susceptible to relatively small changes in temperature that
accompanied El Niño 1982-83 (Glynn 1988-1991, Glynn
and de Weerdt 1991), one would expect tridacnines to be
more susceptible to peripheral restriction than reefbuilding scleractinians since historically this has been the
case.
It is evident (Appendix) that the distributions of
several contemporary species have been restricted and, as
noted above for T. gigas and squamosa there appear to be
marked disjunctions. While some restriction has been
going on since at least the onset of the Pleistocene, the
activities of man are evident. The medium-range species,
H. hippopus, T. gigas, and T. crocea indicate peripheral
restriction, largely toward the Indo-Malayan center of
distribution. Furthermore, Munro (1999), while not
discounting climatic and natural changes in habitat
availability, notes that "Stocks of the smaller giant clam
species, T. maxima, T. squamosa, T. crocea, and
Hippopus hippopus, are heavily exploited near all
population centers, and H. hippopus appears to have been
extinguished relatively recently in Samoa, Fiji, and
Tonga". Considering the range of H. hippopus and T.
gigas, from as far north as the Ryukus and the northern
Marshalls, their present apparent absence from the
Marianas, and Taiwan (M. Chen pers. com.), is notable.
Populations of giant clams of the western Pacific have
not been exempt from episodic mass deaths, but whether
these events are climatically related is not clear. On the
Great Barrier Reef of Australia, populations of Tridacna
gigas and T. derasa were decimated in the last decade. A
total mortality of over 30% was recorded in 1985
reaching a high of over 50% in 1987 (Alder and Braley
1989). More recently, in 1992 in the Solomon Islands,
large numbers of T. gigas, both adults and juveniles, died
from unknown causes. A small number H. hippopus were
also affected (Gervis 1992). Although some of the
affected clams in the Solomon Islands were being held in
captivity or were hatchery-reared, natural populations
were also affected. It is not unusual to have mass
mortalities of larval or juvenile clams under culture
conditions. However, when adult clams held in captivity
die off in large numbers, the phenomenon is no easier to
explain than the events that occurred on the Great Barrier
Reef or in the Solomon Islands.
Is all this an indication of a natural fragility of the
giant clam or are contemporary environmental conditions
changing so rapidly as to increase mass mortalities? The
species are currently under stress, and the impact of man
has been well documented (Govan et al. 1988, Juinio et
al. 1989, Sims and Howard 1988, Taniera 1988, Zann and
Ayling 1988). Since extensive surveys have revealed
only a few juveniles of T. gigas in the Philippines, it
appears to be on the verge of extinction there (Gomez and
Alcala 1988). The most important cause for these
reductions has been fishing pressure.
Recently, a
relationship between exploitation and reduction in the
sizes of individuals has been shown by Planes et al.
(1993). They demonstrate that tourist-related fishing of T.
maxima in French Polynesia resulted in the reduction of
the mean size of individuals in the population. Likewise,
Gomez et al. (1994) provide evidence that continued
exploitation of T. crocea is reducing the mean size of this
species. However, the species is plentiful where it occurs,
and for this reason it was removed from the IUCN Red
List (IUCN 1996).
Other anthropogenic activities may be altering the
environment of some clam populations. Acceleration of
natural sedimentation rates, due to farming and other
activities, is changing coastal environments. A related
factor, elevated nutrient loads, have been found to
promote the growth rate of the clams, but shell density is
reduced, rendering the clams more vulnerable to damage
under natural conditions. Nonetheless, elevated nutrient
levels may be of value under culture conditions (Belda et
al. 1993).
Management policies
Since tridacnines appear to be widely declining, due to
over-exploitation and other anthropogenic impacts as well
as natural processes, should man intervene and attempt to
reverse the trend? A recent regional project had the
restocking of depleted reefs as one of its goals (Copland
and Lucas 1988), but the results have yet to be
ascertained. Furthermore, T. derasa has been introduced
to the Cook Islands (Sims and Howard 1988), and the
same species has been reintroduced to Yap State in
Micronesia (Price and Togolimul 1988). This has
stimulated a vigorous debate on the merits of
introductions, the concern being the inadvertent
introduction of exotic parasites, diseases and other biota,
as the giant clams themselves are not perceived as
potential pests.
After an initial importation of juvenile T. gigas from
Australia and T. derasa from Palau, the Philippines opted
to import only larvae and newly settled spat raised under
aseptic conditions. Imported clams have been quarantined
at land-based nurseries before transfer to the ocean. It is
anticipated that breeding aggregations established at
strategic locations will stimulate the natural recruitment
process. Since the report of Mingoa-Licuanan (1993),
several marine reserves have been identified and
substantial numbers of clams have been deployed in two
of them, Hundred Islands National Park and Puerto
Galera (Man and the Biosphere, Unesco Reserve
Programme). As T. gigas may take more than a decade to
reach sexual maturity, the outcome of the transplants
remains to be seen. However, individuals of a more
rapidly maturing species, T. derasa, were deployed with
them, and the extent of their success should soon be
evident. Densities as high as those discovered in a natural
population by Sirenko (1991) are possible.
With today's fervor over endangered species and the
preservation of biodiversity, it has been relatively easy to
enact legislation to limit fisheries for giant clams. In
virtually all countries having natural stocks, the harvest of
clams is now coming under management and all the
species known prior to 1990 now fall under the
Convention on the International Trade in Endangered
Species (CITES). However, while poaching is on the
decline, it will not likely be completely stopped. On the
other hand, the farming of giant clams has become more
attractive, especially for the aquarium trade. With the
successful culture and sale of various species, an
unplanned expansion in biogeographic ranges could occur
should some hobbyists weary of their pets and set them
free on appropriate reefs.
Acknowledgments We thank R. Cowei of the BP Bishop
Museum for the loan of the type specimen of Tridacna
mbalavuana and, together with W. Emerson of the
American Museum of Natural History and J. Harasewych
of the U.S. National Museum of Natural History, for help
with the fruitless search for the original source and/or
specimen of H. hippopus from French Frigate Shoals; J.
Harasewych for digital photos of a specimen of T.
squamosa from the Pitcairn Islands; R. Richmond for
sounding out colleagues in Guam on the fossil and recent
status of the tridacnines there; M. Grygier for excerpts
from his discussions on the status of Japanese tridacnines
with K. Yamazato and M. Murakoshi and for sending
relevant translations from Kubo and Kurozumi (1995),
TJH Adams for general information; M. Chen for
information on fossil and extant species in Taiwan, and P
Skelton for discussions on Fijian, Tongan and Samoan
forms. And lastly, thanks are due 1) G. Paulay for
alerting us to the distributional enigma involving T.
squamosa and for spirited discussions on points of
disagreement; and 2) two referees for many helpful
comments and criticisms.
APPENDIX Distributional data and comments on species
of Tridacninae arbitrarily grouped according to range;
long, medium and short:
Long-range species
The two long-range species, 1) T. maxima and 2) T.
squamosa, tend to inhabit shallow and deep-water
respectively but the former can be found with the latter (G
Paulay, pers. com.). However, while they have very
similar geographical ranges, from the Red Sea to Pacific
Oceania, there is an apparent previously unrecognized
longitudinal disjunction in the distribution of T. squamosa
in the central Pacific (see section on Reliction for
discussion).
1) Tridacna (Chametrachea) maxima (Röding 1798):
Throughout the Indo-West Pacific where reef building
occurs (except for the Hawaiian Archipelago), from the
Gulfs of Suez, Aqaba, and Persia south to Durban,
spreading east to southern Japan, Australia and Lord
Howe Is., and further east to Pitcairn Is. (Paulay 1989),
but not reaching Easter, Johnston (R. McConnaughey,
pers. com.) or the Hawaiian Islands. Occurs in lagoons
and on seaward reefs to depths of 10 meters or so,
byssally attached among corals and rubble into which it
can burrow, sometimes on sandy bottoms. Largest
recorded specimen 417 mm (Fanning Is., Stasek 1965).
Benzie and William (1997), Ayala et al. (1975) and
Campbell et al. (1975), studied genetic variation in T.
maxima. Ayala et al. (1975) assumed that since the genus
had been restricted to the Indo-West Pacific, where it was
living in a relatively stable environment, its species would
have little genetic variation and therefore would be
relatively vulnerable to extinction, "... a plausible analog
of the sorts of fossil species that have commonly become
extinct." As it turned out, T. maxima was "... one of the
genetically most polymorphic organisms studied so far",
and therefore their results did not "... support the
hypothesis that some massive extinctions... may have
been due to the scarcity of genetic variation in
populations adapted to stable environments" (for more
contrary evidence, see Schopf and Gooch 1972). In their
study of T. gigas, Benzie and Williams (1997) not only
corroborated the Ayala et al. (1975) findings re T.
maxima, as far as local heterozygosity was concerned,
they found its widely separated populations lack
significant genetic differentiation. So, T. maxima, the
most wide ranging species, must at least until relatively
recently had good planktonic dispersal capabilities. It
would be instructive to know what a comparable genetic
study of the short-range species, T. mbalavuana, would
reveal.
Tridacna maxima is morphologically similar to T.
crocea whose habitat it exploits on Niue, Takelau and the
Cooks Islands (T J H Adams, pers. com.), where T.
crocea is absent. Whether this shift in habitat preference
is simply due to availability or to a genetic difference is
unknown, but apparently there is presently little gene flow
between the Pacific Plate (Central Pacific) populations
and those to the west of the plate boundary (Benzie 1993a
and b). Fossil occurrences all fall within the present range
of the species and include the Upper Miocene of Java,
Neogene of Palau, Plio-Pleistocene of Guam and the
Pleistocene of East Africa, Saipan and Tonga; no local
extinctions have been noted.
2) Tridacna (Chametrachea) squamosa Lamarck, 1819:
Red Sea (uncommon compared to T. maxima at Elat, Y.
Achituv pers. com.), south nearly to Durban, South
Africa, east to Taiwan and southern Japan (Kaiki Sima),
Guam and the Marshall Islands, Australia, New Caledonia
and Cook Island (Paulay 1987) where it is rarely found on
outer reef slopes (Sims and Howard 1988), and the
Pitcairn Islands (Paulay 1989), a substantial range
extension. What may have been a single individual was
seen on Kingman Reef, Line Islands (R Brainard, J
Maragos and D. Minton, pers. comms.), but none are
known from the remainder of the Line Islands, nor from
French Polynesia (B. Richer de Forges and P. Laboute,
and B. Salvat pers. comms.). Saquet (1992), who makes
no pretense of being comprehensive regarding the marine
invertebrates of French Polynesia, notes such recent
findings as the Easter Island lobster, Panulirus
pascuensis, in deep waters there. Yet only one species of
giant clam is mentioned (from shallow water and
therefore T. maxima). Thus, the disjunction for T.
squamosa sounds real. Usually in protected localities on
surface of corals to 18 m of depth, deeper on outer reef
slopes, does not burrow, large adults often unattached, to
40 cm in length.
Fossil record includes the Plio-Pleistocene of Kita
Daito Jima, Celebes, Pleistocene of E. Africa, Tonga, and
the ?Tuamotus (the last being in the disjunction discussed
in the text).
No local extinctions recorded, but
uncommon in the New Hebrides (Vanuatu; absent on 9 of
13 islands surveyed perhaps due to limited sheltered
situations, Zann and Ayling 1988).
Medium-range species
While 3) Tridacna derasa, 4) Hippopus hippopus, 5)
T. gigas, and 6) T. crocea range as far west as Sumatra
(the first to Cocos-Keeling), pronounced differences in
their longitudinal ranges occur in the east:
3) Tridacna (Tridacna) derasa (Röding, 1798): Southern
Sumatra (Pasaribu 1988) and Cocos-Keeling, rare in
Solomon Islands (Govan et al. 1988), rare or absent in the
New Hebrides (Vanuatu, Zann and Ayling 1988); no
records in Taiwan (M Chen pers. com.) but occasionally
present in southern Japan [Sakisima Guntô; more northern
record from Muroto Saki, Japan unreliable (M Grygier
pers. com.)], south to Australia and Lord Howe Is., and
east to Tonga (where none found in survey of 1987, Langi
and Aloua 1988); introduced to the Cook Is. in 1986
(Sims and Howard 1988). Lagoons and outer reef slopes
to 20 m, unattached as adults, up to 60 cm in length.
While Macaranas et al. (1992) found no significant
genetic differences among populations on the Great
Barrier Reef, they found large genetic distances between
populations from the Great Barrier Reef, Fiji and the
Philippines. Tridacna derasa is morphologically closest
to T. mbalavuana (Lucas et al. 1991) and genetically to T.
gigas, and together these three species constitute Tridacna
s.s. (Schneider and Ó Foighil 1999).
Susceptible to over-fishing, over-fished on Palau and
now farmed, attempted re-introductions to Yap in 1984
(Price and Fagolimul 1988) and to the Philippines where
it was virtually extinct (Gomez and Alcala 1988).
Apparently no fossil record.
4) Hippopus hippopus (Linn., 1758): Sumatra to Pratas Is.
but not Taiwan (M. Chen pers. com.), Ryukus (Amami-Ô
Shima, M. Grygier pers. com.), Caroline and Marshall but
not Mariana Is. and south from Australia, New Caledonia,
east to Tonga; on reefs and sandy substrates including
harbors, to 10 m of depth, unattached as adults, up to 45
cm in length.
Fossil record includes the Miocene of Saipan,
Neogene of Palau, Plio-Pleistocene and Holocene of
Guam (R Richmond pers. com.) and the L. Pleistocene of
Fiji where fossils are common (T J H Adams pers. com.).
While no longer present in the Marianas, one living
specimen is reported from a commercial port on Guam
and a single shell from Hawaii, but it is uncertain whether
these were waifs or artifacts (see below). The Holocene
records on Guam could have been contemporaneous with
man (R Richmond, pers. com.), no specimens taken on
Tonga although one may have been seen in the 1978-79
survey (Langi dan Aloua 1988), found in coastal middens
but extinct on Fiji (Lewis et al. 1988), protection
recommended in New Hebrides (Vanuatu, Zann and
Ayling 1988).
The living specimen of Hippopus hippopus noted
above from Guam was reputed to have been thrown
overboard from a ship (R Richmond, pers. com.).
However, considering that H. hippopus 1) is known to
have ranged as far north as Saipan in the Miocene and
was in Guam as recently as the Holocene, 2) presently
ranges east into the northern Marshalls and northwest into
the southern Ryukus, 3) commonly occurs in harbors, 4)
is consumed as food when fresh, and 5) does not survive
long out of water, it seems more likely this living
specimen was a waif than an artifact.
Rosewater (1965) reports a single valve of Hippopus
from Tern Island, French Frigate Shoal, but we have been
unable to locate the specimen. It has been suggested that
it may have been discarded there by service personnel (A
Kay, pers. com.), or dredged up when the island was
enlarged (R Grigg, pers. com.), both during World War II.
There is no other record of other tridacnines in the
Hawaiian Archipelago, but if there were, T. maxima
and/or squamosa would seem the more likely candidates.
Thus, while a waif of Hippopus, like waifs of three
species of Acropora (Grigg 1981), might occasionally get
to the Hawaiian Islands, it seems more likely an artifact.
5) Tridacna (Tridacna) gigas (Linn., 1758): Sumatra to
southernmost Ryukyus (M. Grygier, pers. com.) and
northern Marshalls but not Mariana Is. (R. Richmond,
pers. com.) or Taiwan (M. Chen pers. com.), south to
New Caledonia and east to Fiji (not present and no fossil
record on Tonga, Langi and Aloua 1988). On reefs to
about 15 m of depth, does not burrow, adults unattached,
lengths to 1.37 m and shells to over 230 kg (500 kg total
weight likely an exaggeration, Rosewater 1965). Benzie
and Williams (1992) found high heterozygosity within
populations of T. gigas and a lack of variation between 6
widely separated populations on the Great Barrier Reef,
so there appears to be gene flow despite apparent disjunctions (Benzei 1993).
Fossil record includes the Miocene of Java and
Plio-Pleistocene of Guam. Recent extinctions include the
Philippines (Gomez and Alcala 1988; reintroduction in
progress, EDG), and Fiji by 1970 (Langi and Aloua
1988). Govan (1983, appendix) indicates T. gigas is
extinct on Yap, Woleai, Ponape, Truk and Fiji (fossils
never seen in Fiji, T J H Adams pers. com.), extinct in
Taiwan (M. Chen pers. com.). Noted as very rare or
absent from New Hebrides where it has been reintroduced
(Zann and Alying 1988), and Pasaribu (1988, citing Usher
1984) states it has been eliminated from western
Indonesia.
6) Tridacna (Chametrachea) crocea Lamarck, 1819:
Sumatra to Taiwan and the Ryukyus (Takara Sima, M.
Grygier pers. com.) and the southern Great Barrier Reef,
as far east as Palau and Yap north of the equator and the
Solomon Islands south of the equator; not likely in the
New Hebrides and not in Fiji (Lewis et al. 1988:67),
possible sighting on Palmyra (R. Brainard, J Maragos and
D Minton, pers. com.). Deep burrower in corals and
rubble in shallow water, attached for life; smallest species
of the subfamily, up to 15 cm in length. Morphologically
most similar to T. maxima and genetically similar to both
T. maxima and T. squamosa (Schneider and Ó Foighil
1999). Fossil record includes the Pleistocene of
Enewetak, well east of its documented range.
Short-range species
7) Hippopus porcellanus Rosewater, 1982: Restricted to
central equatorial waters; Philippines (mostly from Sulu
archipelago, southern Sulu Sea, one specimen from
Masbate Is., Gomez and Alcala 1988), Palau (Heslinga
1993), Celebes and West Irian, New Guinea (Lucas 1988,
Fig. 7), and all of Indonesia (Pasaribu 1988, citing
Romimohtarto et al. 1987). Coral rock and sand, not
attached as adults, sympatric with and same size as H.
hippopus, no habitat and/or depth partitioning reported.
No fossil record. Recently virtually extinct in the
Philippines, due to shell trade stimulated by the newly
discovered species (Gomez and Alcala 1988).
8) Tridacna (Tridacna) mbalavuana Ladd, 1934 (p. 185
and pl. 31 2-3) [=T. (Persikima) tevoroa Lucas et al.
1991, 1992; see Taxonomic Considerations for discussion]. The synonymy is as follows:
Tridacna mbalavuana Ladd, 1934:185.
T. (Chametrachea) mbalavuana, Rosewater 1965:379.
Tridacna sp. Lewis et al.. 1988:67 ("tevoro", devil clam
in Fijian) (Lewis and Ledua 1988:82, who credit A.
Robinson of Suva for calling it to their attention, first to
recognized it might be distinct from T. derasa).
T. (Persikima) tevoroa Lucas, Ledua and Braley, 1991:94
Ledua et al. 1993: 147 and 151. Tongan language;
"vasuva ngesi manifi" or "vasuva ngesi sio ata" (thin shell
clam or window pane shell clam).
T. tevoroa Lucas, Ledu and Braley, 1990, includes T.
mbulvuana (sic), CITES 1999: 20
T. (Tridacna) tevoroa, Schneider and Ó Foighil 1999:62.
not T. squamosa, Schneider and Ó Foighil 1999: 64
Table 2.
Tridacna mbalavuana, apparently long known to Fijians
as "tevoro", the devil clam, was first described by Ladd
(1934) from the Upper Tertiary (Neogene) of Vitilevu,
Fiji. In recent years the deep-living populations of this
species caught the attention of reef biologists in Fiji
where, curiously, it is reported as very rare today (T. J. H.
Adams pers. com.). Unaware that the "tevoro" had been
previously described as a fossil (Ladd 1934), Lucas et al.
(1990) proposed the name, Tridacna tevoroa. Its
recognition as a "living fossil" herein substantiates
Schneider and Ó Foighil's (1999) suggestion this species
occurred in the Neogene. Scattered individuals of what
was thought might be this species were observed on
Holmes Reef, 120 nautical miles east of Cairns, Australia
(Lucas pers. com.). Generally found on outer reef slopes
from 20-30 m of depth, deepest living species and only
Tridacna without hyaline organs, to 53 cm in length.
9) Tridacna rosewateri Sirenko and Scarlato, 1991: Saya
de Malha Bank (9o47'S, 61o25'E), Massacring Plateau,
Western Indian Ocean, from 12-13 m of depth.
Considered closely related to if not an ecotype of T.
squamosa (see Taxonomic Considerations for discussion).
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