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Home , Giant clam, Hippopus, Hippopus hippopus, Hippopus porcellanus, Tridacna, Tridacna crocea

Proceedings 9th International Reef Symposium, Bali, 23-27 October 2000, Vol. 2.

On the status of giant , relics of Tethys (: : )

W. A. Newman1 and E. D. Gomez2 ABSTRACT The giant clams, subfamily Tridacninae, include two extant genera, and , represented by three and twelve extinct and two and seven extant species, respectively. The extant species are presently restricted to the Indo- West 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, The distribution of extinct and extant Tridacninae Chametrachea, Endemism, Reliction, Human impact, (Rosewater 1965, Cox et al. 1969, Lucas 1988 and herein) Conservation shows this subfamily underwent a marked decline during the breakup of Tethys. Like a number of other relic Introduction family-group taxa (cf. Houbrick 1984a-c for some extreme examples), it is now contributing to the If there were no fossil record, it would appear the remarkably high marine endemism of the Southwest tridacnines evolved in the Indo-Malayan region and the Pacific and (Newman 1991). This is in contrast species radiated to varying degrees throughout the to many other reef organisms at the family-group level, Indo-West Pacific. However, from the fossil record such as many reef (Veron 2000), and coral (Table 1), we know this is not the case. Thus, while barnacles (Ross and Newman, this symposium) which knowledge of the recent history of the species is very have undergone dramatic radiations during the same important, understanding the broader historical aspects period. can add significantly to our appreciation of their present demise.

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.

IWP WI Europe E/E †Goniocardium M. Eoc.-U. Eoc. - - † †Avicularius M. Eoc.-L. Oligo. - † † †Byssocardium M. Eoc.-L. Mio. - - † Hippopus Mio.-Recent + † - 3/2 Tridacna(Tridacna) L. Mio.-Recent + - † 4/3 T. (Chametrachea) Mio.-Recent + - † 8/4

The Tridacninae appeared in the Paleogene when a high latitudes beginning in the Oligocene, and warming of number of genera were to be found in Western Tethys the tropics particularly in the Miocene, followed by the (Table 1). However, most of these became extinct before perturbations of the Pleistocene (Shackleton 1984, the Neogene, apparently due to climatic change Valentine 1984, Newman 1986, Stanley 1986, Paulay concomitant with the breakup of the Tethys Sea. These 1996). changes included restriction of the tropics by cooling of

1 The Scripps Institution of Oceanography, La Jolla, CA 92993-0202,USA. email: [email protected] 2 The Marine Science Institute, University of the , Quezon City, The Philippines

Of the 15 extinct species recorded for the only normalize the data between species as well as individuals surviving genera, Hippopus and Tridacna, nearly half and thus afford a ready means of comparison. To went extinct in the Neogene and the survivors were facilitate such comparisons we calculated their means further restricted, at least peripherally, up into the (Table 2). The mean V.m/H.l ratios for these three Holocene. Since the process is apparently being populations, being closest, are the least informative. intensified by human activities, largely due to exploitation Those for T. rosewateri are intermediate between the for meat and shell, some management policies have been other two, there is a slight overlap between those for proposed (Gomez and Alcala 1988). But before rosewateri and squamosa, and the difference between discussing these, some taxonomic considerations need them is small, so their significance is low. Nonetheless, attention. as we shall see, this is the only one of the four sets of means in which rosewateri is more similar to squamosa Taxonomic considerations than it is to maxima!

We follow Schneider and Ó Foighil (1999) in Table 2. Mean ratios for ratios on shell measurements for considering the subgenus Persikima Iredale, 1937 a junior three species of Tridacna (Chametrachea), calculated synonym of Tridacna s.s., and the taxonomic status of the from ratios of Sirenko and Scarlato (1991:7). B.o = recently described species, T. mbalavuana and T. length of byssal orifice, H.l = length of hinge line, I.p = rosewateri, is discussed below. height of interdigital projections, L = length of shell, W = 1) Tridacna (Tridacna) mbalavuana Ladd, 1934: We width without scales, V.m = length of ventral margin. borrowed the fossil type material of this Fijian species W/I. V.m/ V.m/ L/W from the Bernice P. Bishop Museum, compared it to the p B.o H.l valves of extant T. tevoroa Lucas et al. 1991 of T.rosewateri (n=9) 3.0 1.5 2.3 1.2 comparable size from Tonga, and they are virtually T.squamosa (n=4) 2.4 3.1 3.8 1.0 identical. Therefore we consider the two species synonymous (see Appendix for synonymy). While Ladd T. maxima (n=8) 2.4 3.3 1.8 1.5 (1934) correctly determined the anterior-posterior axis of T. mbalavuana, whereby the byssal gape is anterior In the second set (V.m/B.o, Table 2) there is a slight (Stasek 1965, Lucas et al. 1991), Rosewater (1965:380) overlap between T. rosewateri and T. maxima, and both reversed it, and Norton and Jones (1992, key, dichotomy differ appreciably from squamosa. Thus T. rosewateri 5a) did likewise. appears more similar to T. maxima than T. squamosa by Lewis and Ledua (1988) and Lucas et al. (1990, 1991) this character, but this alone is of no great weight. recognized close affinities between T. mbalavuana, and In the remaining two sets (L/W and W/I.p), the means the only other species then assigned to Persikima, T. for rosewateri and maxima are not only virtually identical derasa, Lucas et al. (1990) used the radial sculpture in the but are quite distinct from that for T. squamosa. By these former as one of the several characters distinguishing two ratios as well as the previous one, rosewateri is more them. However, the genetic analysis of Schneider and Ó similar to maxima than it is to squamosa. By these ratios Foighil (1999) did not provide a genetic basis for then, in addition to the characters given by Sirenko and distinguishing Persikima and Tridacna s.s., and therefore Scarlato (1991), T. rosewateri appears to be distinct from they made Persikima a subjective junior synonym of T. squamosa. Therefore it seems to represent a good Tridacna s.s. While we follow the latter authors in species. Nonetheless, a genetic comparison between placing T. mbalavuana in the subgenus Tridacna, how rosewateri and squamosa could be instructive, and there this will be accepted remains to be seen. is a comparable problem with a population attributed to 2) Tridacna (Chametrachea) rosewateri Sirenko & squamosa near the eastern extend of its range that also Scarlato, 1991: These authors compare their new species appears worthy of attention (see Reliction below). to the other two species known from the region, wide-ranging T. squamosa and T. maxima which are in Temporal and spatial distributions the same subgenus (Chametrachea). They note T. The principal sources on spatial and temporal rosewateri differs from the former in having 1) a thinner distributions for tridacnines are Rosewater (1965, 1982), shell, 2) larger byssal orifice, 3) scales more densely Cox et al. (1969), Copland and Lucas (1988), and Lucas arranged on the primary ribs, and 4) larger interdigitating et al. (1991). Despite there being relatively few, occludent projections. Its soft parts are unknown, as are conspicuous species, range limits and patchiness need to its burrowing capabilities, but since the authors can be better documented if we as well as posterity are to readily distinguish its shells from those of T. squamosa, recognize changes that are likely to continue to occur. they concluded it is a distinct species. Therefore these published records are supplemented here Sirenko and Scarlato (1991:7) give a table of shell with data from more specialized papers and with personal measurements for Tridacna rosewateri, as well as for communications from a number of workers (see also some specimens of T. squamosa and T. maxima, the only Appendix & Acknowledgments). These data are other tridacnines known from the western Indian . summarized in Appendix where we arbitrarily list the Their table includes ratios for various measurements living species in three groups according to geographic (L/W, W/I.p, V.m/B.o, and V.m/H.l). Such ratios range rather than by their taxonomic status or relative

size; 1) two long-range species (Tridacna maxima and T. e.g., was regressive. Therefore the fossil record for T. squamosa, 2) four medium-range species (Hippopus mbalavuana not only corroborates the relic hypothesis of hippopus, T. gigas, T. derasa, and T. crocea), and 3) three Lewis and Ledua (1988), but the genetic evidence which short-range species (H. porcellanus, T. mbalavuana, and suggested it should be at least early Neogene in age T. rosewateri). (Schneider and Ó Foighil 1999). The Tridacninae was originally Tethyan (Table 1). As noted in the introduction, similar relic patterns are All three extinct genera attributed to the subfamily known for a number of formerly Tethyan groups that now appeared in the Eocene, were apparently limited to the have their centers of distribution in the West Pacific, tropical Atlantic, and all but one apparently died out particularly the Southwest Pacific (Fleming 1979, before the Miocene. An Upper Cretaceous record for a Newman 1991). In addition, it is noteworthy that T. Tridacna from Madagascar (Rosewater 1965, Cox et al. mbalavuana is found at the eastern limit of its larger 1969) seemed doubtful, and it has since been concluded subcongeners, T. derasa and T. gigas (the latter not being that it is more likely Neogene (Schneider 1998:352). We known to occur east of Fiji; Lewis and Ledua 1988). concur with this conclusion, as we do with Taviani's Tridacna mbalavuana is most similar to T. derasa, but (1994) rejection of the Pliocene record for Tridacna in these two species for the most part segregate by depth, the Italy. Of the two extant genera, Hippopus, known from former below and the latter above the 20 m line (Lucas et the Early Miocene of the West Indies and the Marianas al. 1991). Of all the species in the region, only T. maxima Islands, is considered morphologically more primitive and squamosa, which range all the way to the Red Sea, (less derived) than Tridacna (Upper Miocene). This is are presently known to range further east. Thus T. consistent with the fossil record and recent morphological mbalavuana is not only the most primitive (least derived) and molecular phylogenetic analyses (Schneider 1998, of the living Tridacna (Lucas et al. 1991), it is apparently Schneider and Ó Foighil 1999). Schneider and Ó Foighil making its last stand so to speak in a quadruple refuge [1) (1999, Fig. 3) combine their inferred phylogeny and the the southern hemisphere, 2) insular outposts, 3) eastern stratigraphic record of extant species in an instructive limit of three other tridacnine species (H. hippopus, T. figure. gigas and T. derasa), and 4) inhabiting depths largely unexploited by them and the more wide-ranging species Relic hypothesis (T. maxima and T. squamosa)].

"Relic" populations are survivors of ancient radiations Reliction having no living ancestors, in contrast to "relicts" or populations isolated from parent populations. In light of "Relict" populations, in contrast to a "relic" the geographical distribution of the fossils, there is no populations (see above), are those separate by some question the Tridacninae is a relic subfamily. vicariant event, and for the Tridacninae, the disjunction in Furthermore, T. mbalavuana shares some characters, such the distribution of T. gigas between western Carolines and as the absence of hyaline organs, a pedal gape and mantle the northern Marshalls is an obvious example (see projections, with the more generalized tridacnines, Rosewater 1965, Plate 279). However, a more notable Hippopus, and therefore appears relatively primitive (less one became evident with the discovery of T. squamosa in derived) (Lucas et al. 1991). However, without a fossil the Pitcairn Islands (Paulay 1989). record, it could be argued it had lost advanced features; 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 (data from Veron 2000). Family Genera No. Species Example Volume: page (in Veron 2000) Acroporidae Montipora 5 M. grisea 1:94 Acropora 8 A. listeri 1:334 Astreopora 1 A. myriophthalma 1:442 Pocilloporidae Pocillopora 1 P. damicornis 2:26 Siderastreidae Psammnocora 1 P. profundacella 2:149 Coscinaraea 1 C. columna 2:160 Agariciiade Pavona 1 P. maldivensis 2:192 Leptoseris 1L. mycetoseroides 2:213 Fungiidae Cycloseris 1 C. vaughani 2:244 Mussidae Lobophyllia 1 L. hemprichii 3:44 Faviidae Favia 3 F. stelligera 3:102 Goniastrea 1 G. australensis 3:170 Montastrea 1 M. curta 3:216 Plesiastrea 1 P. versipora 3:226 Leptastrea 2 L. pruinosa 3:237 Poritidae Porites 2 P. lobata 3:284 Totals 16 31

restriction, largely toward the Indo-Malayan center of Discovery of the Pitcairns population revealed a distribution. Furthermore, Munro (1999), while not disjunction in the known distribution of this species, from discounting climatic and natural changes in the Marshall and Cook Islands to the Pitcairns, a hiatus availability, notes that "Stocks of the smaller giant that includes the Lines Islands and French Polynesia species, T. maxima, T. squamosa, T. crocea, and where T. squamosa must be either absent or extremely , are heavily exploited near all rare (Appendix). population centers, and H. hippopus appears to have been Instructively, this disjunction is not limited to T. extinguished relatively recently in Samoa, Fiji, and squamosa (see Newman 1986 for examples). Tonga". Considering the range of H. hippopus and T. Furthermore, Veron (2000:432) notes such disjunctions gigas, from as far north as the Ryukus and the northern under "Units within species." At least 31 species Marshalls, their present apparent absence from the representing 8 families and 16 genera of reef corals have Marianas, and Taiwan (M. Chen pers. com.), is notable. disjunct distributions in this region (Table 3). Granted, Populations of giant clams of the western Pacific have some of these are rare where known to occur, but others not been exempt from episodic mass deaths, but whether are abundant or common, and still others not included in these events are climatically related is not clear. On the the table are found in, as well as on either side of the of Australia, populations of Tridacna disjunction. So, it is not likely to be simply a sampling gigas and T. derasa were decimated in the last decade. A artifact. While the cause of this distributional hiatus total mortality of over 30% was recorded in 1985 remains to be determined, it probably has a bearing on our reaching a high of over 50% in 1987 (Alder and Braley understanding of the high endemism found to the east of 1989). More recently, in 1992 in the , the Pacific plate (Newman and Foster 1987, Foster and large numbers of T. gigas, both adults and juveniles, died Newman 1987) as well as on our appreciation of the from unknown causes. A small number H. hippopus were demise of the tridacnines. also affected (Gervis 1992). Although some of the

affected clams in the Solomon Islands were being held in Demise of the Tridacninae captivity or were hatchery-reared, natural populations In addition to climatic deterioration and the physical were also affected. It is not unusual to have mass breakup of Tethys accompanying it (Newell 1970), there mortalities of larval or juvenile clams under culture have been marked changes in the physiography of reefs, conditions. However, when adult clams held in captivity especially during and following the Pleistocene (Purdy die off in large numbers, the phenomenon is no easier to 1974). It has been suggested some tridacnine distributions explain than the events that occurred on the Great Barrier reflect past rather than present current patterns due to the Reef or in the Solomon Islands. elevation of islands during low stands of the sea (Benzie Is all this an indication of a natural fragility of the and Williams 1997), the latter having been considered or are contemporary environmental conditions important in latitudinal distributions (Newman 1986). changing so rapidly as to increase mass mortalities? The Furthermore, sea level reached its present extent a few species are currently under stress, and the impact of man thousand years ago, following nearly 10,000 years of has been well documented (Govan et al. 1988, Juinio et relatively rapid rise, and back-reefs began to fill al. 1989, Sims and Howard 1988, Taniera 1988, Zann and and low islands began to build more quickly relative to Ayling 1988). Since extensive surveys have revealed sea level. These changes will no doubt continue to alter only a few juveniles of T. gigas in the Philippines, it the available to shallow-water organism such as appears to be on the verge of extinction there (Gomez and tridacnines. Alcala 1988). The most important cause for these Climatic and habitat changes of the Cenozoic led to reductions has been fishing pressure. Recently, a the restriction of tropical reefs (cf. Newell 1970, Stehli relationship between exploitation and reduction in the and Wells 1971), and while physiologically similar by sizes of individuals has been shown by Planes et al. virtue of their symbiotic , tridacnines were (1993). They demonstrate that tourist-related fishing of T. even more restricted. Even though some reef corals are maxima in French Polynesia resulted in the reduction of susceptible to relatively small changes in temperature that the mean size of individuals in the population. Likewise, accompanied El Niño 1982-83 (Glynn 1988-1991, Glynn Gomez et al. (1994) provide evidence that continued and de Weerdt 1991), one would expect tridacnines to be exploitation of T. crocea is reducing the mean size of this more susceptible to peripheral restriction than reef- species. However, the species is plentiful where it occurs, building scleractinians since historically this has been the and for this reason it was removed from the IUCN Red case. List (IUCN 1996). It is evident (Appendix) that the distributions of Other anthropogenic activities may be altering the several contemporary species have been restricted and, as environment of some clam populations. Acceleration of noted above for T. gigas and squamosa there appear to be natural sedimentation rates, due to farming and other marked disjunctions. While some restriction has been activities, is changing coastal environments. A related going on since at least the onset of the Pleistocene, the factor, elevated nutrient loads, have been found to activities of man are evident. The medium-range species, promote the growth rate of the clams, but shell density is H. hippopus, T. gigas, and T. crocea indicate peripheral reduced, rendering the clams more vulnerable to damage

under natural conditions. Nonetheless, elevated nutrient with the fruitless search for the original source and/or levels may be of value under culture conditions (Belda et specimen of H. hippopus from French Frigate Shoals; J. al. 1993). Harasewych for digital photos of a specimen of T. squamosa from the Pitcairn Islands; R. Richmond for Management policies sounding out colleagues in on the fossil and recent status of the tridacnines there; M. Grygier for excerpts Since tridacnines appear to be widely declining, due to from his discussions on the status of Japanese tridacnines over-exploitation and other anthropogenic impacts as well with K. Yamazato and M. Murakoshi and for sending as natural processes, should man intervene and attempt to relevant translations from Kubo and Kurozumi (1995), reverse the trend? A recent regional project had the TJH Adams for general information; M. Chen for restocking of depleted reefs as one of its goals (Copland information on fossil and extant species in Taiwan, and P and Lucas 1988), but the results have yet to be Skelton for discussions on Fijian, Tongan and Samoan ascertained. Furthermore, T. derasa has been introduced forms. And lastly, thanks are due 1) G. Paulay for to the Cook Islands (Sims and Howard 1988), and the alerting us to the distributional enigma involving T. same species has been reintroduced to Yap State in squamosa and for spirited discussions on points of Micronesia (Price and Togolimul 1988). This has disagreement; and 2) two referees for many helpful stimulated a vigorous debate on the merits of comments and criticisms. introductions, the concern being the inadvertent introduction of exotic parasites, diseases and other biota, APPENDIX Distributional data and comments on species as the giant clams themselves are not perceived as of Tridacninae arbitrarily grouped according to range; potential pests. long, medium and short: After an initial importation of juvenile T. gigas from

Australia and T. derasa from , the Philippines opted Long-range species to import only larvae and newly settled spat raised under aseptic conditions. Imported clams have been quarantined The two long-range species, 1) T. maxima and 2) T. at land-based nurseries before transfer to the ocean. It is squamosa, tend to inhabit shallow and deep-water anticipated that breeding aggregations established at respectively but the former can be found with the latter (G strategic locations will stimulate the natural recruitment Paulay, pers. com.). However, while they have very process. Since the report of Mingoa-Licuanan (1993), similar geographical ranges, from the Red Sea to Pacific several marine reserves have been identified and Oceania, there is an apparent previously unrecognized substantial numbers of clams have been deployed in two longitudinal disjunction in the distribution of T. squamosa of them, Hundred Islands National Park and Puerto in the central Pacific (see section on Reliction for Galera (Man and the Biosphere, Unesco Reserve discussion). Programme). As T. gigas may take more than a decade to reach sexual maturity, the outcome of the transplants 1) Tridacna (Chametrachea) maxima (Röding 1798): remains to be seen. However, individuals of a more Throughout the Indo-West Pacific where reef building rapidly maturing species, T. derasa, were deployed with occurs (except for the Hawaiian Archipelago), from the them, and the extent of their success should soon be Gulfs of Suez, Aqaba, and Persia south to Durban, evident. Densities as high as those discovered in a natural spreading east to southern Japan, Australia and Lord population by Sirenko (1991) are possible. Howe Is., and further east to Pitcairn Is. (Paulay 1989), With today's fervor over endangered species and the but not reaching Easter, Johnston (R. McConnaughey, preservation of biodiversity, it has been relatively easy to pers. com.) or the Hawaiian Islands. Occurs in lagoons enact legislation to limit fisheries for giant clams. In and on seaward reefs to depths of 10 meters or so, virtually all countries having natural stocks, the harvest of byssally attached among corals and rubble into which it clams is now coming under management and all the can burrow, sometimes on sandy bottoms. Largest species known prior to 1990 now fall under the recorded specimen 417 mm (Fanning Is., Stasek 1965). Convention on the International Trade in Endangered Benzie and William (1997), Ayala et al. (1975) and Species (CITES). However, while poaching is on the Campbell et al. (1975), studied genetic variation in T. decline, it will not likely be completely stopped. On the maxima. Ayala et al. (1975) assumed that since the other hand, the farming of giant clams has become more had been restricted to the Indo-West Pacific, where it was attractive, especially for the trade. With the living in a relatively stable environment, its species would successful culture and sale of various species, an have little genetic variation and therefore would be unplanned expansion in biogeographic ranges could occur relatively vulnerable to extinction, "... a plausible analog should some hobbyists weary of their pets and set them of the sorts of fossil species that have commonly become free on appropriate reefs. extinct." As it turned out, T. maxima was "... one of the genetically most polymorphic organisms studied so far", Acknowledgments We thank R. Cowei of the BP Bishop and therefore their results did not "... support the Museum for the loan of the type specimen of Tridacna hypothesis that some massive extinctions... may have mbalavuana and, together with W. Emerson of the been due to the scarcity of genetic variation in American Museum of Natural History and J. Harasewych populations adapted to stable environments" (for more of the U.S. National Museum of Natural History, for help contrary evidence, see Schopf and Gooch 1972). In their

study of T. gigas, Benzie and Williams (1997) not only (the first to Cocos-Keeling), pronounced differences in corroborated the Ayala et al. (1975) findings re T. their longitudinal ranges occur in the east: maxima, as far as local heterozygosity was concerned, they found its widely separated populations lack 3) Tridacna (Tridacna) derasa (Röding, 1798): Southern significant genetic differentiation. So, T. maxima, the Sumatra (Pasaribu 1988) and Cocos-Keeling, rare in most wide ranging species, must at least until relatively Solomon Islands (Govan et al. 1988), rare or absent in the recently had good planktonic dispersal capabilities. It New Hebrides (, Zann and Ayling 1988); no would be instructive to know what a comparable genetic records in Taiwan (M Chen pers. com.) but occasionally study of the short-range species, T. mbalavuana, would present in southern Japan [Sakisima Guntô; more northern reveal. record from Muroto Saki, Japan unreliable (M Grygier Tridacna maxima is morphologically similar to T. pers. com.)], south to Australia and Lord Howe Is., and crocea whose habitat it exploits on Niue, Takelau and the east to Tonga (where none found in survey of 1987, Langi Cooks Islands (T J H Adams, pers. com.), where T. and Aloua 1988); introduced to the Cook Is. in 1986 crocea is absent. Whether this shift in habitat preference (Sims and Howard 1988). Lagoons and outer reef slopes is simply due to availability or to a genetic difference is to 20 m, unattached as adults, up to 60 cm in length. unknown, but apparently there is presently little gene flow While Macaranas et al. (1992) found no significant between the Pacific Plate (Central Pacific) populations genetic differences among populations on the Great and those to the west of the plate boundary (Benzie 1993a Barrier Reef, they found large genetic distances between and b). Fossil occurrences all fall within the present range populations from the Great Barrier Reef, Fiji and the of the species and include the Upper Miocene of Java, Philippines. is morphologically closest Neogene of Palau, Plio-Pleistocene of Guam and the to T. mbalavuana (Lucas et al. 1991) and genetically to T. Pleistocene of East Africa, Saipan and Tonga; no local gigas, and together these three species constitute Tridacna extinctions have been noted. s.s. (Schneider and Ó Foighil 1999). Susceptible to over-fishing, over-fished on Palau and 2) Tridacna (Chametrachea) squamosa Lamarck, 1819: now farmed, attempted re-introductions to Yap in 1984 Red Sea (uncommon compared to T. maxima at Elat, Y. (Price and Fagolimul 1988) and to the Philippines where Achituv pers. com.), south nearly to Durban, South it was virtually extinct (Gomez and Alcala 1988). Africa, east to Taiwan and southern Japan (Kaiki Sima), Apparently no fossil record. Guam and the , Australia, New Caledonia and Cook Island (Paulay 1987) where it is rarely found on 4) Hippopus hippopus (Linn., 1758): Sumatra to Pratas Is. outer reef slopes (Sims and Howard 1988), and the but not Taiwan (M. Chen pers. com.), Ryukus (Amami-Ô Pitcairn Islands (Paulay 1989), a substantial range Shima, M. Grygier pers. com.), Caroline and Marshall but extension. What may have been a single individual was not Mariana Is. and south from Australia, New Caledonia, seen on Kingman Reef, Line Islands (R Brainard, J east to Tonga; on reefs and sandy substrates including Maragos and D. Minton, pers. comms.), but none are harbors, to 10 m of depth, unattached as adults, up to 45 known from the remainder of the Line Islands, nor from cm in length. French Polynesia (B. Richer de Forges and P. Laboute, Fossil record includes the Miocene of Saipan, and B. Salvat pers. comms.). Saquet (1992), who makes Neogene of Palau, Plio-Pleistocene and Holocene of no pretense of being comprehensive regarding the marine Guam (R Richmond pers. com.) and the L. Pleistocene of invertebrates of French Polynesia, notes such recent Fiji where fossils are common (T J H Adams pers. com.). findings as the Easter Island lobster, Panulirus While no longer present in the Marianas, one living pascuensis, in deep waters there. Yet only one species of specimen is reported from a commercial port on Guam giant clam is mentioned (from shallow water and and a single shell from , but it is uncertain whether therefore T. maxima). Thus, the disjunction for T. these were waifs or artifacts (see below). The Holocene squamosa sounds real. Usually in protected localities on records on Guam could have been contemporaneous with surface of corals to 18 m of depth, deeper on outer reef man (R Richmond, pers. com.), no specimens taken on slopes, does not burrow, large adults often unattached, to Tonga although one may have been seen in the 1978-79 40 cm in length. survey (Langi dan Aloua 1988), found in coastal middens Fossil record includes the Plio-Pleistocene of Kita but extinct on Fiji (Lewis et al. 1988), protection Daito Jima, Celebes, Pleistocene of E. Africa, Tonga, and recommended in New Hebrides (Vanuatu, Zann and the ?Tuamotus (the last being in the disjunction discussed Ayling 1988). in the text). No local extinctions recorded, but The living specimen of Hippopus hippopus noted uncommon in the New Hebrides (Vanuatu; absent on 9 of above from Guam was reputed to have been thrown 13 islands surveyed perhaps due to limited sheltered overboard from a ship (R Richmond, pers. com.). situations, Zann and Ayling 1988). However, considering that H. hippopus 1) is known to have ranged as far north as Saipan in the Miocene and Medium-range species was in Guam as recently as the Holocene, 2) presently ranges east into the northern Marshalls and northwest into While 3) Tridacna derasa, 4) Hippopus hippopus, 5) the southern Ryukus, 3) commonly occurs in harbors, 4) T. gigas, and 6) T. crocea range as far west as Sumatra is consumed as food when fresh, and 5) does not survive

long out of water, it seems more likely this living archipelago, southern Sulu Sea, one specimen from specimen was a waif than an artifact. Masbate Is., Gomez and Alcala 1988), Palau (Heslinga Rosewater (1965) reports a single valve of Hippopus 1993), Celebes and West Irian, New Guinea (Lucas 1988, from Tern Island, French Frigate Shoal, but we have been Fig. 7), and all of Indonesia (Pasaribu 1988, citing unable to locate the specimen. It has been suggested that Romimohtarto et al. 1987). Coral rock and , not it may have been discarded there by service personnel (A attached as adults, sympatric with and same size as H. Kay, pers. com.), or dredged up when the island was hippopus, no habitat and/or depth partitioning reported. enlarged (R Grigg, pers. com.), both during World War II. No fossil record. Recently virtually extinct in the There is no other record of other tridacnines in the Philippines, due to shell trade stimulated by the newly Hawaiian Archipelago, but if there were, T. maxima discovered species (Gomez and Alcala 1988). and/or squamosa would seem the more likely candidates. Thus, while a waif of Hippopus, like waifs of three 8) Tridacna (Tridacna) mbalavuana Ladd, 1934 (p. 185 species of Acropora (Grigg 1981), might occasionally get and pl. 31 2-3) [=T. (Persikima) tevoroa Lucas et al. to the Hawaiian Islands, it seems more likely an artifact. 1991, 1992; see Taxonomic Considerations for discus- sion]. The synonymy is as follows: 5) Tridacna (Tridacna) gigas (Linn., 1758): Sumatra to Tridacna mbalavuana Ladd, 1934:185. southernmost Ryukyus (M. Grygier, pers. com.) and T. (Chametrachea) mbalavuana, Rosewater 1965:379. northern Marshalls but not Mariana Is. (R. Richmond, Tridacna sp. Lewis et al.. 1988:67 ("tevoro", devil clam pers. com.) or Taiwan (M. Chen pers. com.), south to in Fijian) (Lewis and Ledua 1988:82, who credit A. New Caledonia and east to Fiji (not present and no fossil Robinson of Suva for calling it to their attention, first to record on Tonga, Langi and Aloua 1988). On reefs to recognized it might be distinct from T. derasa). about 15 m of depth, does not burrow, adults unattached, T. (Persikima) tevoroa Lucas, Ledua and Braley, 1991:94 lengths to 1.37 m and shells to over 230 kg (500 kg total Ledua et al. 1993: 147 and 151. Tongan language; weight likely an exaggeration, Rosewater 1965). Benzie "vasuva ngesi manifi" or "vasuva ngesi sio ata" (thin shell and Williams (1992) found high heterozygosity within clam or window pane shell clam). populations of T. gigas and a lack of variation between 6 T. tevoroa Lucas, Ledu and Braley, 1990, includes T. widely separated populations on the Great Barrier Reef, mbulvuana (sic), CITES 1999: 20 so there appears to be gene flow despite apparent dis- T. (Tridacna) tevoroa, Schneider and Ó Foighil 1999:62. junctions (Benzei 1993). not T. squamosa, Schneider and Ó Foighil 1999: 64 Fossil record includes the Miocene of Java and Table 2. Plio-Pleistocene of Guam. Recent extinctions include the Tridacna mbalavuana, apparently long known to Fijians Philippines (Gomez and Alcala 1988; reintroduction in as "tevoro", the devil clam, was first described by Ladd progress, EDG), and Fiji by 1970 (Langi and Aloua (1934) from the Upper Tertiary (Neogene) of Vitilevu, 1988). Govan (1983, appendix) indicates T. gigas is Fiji. In recent years the deep-living populations of this extinct on Yap, Woleai, Ponape, Truk and Fiji (fossils species caught the attention of reef biologists in Fiji never seen in Fiji, T J H Adams pers. com.), extinct in where, curiously, it is reported as very rare today (T. J. H. Taiwan (M. Chen pers. com.). Noted as very rare or Adams pers. com.). Unaware that the "tevoro" had been absent from New Hebrides where it has been reintroduced previously described as a fossil (Ladd 1934), Lucas et al. (Zann and Alying 1988), and Pasaribu (1988, citing Usher (1990) proposed the name, Tridacna tevoroa. Its 1984) states it has been eliminated from western recognition as a "living fossil" herein substantiates Indonesia. Schneider and Ó Foighil's (1999) suggestion this species occurred in the Neogene. Scattered individuals of what 6) Tridacna (Chametrachea) crocea Lamarck, 1819: was thought might be this species were observed on Sumatra to Taiwan and the Ryukyus (Takara Sima, M. Holmes Reef, 120 nautical miles east of Cairns, Australia Grygier pers. com.) and the southern Great Barrier Reef, (Lucas pers. com.). Generally found on outer reef slopes as far east as Palau and Yap north of the equator and the from 20-30 m of depth, deepest living species and only Solomon Islands south of the equator; not likely in the Tridacna without hyaline organs, to 53 cm in length. New Hebrides and not in Fiji (Lewis et al. 1988:67), possible sighting on Palmyra (R. Brainard, J Maragos and 9) Tridacna rosewateri Sirenko and Scarlato, 1991: Saya D Minton, pers. com.). Deep burrower in corals and de Malha Bank (9o47'S, 61o25'E), Massacring Plateau, rubble in shallow water, attached for life; smallest species Western , from 12-13 m of depth. of the subfamily, up to 15 cm in length. Morphologically Considered closely related to if not an ecotype of T. most similar to T. maxima and genetically similar to both squamosa (see Taxonomic Considerations for discussion). T. maxima and T. squamosa (Schneider and Ó Foighil 1999). Fossil record includes the Pleistocene of Enewetak, well east of its documented range. 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