On the status of giant clams, relics of Tethys (Mollusca
Transcription
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). References Alder J, Braley R (1989) Serious mortality in populations of giant clams on reefs surrounding Lizard Island,Great Barrier Reef. Australian J Mar. Freshwater Res. 40:205-213. Ayala FJ, Hedgecock D, Zumwalt G, Valentine J (1973) Genetic variation in Tridacna maxima, an ecological analog of some unsuccessful evolutionary lineages. Evol 27:177-191. Belda CA, Cuff C, Yellowlees D (1993) Modification of shell formation in the giant clam Tridacna gigas at elevated nutrient levels in sea water. Mar Biol 117:251-257. 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