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On the Status of Giant Clams, Relics of Tethys (Mollusca: Bivalvia: Tridacninae)

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On the Status of Giant Clams, Relics of Tethys (Mollusca: Bivalvia: Tridacninae) 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 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 Australia (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 corals (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 Philippines, 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 Ocean. 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.
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