chapter  4
Giant Clams (Bivalvia: Cardiidae: Tridacninae): A Comprehensive Update of Species and their Distribution, Current Threats and Conservation Status
ByMei Lin Neo, Colette C.C. Wabnitz, Richard D. Braley, Gerald A. Heslinga, Cécile Fauvelot, Simon Van Wynsberge, Ser.G.E. Andréfouët, Charles Waters, Aileen Shau-Hwai Tan, Edgardo D. Gomez, Mark J. Costello, Peter A. Todd
Pages 301

Giant clams, the largest living bivalves, play important ecological roles in coral reef ecosystems and provide a source of nutrition and income for coastal communities; however, all species are under threat and intervention is required. Here, we re-examine and update their taxonomy, distribution, abundance and conservation status as a contribution to the protection, rebuilding and management of declining populations. Since the first comprehensive review of the Tridacnidae by Rosewater (1965), the taxonomy and phylogeny of giant clams have evolved, with three new species descriptions and rediscoveries since 1982 represented by Tridacna squamosina (formerly known as T. costata), T. noae and T. lorenzi. Giant clams are distributed along shallow coasts and coral reefs from South 88Africa to the Pitcairn Islands (32°E to 128°W), and from southern Japan to Western Australia (24°N to 15°S). Geographic distribution of the 12 currently recognized species is not even across the 66 localities we review here. Tridacna maxima and T. squamosa are the most widespread, followed by the intermediate-range species, T. gigas, T. derasa, T. noae, T. crocea and Hippopus hippopus, and the restricted-range species, Tridacna lorenzi, T. mbalavuana, T. squamosina, T. rosewateri and Hippopus porcellanus. The larger species, Tridacna gigas and T. derasa are the most endangered, with >50% of wild populations either locally extinct or severely depleted. The smaller and boring species, such as T. maxima and T. crocea, remain relatively abundant despite ongoing fishing activities. Population density also varies across localities. Areas with the lowest densities generally correspond with evidence of high historical exploitation intensity, while areas with the highest densities tend to be within marine reserves, remote from human populations or have low historical fishing pressures. Exploitation continues to be the main threat and conservation challenge for giant clams. Harvesting for subsistence use or local sale remains an important artisanal fishery in many localities; however, increased commercial demand as well as advances in fishing, transport and storage practices, are in large part responsible for the ongoing loss of wild populations. Habitat loss and a suite of other anthropogenic stressors, including climate change, are potentially accelerating stock depletions. Despite these challenges, global efforts to protect giant clams have gained momentum. CITES Appendix II listings and IUCN conservation categories have raised awareness of the threats to giant clams and have contributed to stemming their decline. The continued development of mariculture techniques may also help improve stock numbers and lend populations additional resilience. However, more effective implementation of conservation measures and enforcement of national and international regulations are needed. It is clear that active management is necessary to prevent the extinction of giant clam species as they continue to face threats associated with human behaviours.