Tridacna crocea ranges from the western coast of the Malay Peninsula east to Micronesia and north to the Ryukyu Islands, Japan, including Thailand, Singapore, Philippines, Indonesia, North Borneo, Papua New Guinea, the Solomon Islands, the Caroline Islands (22). It is reported to be common on the Great Barrier Reef, Australia (19) and in 1969 was the most abundant giant clam in Palau in the Caroline Islands (10). T. derasa is known from the Philippines, Indonesia (Irian Jaya), Guam, Cocos-keeling Islands, Australia, Papua New Guinea, New Caledonia, Caroline Islands (22). T. gigas occurs from the Philippines to Micronesia. Records exist for the Ryukyu Islands, Philippines, Indonesia, western Australia and Queensland, Papua New Guinea, Solomon Islands, Vanuatu, (New Hebrides), Caroline Islands, Marshall Islands, Gilbert Islands (22). T. maxima and T. squamosa are the most widespread, extending from the Red Sea and the East African coast to the Tuamotu Archipelago and Pitcairn Island, and from southern Japan to the coast of New South Wales, Australia (16,22). Both species have been recorded from Mozambique, Kenya, Madagascar, Saudi Arabia, Egypt, Seychelles, Mauritius, Maldives, Chagos, Thailand, Malaysia, Japan, Philippines, Indonesia, North Borneo, Australia, Papua New Guinea, New Caledonia, Fiji, Marianas, Caroline Islands, Marshall Islands, Gilbert Islands, Tuvalu (Ellice Islands), Samoa, Tonga, Tuamotu. T. maxima has also been recorded from South Africa, Sri Lanka, Andaman Islands, China, Taiwan, Solomon Islands, Vanuatu (New Hebrides), Lord Howe Island, Wake Island, Line Islands and Henderson Island (22). POPULATION Unknown. The smaller species are still found abundantly in many areas. In certain areas of the Great Barrier Reef T. crocea density regularly exceeds 100 animals per m2 (30). In closed lagoons at Tuamotu, T. maxima has been found at densities of 63 per m, that is up to 90 000 individuals per hectare (31). In Indonesia near Vogelkop as many as 30-50 large specimens of T. derasa and T. gigas have been found on certain reefs (29). In many areas populations are now much reduced (see below), although in shallow reef areas Tridacnidae are often the commonest bivalves, if not in actual numbers, nearly always in biomass (33). HABITAT AND ECOLOGY Restricted to shallow waters of coral reefs. T. gigas is found on sand and among corals on reefs from about 1 to 20 m in depth. Some or all of the shell may be exposed at low tides (22). T. derasa generally occurs on the outer edges of reefs at about 'f-lo m (22); it appears to be restricted to oceanic environments and is not found on reefs adjacent to large land masses (itj). H. hippopus is found on sandy substrates on coral reefs down to 6 m in depth and on sea grass beds near the reef (22). T. maxima is essentially a reef-top inhabitant living on the surfaces of the reef or sand, or partly embedded in coral (16). T. crocea burrows by mechanical, and possibly chemical, means into coral boulders on the reef-top and lives with only the valve margins visible; it is found most often on top of small, detached coral boulders on the interior reef flat (30). T. squamosa usually occurs on coral reef surfaces in depths less than 15 m, most often in protected environments such as reef canyons and fissures, sheltered lagoons and marine lakes (19). It is uncommon in atoll environments (17). Juveniles of all six species are attached to the reef by a gelatinous byssus. This is retained in small species but is gradually lost in T. gigas T. derasa and H. hippopus (33). Giant clams feed by filtering plankton from the sea with their gills. A supplementary source of food is provided by zooxanthellae of the species Symbiodinum (= Gymnodinum ) microadriaticum which are specialised dinoflagellates that live symbiotically in the mantle tissues and produce glycerol, 99