Giant clams in danger

Press, 27 July 1984, Page 17

Giant clams in danger

Despite their undeserved reputation, giant clams do not threaten divers. But divers do threaten these odd creatures, whose bizarre sex lives and eating habits make them particularly vulnerable. “Clam farms” may offer hope.

By

DONATES DE SILVA

of Earthscan

Despite the reputation given them by adventure novels, bad films and comic books, giant clams account for the deaths of very few, if any, divers. In fact, it is the divers who are rapidly doing away with the giant clams. The authoritative Hutchinson 20th Century Encyclopaedia warns: “If the two valves (of the giant clam) close on the foot of the diver, he may not be able to free himself.” True; but he would have to leave his foot between the shells for a very long time before this extremely slow creature could chomp down on it. Giant clams, the world’s largest molluscs, live mainly in shallow South-East Asian and Pacific waters, where they are relentlessly harvested for shell and meat and, as a result, are threatened with extinction. Tridacna gigas, over a metre long and weighing around 250 kilograms is the largest of the seven species. Some specimens are thought to be over 100 years old. They need shallow waters, because much of their food is provided by algae living inside them producing a sugar-like substance

during photosynthesis. Thus they need light, and their exposed position in shallow, well-lighted waters makes them easy prey for fishermen. (The clams themselves feed not on divers but on tiny plankton filtered through their gills.) All the species feed local fishermen. Giant clams have been a major part of the peasant fishery in Tonga, but recently a commercial fishery was established there, taking in most of the clams around Tongatapu Island. Loan schemes for fishermen for the purchase of boats and outboard motors have also increased pressure on the clams. In Manus Province, Papua New Guinea, the big clams are collected from the reefs and placed in “clam gardens” in quieter water, where they continue to grow. They are harvested as needed, especially when a long spell of bad weather prevents fishing. Some nations in the region regard the meat of the giant clam as

a delicacy. The Japanese use it in the raw fish preparations known as “sushi.” Although all the meat is edible, the large white adductor muscles (which open and close the shells) are most in demand. The Taiwanese fish the clams commercially, removing the muscles and leaving the rest on the reefs. Clam meat is reported to fetch SUSB2 to $143 a kilogram in Hong Kong. The supply of frozen clam meat is a thriving industry in some parts of the Philippines. The thin, fluted and brightly coloured shells of some species are snapped up by shell collectors and tourists world-wide. Many are exported to Florida, where they are one of the most popular items in the tourist shell trade. There is a buslting shell trade in the Philippines, where they are used as bird-baths, washbasins and to decorate gardens and even graves. There are warehouses stocked with shells of the larger species awaiting export from the

Philipppine islands of Cebu and Zamboanga. Luxury clam shell floor tiles are fashionable in Indonesia. To feed this recent craze, hundreds of tonnes of old shells are being dug up in the Thousand Islands near Jakarta. The clams’ sedentary habits and odd sex lives make them vulnerable to over-exploitation. The creatures are hermaphrodites — that is, each has male and female sexual organs. They release sperm and eggs at different times, so that one animal does not mate with itself. Successful breeding requires other clams nearby, and there is probably a population density below which the clams cannot breed. Reefs from the northern Marianas to the southern tip of the Great Barrier Reef have been stripped of giant clams. Recent extinctions have been reported from Truk and Cosrae in the eastern Caroline Islands, and from Guam. Taiwanese fishermen took 156,000 giant clams from Australia’s Swain Reefs between 1969 and 1977. Species are also threatened by over-fishing in Japan’s southern Ryuku Islands, and giant clams could disappear from the area within 10 years. There is some hope. Countries such as Australia and Indonesia are expanding their programmes to protect reef areas, allowing giant clams there to breed undisturbed. All species are now protected on Australia’s Great Barrier Reef. Captive breeding and farming are also possible solutions. Workers at a sea-farming station on Palau, in the Caroline Islands, have been successfully mass-breeding giant clams using relatively simple techniques; and researchers at the International Centre for Living Aquatic Resources Management in Manila are developing a commercial farming operation. “Clam farming” should be easy because clams are robust, have no major predators and feed on plankton. They exist as larvae for only a matter of days before settling down to grow shells. Then they grow rapidly, a two to four-year-old T. gigas adding 8 to 12 centimetres a year in length. All this augurs well for the future of the giant clams — and of the people of the small Pacific islands who depend on these giants of the reefs. /