The giant clam (Tridacna gigas) or traditionally, pa’ua, is the largest living bivalve mollusc. One of a number of large clam species native to the shallow coral reefs of the South Pacific and Indian oceans, they can weigh more than 180 kilograms (400 pounds) and measure as much as 1.5 metres (5 feet) across.
Sessile in adulthood, the creature's mantle tissues act as a habitat for the symbiotic single-celled dinoflagellate algae (zooxanthellae) from which it gets its nutrition. By day, the clam spreads out its mantle tissue so that the algae receive the sunlight they need to photosynthesize.
As is often the case with uncharacteristically large species, the giant clam has been historically misunderstood. Known in times past as the killer clam or man-eating clam, reputable scientific and technical manuals once claimed that the great mollusc had caused deaths; versions of the U.S. Navy Diving Manual even gave detailed instructions for releasing oneself from its grasp by severing the adductor muscles used to close its shell.
Today, it is generally acknowledged that the giant clam is neither aggressive nor particularly dangerous; while it is certainly capable of holding one fast in its grip, the shell's closing action is actually a defensive response, and far too slow to pose any reasonable threat. No account of a human becoming trapped in this manner has ever been substantiated.
The IUCN lists the giant clams as vulnerable. There is concern among conservationists for the sustainability of practices among those who use the animal as a source of livelihood. The numbers in the wild have been greatly reduced by extensive overharvesting for food and the aquarium trade. On the black market, giant clam shells are sold as decorative accoutrements, and the meat, called Himejako in Japan, is prized as a delicacy.