Giant clam

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 227 kilograms (500 pounds) and measure as much as 1.2 metres (4 feet) across, and have an average lifespan in the wild of 100 years or more.^[1]

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.

While records of death or injury in this manner are rare and often dubious, they are not entirely unheard of. According to a colorful account by Wilburn Cobb,^[2] the Pearl of Lao Tzu was extracted from a tridacna which had drowned a Dyak diver by closing its shell on his arm. In a more recent incident on Hamilton Island off the coast of Queensland, Australia, a five year old boy was treated for injuries after a giant clam nearly severed his finger by clamping down on it.^[3]

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 too slow to pose a reasonable threat of death.

Information from wikepedia - http://en.wikipedia.org/wiki/Giant_clam