Giant Clam

The largest of all mollusks, the giant clam prefers the warm waters around Australia's Great Barrier Reef.

The giant clam gets only one chance to find a nice home. Once it fastens itself to a spot on a reef, there it sits for the rest of its life.

These bottom-dwelling behemoths are the largest mollusks on Earth, capable of reaching 4 feet (1.2 meters) in length and weighing more than 500 pounds (227 kg). They live in the warm waters of the South Pacific and Indian Oceans.

Giant clams achieve their enormous proportions by consuming the sugars and proteins produced by the billions of algae that live in their tissues. In exchange, they offer the algae a safe home and regular access to sunlight for photosynthesis, basking by day below the water's surface with their fluted shells open and their multi-colored mantles exposed. They also use a siphon to draw in water to filter and consume passing plankton.

Giant clams have a wildly undeserved reputation as man-eaters, with South Pacific legends describing clams that lie in wait to trap unsuspecting swimmers or swallow them whole. No account of a human death by giant clam has ever been substantiated, and scientists say its adductor muscles, used to close the shell, move far too slowly to take a swimmer by surprise. Even the largest specimen would simply retreat into its shell rather than attempt to sample human prey.

The adductor muscle of the giant clam is actually considered a delicacy, and overharvesting of the species for food, shells, and the aquarium trade have landed it on at least one group's "vulnerable" list.