What is the largest bivalve mollusk?

The largest living bivalve mollusk is the giant clam, especially Tridacna gigas. This enormous reef-dwelling clam lives in warm Indo-Pacific waters and can reach roughly 120 centimetres across and weigh more than 200 kilograms.

Giant clams are bivalves because they have two hinged shells, like oysters, mussels and scallops. Their extraordinary size comes partly from a partnership with photosynthetic algae called zooxanthellae. The algae live in the clam's mantle tissue and provide much of its nutrition, while the clam supplies them with shelter and access to sunlight.

The giant clam opens its shell during daylight so its colorful mantle can spread into the light. It also filter-feeds, particularly when young, and remains attached to the reef after its planktonic larval stage. Despite old stories about giant clams trapping divers, their slow closing motion and need to expel water make that danger largely mythical.

The largest recorded shells are even bigger than the usual adult measurements. One specimen measured 137 centimetres and was estimated to have weighed about 250 kilograms alive. Giant clams face pressure from harvesting and are regulated under CITES international trade rules.