In marine biology, which bivalve is the largest living species by mass?

The giant clam is the largest living bivalve species by mass.

Its scientific name is Tridacna gigas, and it lives in shallow coral-reef environments of the Indo-Pacific. Like other bivalves, it has two shells joined by a hinge, but giant clams can become dramatically larger than familiar mussels, cockles, or oysters. Their shells may exceed 1 metre in length, while exceptionally large individuals can weigh more than 200 kilograms.

Giant clams obtain much of their energy from microscopic algae called zooxanthellae, which live symbiotically in their mantle tissue and perform photosynthesis. The clam also filters plankton and organic particles from seawater, so it is both a suspension feeder and a photosynthetic partner.

A common mix-up is confusing the giant clam with the geoduck, another very large bivalve. Geoducks are famous for their long siphons, but the giant clam is generally recognized as the largest bivalve by mass. Giant clams are also culturally important and have faced pressure from harvesting and reef degradation.