Which bivalve is the largest living mollusc, reaching more than 1 metre across?

The giant clam (Tridacna gigas) is the largest living mollusc and the largest bivalve species.

It lives on shallow coral reefs in the Indo-Pacific, where its heavy shell can exceed 1 metre in length. Like corals, giant clams host microscopic photosynthetic algae called zooxanthellae in their tissues. These algae provide energy, while the clam gains additional nutrients by filtering tiny particles from seawater.

The animal’s spectacular mantle is often blue, green, brown, or patterned. Its intense colour comes from reflective cells and pigments, not from a gemstone-like shell. Giant clams do not actively trap divers or people; their shells close slowly and are generally easy for a person to escape.

Giant clams were historically collected for food and shells, and some populations declined sharply. Conservation programs now breed and reintroduce them in parts of the Indo-Pacific. The species is listed as vulnerable because of harvesting, habitat damage, and changes to coral-reef ecosystems.