What Divers Should Know About Giant Clam

Living a sessile life in adulthood, the giant clam, Tridacna gigas, is a fascinating example of marine symbiosis. This largest living bivalve mollusc cultivates single-celled algae within its mantle tissues, extending them by day to ensure the algae receive sunlight for photosynthesis. An impressive creature, it can weigh over 200 kilograms, measure up to 120 cm across, and boasts an average lifespan exceeding 100 years in the wild.

You'll typically find these magnificent bivalves in flat coral sand or broken coral on shallow coral reefs across the Indo-Pacific, including the South Pacific, Indian Oceans, the Philippines, and Malaysia. They inhabit depths up to 20 meters. A key identifier for adult T. gigas is its inability to completely close its shells, leaving part of its brownish-yellow mantle visible.

Hundreds to thousands of pinhole eyespots dot the mantle border. These specialized receptors enable T. gigas to detect changes in light, such as dimming or direction shifts, or even the movement of an object, prompting a partial closure of its shell.