Giant clam

Tiny living things called algae live inside the giant clam's body.

They use sunlight to make food, and they share it with the clam. The clam also catches tiny bits of food from the water.

Giant clams can filter tiny food from the water, but most of their food comes from tiny algae called zooxanthellae living in their bodies. In sunlight, the algae make sugars, and the clam uses them. In some clams, the algae supply about 90 percent of the clam's carbon, a building block of food.

Giant clams need those algae: without them, or if they are kept in the dark, they die.

Tridacna gigas has an established symbiosis with dinoflagellate algae (zooxanthellae): SeaLifeBase notes that the clam ingests the algae at an early stage and they then propagate inside it. Animal Diversity Web reports that the algae have been shown to provide 90 percent of the carbon metabolized in some individuals, that the clams also filter plankton, and that they die without their symbionts or in darkness.

In a 2025 common-garden experiment with six giant clam species, Guibert and colleagues found that T. gigas and T. derasa were the most autotrophic and fastest-growing; they note that such heavy reliance on symbionts brings faster growth but may increase vulnerability to disturbances.

A grown-up giant clam stays in the same place all its life.

It opens its shell and spreads out its colorful mantle. If something dark moves close by, it pulls its mantle in.

Once a giant clam settles, it stays in that spot for the rest of its life. When it is open, its colorful mantle spreads out over the edges of its shell.

You may have heard old stories that giant clams can trap a swimmer's foot. That is not true. Scientists say there are no real cases of people being trapped by giant clams. And grown-up giant clams cannot even shut their shells all the way.

Adults are sessile; their visible behavior is mostly extending and retracting the mantle. On the Great Barrier Reef, Braley (1984) suggested that clams which did not respond to a spawning neighbor may have been reproductively inactive.

The "killer clam" story is unfounded. Animal Diversity Web states that there are no authentic cases of people being trapped and drowned by giant clams; the real injuries come from people lifting heavy adult clams out of the water.

There are very few giant clams left.

People took too many for food and for their shells. Now scientists raise baby clams and put them back on reefs.

Tridacna gigas is Critically Endangered. People have collected so many for their meat and shells that they have disappeared from many reefs. Pollution and very warm water harm them too.

There is good news. Giant clams can be raised in farms, and young clams have been put back on reefs in places such as Fiji and Vanuatu. International trade in giant clams is watched under an agreement called CITES, and some reefs where they live are protected.

The IUCN assessed T. gigas as Critically Endangered (criteria A2acd) on 23 May 2024, after an estimated decline of more than 80 percent over three generations, with a decreasing population and a generation length of 33.5 years. Listed threats are fishing and harvesting, pollution from domestic, urban and agricultural effluents, and temperature extremes; it is used for food, handicrafts, research and ex-situ production.

Declines were documented decades ago. On Helen Reef, Palau, a 1975 survey found 4 live and 206 dead T. gigas, a fall attributed to human exploitation (Bryan and McConnell 1976). The species is on CITES Appendix II, has a recovery plan, has been reintroduced in several places, and occurs in at least one protected area.