Like corals, giant clams live in partnership with tiny plantlike algae (called zooxanthellae) that live inside the clams' tissues. And as with corals, the arrangement helps both creatures. The algae gain protection from grazing animals; the clams grow large with the benefit of food produced by the algae.
At home on reefs throughout the Indian Ocean, the South Pacific and parts of South Africa, giant clams live on shallow reef flats down to depths of around 66 feet (20 m). Below that, the algae they depend on to survive wouldn't have enough sunlight to grow.
The giant clam, Tridacna gigas, is rare due to overharvesting by people. Giant clams are now being farmed, which can cut down on the numbers taken from the wild.
As their name implies, giant clams are the largest clams in the world. The largest grow more than four feet (1.2 m) long.
Once a giant clam settles into a place and begins to grow, it stays permanently attached to that spot for life.