Marine Wildlife Encyclopedia
Giant Clam Tridacna gigas
The largest and heaviest of all mollusks is the giant clam. Like other bivalves, the giant clam feeds by filtering small food particles from the water using its ingoing, or inhalant, siphon, which is fringed with small tentacles. However, the giant clam differs in obtaining most of its nourishment from zooxanthellae (unicellular algae that live within its tissues)—a type of relationship also associated with coral polyps. The algae have a constant and safe environment in which to live; in return, they provide the clam with essential nutrients, the carbon-based products of photosynthesis. In fact, so dependent is the giant clam on these algae that it will die without them.
The adult giant clam is sessile (immobile) and its inhalant and exhalant (outgoing) siphons are the only openings in its mantle. Although the scalloped edges of their shell halves are mirror images of one another, larger giant clams may be unable to close their shells fully, so their brightly colored mantle and siphons remain constantly exposed. Many giant clams appear iridescent due to an almost continuous covering of purple and blue spots on their mantles, while others look more green or gold, but all have a number of clear spots, or “windows,” that let sunlight filter into the mantle cavity. Fertilization is external and the eggs hatch into free-swimming larvae before settling onto the seabed. The exhalant siphon expels water and at spawning time provides an exit point for the eggs or sperm.
Giant Clam Spawning
Reproduction in giant clams is triggered by chemical signals that synchronize the release of sperm and eggs into the water. Giant clams start life as males and later become hermaphroditic, but during any one spawning event, they release either sperm or eggs in order to avoid self-fertilization. A large giant clam can release as many as 50 million eggs in 20 minutes.