The largest and heaviest of all mollusks is the giant clam (Tridacna gigas). Like other bivalves, it feeds by filtering small food particles from the water using its ingoing, or inhalant, siphon, which is fringed with small tentacles. However, it differs in obtaining most of its nourishment from zooxanthellae (the same dinoflagellate algae that form a symbiotic relationship with many species of coral). 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.
Photo © Christoph Specjalski