clam, common name for certain bivalve mollusks, especially for marine species that live buried in mud or sand and have valves (the two pieces of the shell) of equal size. The oval valves, which cover the right and left sides of the animal, are hinged together at the top by an elastic ligament. Clams burrow by means of a muscular foot, located at the front end, which can be extruded between the valves. The head, located within the shell, is rudimentary, without eyes or antennae. Water containing oxygen and food particles enters through an incurrent siphon; waste-containing water is expelled through an excurrent siphon. The two tubes project from the end opposite the foot and may be united in a single structure called the neck. The sexes are usually separate. Eggs and sperm are deposited in the water; the fertilized egg develops into a free-swimming larva without a shell, which may not attain the adult form for several months.

Clams are highly valued as food. The soft-shell clam, or steamer (Mya arenaria), of both coasts of North America, is one of the most popular eating clams. The hard-shell clam (Mercenaria mercenaria), abundant from the Gulf of St. Lawrence to Texas, was called quahog by some Native Americans, who used the violet portion of the shell for wampum. Small hard-shell clams are called littlenecks, or cherrystones. The razor clam (Ensis), shaped like an old-fashioned straight razor, burrows rapidly and swims by means of its foot. The Atlantic razor clam, found from Labrador to W Florida and prized for its flavor, may attain lengths of 10 in. (25 cm). The Eastern surf clam (Spisula solidissima) frequents sandy bottoms in shallow water from Labrador to North Carolina and is much used for bait. There are also several Pacific surf clams. Other Pacific clams include the succulent Pismo clam (Tivela stultorum), found from mid-California southward and protected by law from overdigging, and the geoduck of the Pacific Northwest, which may weigh as much as 12 lb (5.4 kg). The valves of many small clams are familiar seashells, such as those of the pea-sized amethyst gem clam. The giant clam of the S Pacific Ocean may reach a weight of 500 lb (227 kg) and a length of 5 ft (150 cm).

There are two families of freshwater bivalves called clams. The small freshwater clams (family Sphaeriidae) are hermaphroditic; they retain the fertilized eggs in a brood pouch and bear young with shells. The large freshwater clams (family Unionidae) are also called freshwater mussels; the nacreous inner layer of their shells is a source of mother-of-pearl. The larvae of these clams are parasitic on the gills of fish.

The term clam is sometimes used synonomously with bivalve; in this sense it includes the oysters, scallops, and marine mussels. Clams are classified in the phylum Mollusca, class Pelecypoda or bivalvia.