giant clam, common name for the largest bivalve mollusk in the world, Tridacna gigas, also known as the bear's paw clam. The giant clam may weigh over 500 lb (225 kg) and attain a length of over 4 ft (120 cm). The heavy shell is coarsely fluted and toothed. Giant clams are found in the South Pacific and Indian oceans, especially in the Great Barrier Reef. They lie with the hinge downward in the coral reefs, usually in shallow water. The adductor muscles, which cause the shell to close, are a source of food for people of the South Pacific. The shell closes very slowly; stories of human beings trapped within giant clams have never been substantiated. Small giant clam shells have been used as birdbaths and baptismal fonts. An interesting symbiosis occurs between a unicellular green alga (Zooanthella) and the clam. The algae live in the tissues of the clam's siphon and mantle; they are able to obtain the sunlight needed for photosynthesis because the clam lies with its valves opening upward and part of the thick, purple mantle extruding over the shell. In addition, there are crystalloid vesicles on the mantle surface that let in sunlight, thus allowing the algae to live deep within the tissues. The clam uses the algae as a supplementary or perhaps even a major source of food. Tridacna gigas is classified in the phylum Mollusca, class Pelecypoda or bivalvia, order Eulamellibranchia, family Tridacnidae.

clam shrimp: see shrimp.

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.

Any of several species of marine clams (family Solenidae) common in intertidal sands and muds, particularly of temperate seas. Razor clams have narrow and elongated shells (shaped like straight razors) up to 8 in. (20 cm) long. A large active foot enables them to move rapidly up and down within their burrow and retreat quickly when disturbed. With their short siphons (tubes) they feed on particulate material in seawater. Some species can swim short distances by jetting water through their siphons.

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Any clam of the genus Donax, inhabiting sandy beaches worldwide. Coquinas are very active; they migrate up and down beaches with the tide and can reburrow between waves. They have short siphons and feed on suspended plant material and detritus. A typical species, D. variabilis, measures about 0.4–1 in. (10–25 mm) in length. Its shell is wedge-shaped and varies from white, yellow, and pink to blue and mauve.

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In general, any bivalve mollusk. True clams, in the strict sense, have equal shells, closed by two opposing muscles, and a powerful, muscular, burrowing foot. They usually lie buried in the sand in shallow marine waters. Clams draw in and expel water for respiration and feeding through two tubes, the siphons. Species range in size from 0.004 in. to 4 ft (0.1 mm–1.2 m) across. Many species are edible, including the coquina clam, geoduck, quahog, and soft-shell clam.

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