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 ), also known as the northern quahog, is abundant from the Gulf of St. Lawrence to Texas. The name quahog is from the Naragansett; some Native Americans used the violet portion of the shell for wampum. Small hard-shell clams are called littlenecks; somewhat larger ones, cherrystones. The ocean quahog ( Artica islandica ) is among the longest-lived animals; one was estimated to be between 405 and 410 years old in 2007. 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.
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