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The most abundantly obtained clam in the United States is the eastern surf clam, Spisula solidissima. It ranges from Maine to South Carolina and is found from the intertidal zone to a depth of about 100 feet (31 meters). Its smooth, tan-colored, oval shell may reach a length of 7 inches (18 cm).
The hard-shell, or quahog, clam is popular in North America either raw on the half shell or minced in hot chowders. Small quahogs, usually served raw on the half-shell, are called cherrystones. The quahog clam is a heart-shaped clam that lives in the shallow waters of estuaries from southern Massachusetts to Texas. The northern form is Mercenaria mercenaria; the southern subspecies is M. mercenaria campechiensis.
The soft-shell, or steamer, clam, Mya arenaria, has a relatively thin, fragile shell and a long, tubular sheath around its siphon. It lives along the Atlantic coast of North America, usually buried in the sandy mud to a depth of about 8 inches (20 cm).
Along the Pacific coast a number of sand-dwelling, shallow-water clams are used as food. The pismo clam of California, Tivela stulto-rum, and the Washington clam, Saxidomus, are abundant at certain seasons. The large geoduck, Panopea, of the Pacific Northwest, resembles the eastern Mya, but may weigh 5 pounds (2.3 kg).
The largest known living bivalve is the giant clam, Tridacna gigas, of the tropical Indo-Pacific area. Its two shell valves may weigh as much as 500 pounds (227 kg) and have a length of 50 inches (127 cm). This clam lives on reefs or in shallow lagoons on sand, with the hinge lying on the bottom and the gaping open end facing up. The swollen, wavy mantle edge resembles a green and blue snake. The giant clam feeds on colonies of marine algae, which it permits to grow under the epidermis of its mantle. The clams are very slow in closing their valves, and there are no authentic cases of persons being trapped by them.
There are several hundred species of freshwater clams, or river mussels, belonging to the family Unionidae. The thick shell is pearly within and the outside is covered with a varnishlike, green to blackish periostracum, or horny coating. The meat is edible but not very palatable. The largest and more colorful species come from the Mississippi River and its tributaries.
Both marine and freshwater clams are vegetarians. They take food-laden water in through an inhalant siphon (a tube-shaped extension of the mantle) and pass it over the gills. Mucus strands in the gills entangle the microscopic plant particles and carry them to the small, toothless mouth where flaplike palps push the food into the esophagus. An exhalant siphon expels used water and wastes.
Clams move mainly by using their single, tongue-shaped foot, which is maneuvered through the sand. The tip of the foot, when swollen by an influx of lymph and blood, serves as an anchor by which the clam pulls itself forward. The tip is then deflated, and the thin foot is thrust forward to repeat the operation. Clams with short siphons usually move about a great deal and keep within an inch (2.5 cm) of the surface. Clams with long siphons usually remain buried deeply in one place most of their lives.
Clams reproduce sexually. In most species the sexes are separate, but some species are hermaphroditic (one individual contains both male and female sex organs). Fertilized eggs of freshwater clams develop within the adult's gills into small larvae called glochidia. These larvae are expelled into the water; to develop further, they must live as parasites. With small hooks they clamp onto the gills of a fish and suck its blood for several weeks. When their shells become too heavy, the young clams drop to the bottom and begin the normal life of a clam. Many marine clams have a free-swimming larval stage, called the veliger; other clams brood their young inside the mantle cavity.
Clams reach maturity in 1 to 3 years and may live for as long as 20 years. The rings in the shell are not necessarily annual but indicate periods of slow growth in extra cold weather or drought conditions.