Clam is a common name for several kinds of bivalve mollusks. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shells of equal size connected by two adductor muscles and have a powerful burrowing foot. They live in freshwater and marine environments; in saltwater, they prefer to burrow down into the mud. The turbidity of the water required varies with species and location; the greatest diversity of these is in North America.
A clam's shell consists of two (usually equal) valves connected by a hinge joint and a ligament external or internal. The ligament provides tension to bring the valves apart, while one or two adductor muscles can contract to close the valves. Clams also have kidneys, a heart, a mouth, a stomach, and a nervous system. Many have a siphon.
Clams are shellfish that make up an important part of the web of life that keeps the seas functioning, both as filter feeders and food sources for many different animals. Cultures around the world eat clams along with many other types of shellfish.
Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of mollusks, crustaceans, and echinoderms.
Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. Shellfish are among the most common food allergens.