A clam is any of various mostly edible marine bivalve molluscs. The word comes from the Middle English clam, meaning both the hard-shelled fruit of the watermelon (Citrullus lanatus) and "mussel" as well as clape, a Northern English dialect name for the common periwinkle (Littorina littorea) as well as "mussel". Some edible clams such as Manila clams and razor clams are not members of the family Veneridae but the superfamily Lucinoidea. A clam is an edible marine bivalve mollusc with two hinged shells. A large clam has a heavy shell which may be over 5 centimetres (2 inches) long at the largest size. A few species have got such huge shells that people use them as utensils or decoration. Some clams live buried in mud or sand and can dig themselves down into the substrate.
A clam typically has two shells, one upper and one lower, held together by a hinge made of muscle. It also has two adductor muscles that close the shells. It uses its muscular foot to burrow into sand or mud in aquatic environments. Clams are well known for their excellent meats, which only need to be rinsed before cooking. They are used in soups, salads, quiches, and more. Other types of clams, such as razor clams, do not burrow but live above ground where they are subject to predation by gulls, sea otters, and humans.
Clams are filter feeders, feeding on plankton. They draw water in over their gills through the beating of cilia. Suspended particles (including phytoplankton) and even small organisms like copepods or rotifers can be trapped against a sticky mucus layer on the gill surface and transported to the mouth, where they are swallowed, digested, and expelled as feces.
Clams show remarkable anatomical adaptations to deal with predators such as starfish and humans. The clams' heavy shells protect against attack from below, but do not make it impossible for a starfish (for example) to prey on them; rather, strong adductor muscles prevent the shells from being opened. The shell has no grooves, making it harder for a starfish to grip onto its surface with its tube feet. A starfish usually engulfs its prey in an acidic environment of high osmotic pressure, which causes water to move passively along a pressure gradient into its body cavity, instead of being pumped by the pharyngeal muscles. When this is done enough, the starfish can actually pull the clam's shell apart to reach its soft body.
The soft parts of a clam are more abundant than its hard parts, so clams have evolved many defenses against predation. Clams have two siphons, one for the intake of water and the other for the intake of, or exit from, their gut. The siphons can be closed from below by a pair of labial palps during feeding in order to block food particles from entering the digestive cavity. In any given clam, one is larger than the other and both are held permanently open by strong muscles that attach directly to the shell.