Clam is a word which can be used for all, some, or only a few species of bivalve mollusks; the word is a common name which has no real taxonomic significance in biology. It is however quite widely used as part of the common names of bivalves, and also has significance in fisheries and cuisine.
In the USA, the word "clam" can be used in several different ways: one is as a general term covering all bivalve mollusks. The word can also be used in a more limited sense, to mean bivalves which burrow in sediment, as opposed to ones which attach themselves to the substrate (for example oysters and mussels), or ones which can swim and are migratory, like scallops. In addition the word "clam" can be used in an even more limited sense, to mean one or more species of commonly consumed edible marine bivalves, as in the phrase clam chowder for a thick shellfish soup usually made using the hard clam. Many edible bivalves have a more-and-less oval shape, however, the edible razor clam has an elongated, parallel-sided shell, whose shape suggests that of an old-fashioned straight razor.
In the UK, the word is not as widely used: it forms part of the common names of various species of bivalve mollusk, but it is not used as a general term to cover edible clams that burrow, and it is not used as a general term for all bivalves.
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.
Numerous edible marine bivalve species live buried in sand or mud, and respire by means of siphons which reach to the surface. In the USA, these clams are collected by "digging for clams" or clam digging. Again, in the USA, clam diggers is a term that can mean people who are searching for clams, or it can also mean one variety of three-quarter length pants or trousers.
In October 2007 an Arctica islandica clam, caught off the coast of Iceland, was discovered to be at least 405 years old, and was declared the world's oldest living animal by researchers from Bangor University, see Ming (clam).
In regard to the concept of edible clams, most species of bivalves are at least potentially edible, but some are too small to be useful, and not all species are considered palatable.
The word "clam" has given rise to the metaphor "clamming up", meaning refusing to speak, at least on a certain topic. A "clam shell" is the name given to a plastic container which is hinged, and which consists of two equal halves that lock together.
A clam's shell consists of two (usually equal) valves, which are connected by a hinge joint and a ligament which can be external or internal.
In most clams, two adductor muscles contract to close the shells. The clam has no head, and usually has no eyes, (scallops are a notable exception), but a clam does have kidneys, a heart, a mouth, and an anus. For more information see bivalve and pseudofeces.
Clams, like most mollusks, also have open circulatory systems, which means that their organs are surrounded by watery blood that contains nutrients and oxygen.
In culinary use, within the USA, the term "clam" most often refers to the hard clam Mercenaria mercenaria. It may also refer to several other common edible species, such as the soft-shell clam Mya arenaria, and the ocean quahog, Arctica islandica.
Clams can be eaten raw, steamed, boiled, baked or fried; the method of preparation depends partly on the size and species. They can also be made into clam chowder (a popular soup in the U.S. and Canada) or they can be cooked using hot rocks and seaweed in a New England clam bake.
In Italy, clams are often an ingredient of mixed seafood dishes, or are eaten together with pasta.
Not usually considered edible: