|Edible clams in the family Veneridae|
In the USA, the word "clam" can be used in several different ways: one, is as a general term covering all bivalve molluscs. 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 marine bivalves, as in the phrase clam chowder, meaning a thick shellfish soup usually made using the hard clam. Many edible bivalves have a roughly 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 clam is not as widely used: it forms part of the common names of various species of bivalve mollusc, 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.
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.
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. However 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) halves, which are connected by a hinge joint and a ligament which can be external or internal, much like a Venus Flytrap.
In 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 molluscs, 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 eastern coast of 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. Another species which is commercially exploited on the Atlantic Coast of the US is the surf clam Spisula solidissima.
Clams can be eaten raw, steamed, boiled, baked or fried; the method of preparation depends partly on the size and species of the clam. 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. The more commonly used varieties of clams in Italian cooking are the Vongola (Venerupis decussata), the Cozza (Mytilus galloprovincialis) and the Tellina (Donax trunculus). A variety of mussel called Dattero di mare (Lithophaga lithophaga) was also once widely popular as seafood. However, since overfishing drove it on the verge of exctintion (it takes 15 to 35 years to reach adult size and could only be harvested by smashing the calcarean rocks that form its habitat), it has been declared an endangered species by the Italian government since 1998, and its harvest and sale are forbidden.
Not usually considered edible: