The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.
In the United States, "clam" can be used in several different ways: one, 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 "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 United Kingdom, "clam" is one of the common names of various species of marine 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.
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