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 that burrow in sediment, as opposed to ones that attach themselves to the substrate (for example oysters and mussels), or ones that 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 shellfish soup usually made using the hard clam. Many edible bivalves have a roughly oval shape; however, the edible Pacific razor clam has an elongated, parallel-sided shell, whose shape suggests that of an old-fashioned straight razor.
Numerous edible marine bivalve species live buried in sand or mud, and respire by means of siphons, which reach to the surface. In the United States, 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.
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 “clam up”, meaning to refuse to talk or answer, based on the clam behavior of quickly closing its shell when threatened. A “clam shell” is the name given to a hinged container consisting of two equal halves that lock together. Clams have also inspired the phrase “happy as a clam”, short for “happy as a clam at high tide” (which should be happy because it cannot easily be dug up and eaten).