Clam is the general common name of any mollusks within the class Bivalvia. More specifically, true clams are bivalves having two shells of equal size connected by two adductor muscles and having a powerful burrowing foot. They are infaunal, spending most of their lives partially buried in the sand of the ocean floor. These characteristics distinguish the true clams from other bivalves, such as oysters, mussels, and scallops.
Remains first appeared in Cambrian age rocks 510 million years ago. They presently live in both freshwater and marine habitats, and range in adult size from nearly microscopic to the giant clam, which can weigh 200Â kg (440Â lb). Some have life cycles of only one year, while at least one has been discovered that may be over 500 years old. They lack heads but most can react to changes in light and some, such as the scallops, have rudimentary eyes. Though a common food item, many are too small to be useful as food, and not all species are considered palatable. All clams have two calcareous shells or valves joined near a hinge structure with a flexible ligament, and all are filter feeders.
In the United States, the word "clam" has several different meanings. First, it can generally refer to all bivalve molluscs. In the more limited sense, the term refers to the large subset of bivalves living as infauna, rather than those attached to a substrate (like oysters and mussels) or those that lie and move near the bottom or swim (like scallops). It can also refer to one or more kinds of commonly consumed marine bivalves, such as in the phrase clam chowder, which refers to shellfish soup. Many edible clams are roughly oval-shaped or triangular; however, razor clams have an elongated parallel-sided shell, suggesting an old-fashioned straight razor.
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