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|Edible clams in the family Veneridae|
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|Edible clams in the family Veneridae|
A clam is a bivalve mollusc. The first fossils of them appear in rocks dated 510 million years ago in the Cambrian period. There are freshwater and marine varieties ranging in size from those that even as adults are nearly microscopic to others, such as the giant clam, which can weigh 200 kilograms (440 lbs). Some have lifecycles of only one year, while at least one has been discovered that may be over 500 years old. They do not have heads and most are blind but some, such as the scallops, have rudimentary eyes. Though a common food item, many species of clam are too small to be useful as food, and not all species are considered palatable. However, all clams have two calcareous shells or valves, 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.
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 term covering either edible clams that burrow or bivalves in general.
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.
The word "clam" is used in the idiom "to clam up," meaning to refuse to talk or answer, based on the clam behavior of quickly closing the shell when threatened. A "clamshell" is the name given to a container or mobile phone consisting of two hinged halves that lock together. Clams have also inspired the phrase "happy as a clam," short for "happy as a clam at high tide" (when it can't easily be dug up and eaten).
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A clam's shell consists of two (usually equal) valves, which are connected by a hinge joint and a ligament that can be external or internal. The ligament provides tension to bring the valves apart, while one or two adductor muscles can contract to close the valves. Clams also have kidneys, a heart, a mouth, a stomach, a nervous system, and an anus. Many have a siphon.
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In culinary use, within the eastern coast of the United States, the term "clam" most often refers to the hard clam Mercenaria mercenaria. It may also refer to a few other common edible species, such as the soft-shell clam, Mya arenaria, and the ocean quahog, Arctica islandica. Another species commercially exploited on the Atlantic Coast of the United States is the surf clam Spisula solidissima. Scallops are also used for food.
In Japan, clams are often an ingredient of mixed seafood dishes. They can also be made into Hot pot, Miso soup, Tsukudani. The more commonly used varieties of clams in Japanese cooking are the Asari (Venerupis philippinarum) and the Hamaguri (Meretrix lusoria).
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). Though Dattero di mare (Lithophaga lithophaga) was once eaten, overfishing drove it to the verge of extinction (it takes 15 to 35 years to reach adult size and could only be harvested by smashing the calcarean rocks that form its habitat) and the Italian government has declared it an endangered species since 1998 and its harvest and sale are forbidden.
Clams or shellfish are locally called chipchip and local fishermen sell those in rural markets.
Not usually considered edible:
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