clam

clam, common name for certain bivalve mollusks, especially for marine species that live buried in mud or sand and have valves (the two pieces of the shell) of equal size. The oval valves, which cover the right and left sides of the animal, are hinged together at the top by an elastic ligament. Clams burrow by means of a muscular foot, located at the front end, which can be extruded between the valves. The head, located within the shell, is rudimentary, without eyes or antennae. Water containing oxygen and food particles enters through an incurrent siphon; waste-containing water is expelled through an excurrent siphon. The two tubes project from the end opposite the foot and may be united in a single structure called the neck. The sexes are usually separate. Eggs and sperm are deposited in the water; the fertilized egg develops into a free-swimming larva without a shell, which may not attain the adult form for several months.

Clams are highly valued as food. The soft-shell clam, or steamer (Mya arenaria), of both coasts of North America, is one of the most popular eating clams. The hard-shell clam (Mercenaria mercenaria), also known as the northern quahog, is abundant from the Gulf of St. Lawrence to Texas. The name quahog is from the Naragansett; some Native Americans used the violet portion of the shell for wampum. Small hard-shell clams are called littlenecks; somewhat larger ones, cherrystones. The ocean quahog (Artica islandica) is among the longest-lived animals; one was estimated to be between 405 and 410 years old in 2007. The razor clam (Ensis), shaped like an old-fashioned straight razor, burrows rapidly and swims by means of its foot. The Atlantic razor clam, found from Labrador to W Florida and prized for its flavor, may attain lengths of 10 in. (25 cm). The Eastern surf clam (Spisula solidissima) frequents sandy bottoms in shallow water from Labrador to North Carolina and is much used for bait. There are also several Pacific surf clams. Other Pacific clams include the succulent Pismo clam (Tivela stultorum), found from mid-California southward and protected by law from overdigging, and the geoduck of the Pacific Northwest, which may weigh as much as 12 lb (5.4 kg). The valves of many small clams are familiar seashells, such as those of the pea-sized amethyst gem clam. The giant clam of the S Pacific Ocean may reach a weight of 500 lb (227 kg) and a length of 5 ft (150 cm).

There are two families of freshwater bivalves called clams. The small freshwater clams (family Sphaeriidae) are hermaphroditic; they retain the fertilized eggs in a brood pouch and bear young with shells. The large freshwater clams (family Unionidae) are also called freshwater mussels; the nacreous inner layer of their shells is a source of mother-of-pearl. The larvae of these clams are parasitic on the gills of fish.

The term clam is sometimes used synonomously with bivalve; in this sense it includes the oysters, scallops, and marine mussels. Clams are classified in the phylum Mollusca, class Pelecypoda or bivalvia.

Any of several species of marine clams (family Solenidae) common in intertidal sands and muds, particularly of temperate seas. Razor clams have narrow and elongated shells (shaped like straight razors) up to 8 in. (20 cm) long. A large active foot enables them to move rapidly up and down within their burrow and retreat quickly when disturbed. With their short siphons (tubes) they feed on particulate material in seawater. Some species can swim short distances by jetting water through their siphons.

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Any clam of the genus Donax, inhabiting sandy beaches worldwide. Coquinas are very active; they migrate up and down beaches with the tide and can reburrow between waves. They have short siphons and feed on suspended plant material and detritus. A typical species, D. variabilis, measures about 0.4–1 in. (10–25 mm) in length. Its shell is wedge-shaped and varies from white, yellow, and pink to blue and mauve.

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In general, any bivalve mollusk. True clams, in the strict sense, have equal shells, closed by two opposing muscles, and a powerful, muscular, burrowing foot. They usually lie buried in the sand in shallow marine waters. Clams draw in and expel water for respiration and feeding through two tubes, the siphons. Species range in size from 0.004 in. to 4 ft (0.1 mm–1.2 m) across. Many species are edible, including the coquina clam, geoduck, quahog, and soft-shell clam.

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Clam is a word which can be used for all, some, or only a few species of bivalve mollusks; the word is a common name which has no real taxonomic significance in biology. It is however quite widely used as part of the common names of bivalves, and also has significance in fisheries and cuisine.

In the USA, the word "clam" can be used in several different ways: one is as a general term covering all bivalve mollusks. 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 edible marine bivalves, as in the phrase clam chowder for a thick shellfish soup usually made using the hard clam. Many edible bivalves have a more-and-less 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 is not as widely used: it forms part of the common names of various species of bivalve mollusk, 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.

The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine 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. Again, in the USA, clam diggers is a term that can mean people who are searching for clams, or it can also mean one variety of three-quarter length pants or trousers.

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, but 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.

Anatomy

A clam's shell consists of two (usually equal) valves, which are connected by a hinge joint and a ligament which can be external or internal.

In most 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 mollusks, also have open circulatory systems, which means that their organs are surrounded by watery blood that contains nutrients and oxygen.

Clams eat plankton by filter feeding, and they themselves are eaten by small sharks and squid.

Human use

As food items

In culinary use, within 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.

Clams can be eaten raw, steamed, boiled, baked or fried; the method of preparation depends partly on the size and species. 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.

In an aquarium

The Maxima clam Tridacna maxima, a species of giant clam, is a popular species with saltwater aquarium hobbyists.

In a religious context

The Moche people of ancient Peru worshiped the sea and its animals. They often depicted clams in their art.

Some examples of clams

Edible:

• Grooved carpet shell: Ruditapes decussatus
• Hard clam or Northern Quahog: Mercenaria mercenaria
• Soft clam: Mya arenaria
• Surf clam: Spisula solidissima
• Ocean quahog: Arctica islandica
• Pacific razor clam: Siliqua patula
• Pismo clam: Tivela stultorum (8 inch shell on display at the Pismo Beach Chamber of Commerce)
• Geoduck clam: Panopea abrupta or Panope generosa (largest burrowing clam in the world)
• Atlantic jackknife clam: Ensis directus

Not usually considered edible:

• Ark clams, family Arcidae
• Nut clams or pointed nut clams, family Nuculidae
• Duck clams or trough shells, family Mactridae
• Marsh clams, family Corbiculidae
• File clams, family Limidae
• Giant clam: Tridacna gigas
• Asian or Asiatic clam: genus Corbicula
• Peppery furrow shell: Scrobicularia plana

See also

• Clam bake
• Clam boil
• Fried clams
• Clam Chowder

References

External links

• Nutrition Facts for Clams
• Deep In The Ocean A Clam That Acts Like A Plant Science Daily March 2 2007
• How to Shuck Hard Shell Clams