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aquatic mollusk of the class Pelecypoda ("hatchet-foot") or Bivalvia, with a laterally compressed body and a shell consisting of two valves, or movable pieces, hinged by an elastic ligament. Bivalves, which include clams, cockles, mussels, oysters, and scallops, are an important food source for humans, as well as for gastropods, fish, and shore birds.

Bivalve Shells

The two valves or a bivalve shell cover the right and left sides of the animal; they are hinged dorsally (above the body) and open ventrally (below the body). Usually the two valves are similar and equal in size, but in some forms, such as the oysteroyster,
bivalve mollusk found in beds in shallow, warm waters of all oceans. The shell is made up of two valves, the upper one flat and the lower convex, with variable outlines and a rough outer surface.
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, that attach to the substratum by one valve (i.e., lying on their sides), the left-hand (or upper) valve is larger than the right-hand (or lower) one. Two muscles, called adductors, run between the inner surfaces of the two valves; acting antagonistically to the hinge ligament, they enable the shell to close rapidly and tightly.

Because of the enormous variety of sizes, shapes, surface sculpturing, and colors, shell characteristics are of great importance in the identification and classification of bivalves. Shells range in size from the tiny ( 1-16-in./2-mm) seed shells characteristic of members of the freshwater family Sphaeriidae to the giant clamgiant clam,
common name for the largest bivalve mollusk in the world, Tridacna gigas, also known as the bear's paw clam. The giant clam may weigh over 500 lb (225 kg) and attain a length of over 4 ft (120 cm). The heavy shell is coarsely fluted and toothed.
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, Tridacna, of the South Pacific, which attains a length of over 4 ft (120 cm) and may weigh over 500 lb (225 kg).

Bivalve Anatomy

Within the shell is a fleshy layer of tissue called the mantle; there is a cavity (the mantle cavity) between the mantle and the body wall proper. The mantle secretes the layers of the shell, including the inner nacreous, or pearly, layer. Sometimes a pearl is formed as a reaction to irritation, by the depositing of nacreous layers around a foreign particle. The head is much reduced, without eyes or tentacles, and a muscular hatchet-shaped foot projects from the front end of the animal, between the valves. The foot is used for burrowing, and, in some bivalves (e.g., razor clams), to swim. Many bivalves have two tubes, or siphons, extending from the rear end: one (the incurrent siphon) for the intake of oxygenated water and food and one (the excurrent siphon) for the outflow of waste products. The two tubes may be joined in a single siphon, or "neck."

The gills, suspended within a mantle cavity, are usually very large and function in food gathering (filter feeding) as well as in respiration. As water passes over the gills, tiny organic particles are strained out and are carried to the mouth. Members of the order Septibranchia, however, lack gills and feed on small crustaceans and worms.

Bivalves have a complete digestive tract; a reduced nervous system; a complete, open circulatory system with a chambered heart, arteries, veins, and blood sinuses; and excretory and reproductive organs. In most species the sexes are separate, and the eggs and sperm are shed into the water, where fertilization occurs. The larval stage is free-swimming and lacks a shell.

Bivalve Specialization

Bivalves differ in their habits: some, such as the oysters and marine musselsmussel,
edible freshwater or marine bivalve mollusk. Mussels are able to move slowly by means of the muscular foot. They feed and breathe by filtering water through extensible tubes called siphons; a large mussel filters 10 gal (38 liters) of water per day.
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, have a reduced foot and are permanently attached to a substratum; some, such as the clamsclam,
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.
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 and freshwater mussels, burrow slowly through the sand or mud using the foot; some, such as the cocklecockle,
common name applied to the heart-shaped, jumping or leaping marine bivalve mollusks, belonging to the order Eulamellibranchia. The brittle shells are of uniform size, are obliquely spherical, and possess distinct radiating ridges, or ribs, which aid the animal in
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 shells, live on or near the surface of the ocean floor; still others, such as the shipwormshipworm
or teredo
, marine bivalve mollusk of the family Teredinidae, specialized for boring in wood. A shipworm is not a worm, but a greatly elongated clam. Its two shells, enclosing only the front end of the body, function as a tool, rather than a protective
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, burrow through rocks or wood seeking protected dwellings and do damage to rock pilings and other marine installations. The scallopsscallop
or pecten,
marine bivalve mollusk. Like its close relative the oyster, the scallop has no siphons, the mantle being completely open, but it differs from other mollusks in that both mantle edges have a row of steely blue "eyes" (which use a mirror consisting of
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 swim with great speed by suddenly clapping the shell valves together and ejecting water from the mantle cavity. Bivalves that are exposed at low tide, such as the marine mussels, keep their gills wet with water retained in the mantle cavity.


Bivalves are classified in the phylum MolluscaMollusca
, taxonomic name for the one of the largest phyla of invertebrate animals (Arthropoda is the largest) comprising more than 50,000 living mollusk species and about 35,000 fossil species dating back to the Cambrian period.
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, class Pelecypoda or Bivalvia.

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(invertebrate zoology)
The common name for a number of diverse, bilaterally symmetrical animals, including mollusks, ostracod crustaceans, and brachiopods, having a soft body enclosed in a calcareous two-part shell.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. any marine or freshwater mollusc of the class Pelecypoda (formerly Bivalvia or Lamellibranchia), having a laterally compressed body, a shell consisting of two hinged valves, and gills for respiration. The group includes clams, cockles, oysters, and mussels
2. of, relating to, or belonging to the Pelecypoda
3. Biology having or consisting of two valves or similar parts
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
2013) dealt with bivalve shell and chemical elements, and considered the only barium/calcium ratios in two bivalves species collected from several sites in the San Francisco Bay (USA) and Oosterschelde estuary (The Netherlands).
The information gained from this study suggests the presence of substantial interspecies variation in the immune responses of marine bivalves and highlights the need for additional studies to fully characterize these differences.
melanogaster, and shell matrix proteins in bivalves. Immunodetection was not performed for the aforementioned bivalve ECM components because specific antibodies were lacking for them, and the bivalve larvae were small and difficult to cultivate.
Nor would the pattern of over- and underrepresentation of bivalve species at different collection sites support prey selection by oyster drills or moon snails.
We have included observations relating to reproduction of bivalves reported in various international publications by scientists in Europe, mainly the Netherlands, because some aspects are more extensive than those reported in the United States.
The model, described in ( a study published in the latest edition of the Proceedings of the National Academy of Sciences, is based on an extensive database of 5,744 marine bivalve species.
Molluscs from marine habitat of India, belonging to 220 families and 591 genera, of which 1900 are gastropods, 1100 are bivalves, 210 are cephalopods, 41 are polyplacophores and 20 are scaphopods [55].
Continuous layers (as opposed to isolated patches) of calcite are found in two major clades of bivalves: the pteriomorphs and the euheterodonts.
(Note: in polluted waters, bivalve mollusks unfortunately take in pollutants at the same rate at which they take in nutrients.
Sandy beaches are inhabited by burrowing bivalves and gastropods.
Diet Analysis--Contents of freshwater drum stomachs and intestines yielded four broad categories of prey items: insects, bivalves, crayfish, and fish.