BIVALVE

class name Bivalvia, member of a class of mollusks characterized by having a two-valved shell. Bivalves include such mollusks as clams, cockles, mussels, oysters, razor shells, and shipworms. They are characterized by laterally compressed shells consisting of two halves, or valves, joined by an elastic ligament. The ligament opens the valves, which are closed by the action of one or two transverse adductor muscles that join the valves. The animal is generally totally enclosed by the shell and by two sheets of tissue called the mantle. Bivalves have no head, and most of their sense organs are concentrated in the mantle margins. Most species feed on suspended phytoplankton by using a large pair of specialized gills that lie on either side of the animal and are folded into lamellae (platelike structures) composed of filaments. Large volumes of water are pumped across the gills by ciliary action, and trapped food particles are transported toward the mouth by specialized cilia. The food is then further sorted by special organs called palps before it is ingested. Some other bivalves feed on organic material in sediment, and a few others are carnivores, subsisting on small crustaceans. Many bivalves burrow into soft substrates; they penetrate the sediment by probing and anchoring with a muscular foot and then rapidly closing the valves, thus fluidizing the sediment. Most burrowing bivalves have inhalent and exhalent siphons through which water passes to and from the sediment surface. The siphons consist of muscular mantle tissue that has been fused into extensible tubelike structures, which in some bivalves (e.g., the geoduck) can extend more than 1 m (3 feet) in length. Other bivalves live on the substrate surface and are attached by tough, horny threads (the byssus), which are secreted by a gland in the foot. In oysters and a number of other bivalves, actual cementation of one valve to the substrates occurs. Several groups of bivalves can bore into hard substrates. These include the date mussel (Lithophaga), which bores into limestone by chemical means, and the piddock and its relatives (Pholadidae), which penetrate rock by mechanical scraping. The shipworm (Teredo) bores into and digests wood. A few bivalves, such as certain scallops (Pecten and Chlamys) and file shells (Lima), are capable of swimming by rapidly flapping the valves together as an escape reaction to predators. In most bivalves, male and female reproductive organs occur in separate individuals, but some are hermaphroditic (i.e., have the organs of both sexes in the same individual). Still others experience sex changes through life. Both the eggs and sperm are shed into the water, and larval development usually takes place in the open sea. Bivalve larvae drift among plankton for several weeks before settling to the sea bottom. There are about 8,500 species of marine bivalves. They are found in most parts of the ocean from the intertidal zone to abyssal depths. The largest bivalve is the giant clam Tridacna gigas, which reaches 137 cm (about 55 inches) in length. The Unionacea, consisting of freshwater mussels, is particularly diverse and abundant in eastern North America, with many endemic species inhabiting individual river systems. For more information on members of the bivalve group, see clam ; cockle; mussel; oyster; scallop; shipworm. also called Pelecypoda any member of the class Bivalvia, of the phylum Mollusca, characterized by a shell that is divided from front to back into left and right valves. Enclosure in a shell has resulted in loss of the head. Similarly, the adoption of deposit-feeding using labial palps and, later, suspension feeding utilizing the respiratory gills modified into organs of filtration called ctenidia have resulted in loss of the radula from the mouth. Bivalves include clams, oysters, and shells. Additional reading R.D. Purchon, The Biology of the Mollusca, 2nd ed. (1977), contains essays on various aspects of bivalve ecology and physiology. Raymond C. Moore (ed.), Treatise on Invertebrate Paleontology, pt. N, Mollusca 6: Bivalvia, by L.R. Cox et al., 3 vol. (196971), provides a comprehensive review of bivalve structure, history, and classification. Steven M. Stanley, Relation of Shell Form to Life Habits in the Bivalvia (Mollusca) (1970), investigates the relationships between shell structure, form, habitat, and life-style.