Bivalves

Bivalves belong to the invertebrate phylum Mollusca, which also includes snails, squids, and octopuses. Some well-known bivalves include clams, scallops, mussels, and oysters. More than 15,000 species of bivalves exist. All bivalves are aquatic, encompassing both marine and fresh-water species.

Characteristics of Bivalves

The name "bivalve" refers to the two-part shell that characterizes these mollusk species. The two halves of the shell are joined by a ligamentous hinge and held shut by a pair of strong adductor muscles. The shell is made of calcium carbonate and is secreted by the mantle (soft body wall). Shells grow with the organisms, extending out from the hinge area. Most bivalve species go through a free-swimming larval stage before taking on their characteristic adult form and lifestyle.

Most species of bivalves are filter feeders. Currents of water are drawn into the body and through the gills, where tiny food particles are caught in the gill mucus. This flow of water also functions in respiration, allowing organisms to obtain fresh oxygen. A few bivalve species are predatory, including some deep-sea scallops.

Bivalves make use of a variety of lifestyles. Sedentary species (e.g., mussels and oysters) spend their lives attached to a substrate , whereas others burrow underground (e.g., clams) or live on the water bottom and swim (e.g., scallops). Bivalves have highly reduced heads and simple nervous and sensory systems. Most species have some chemosensory cells at the edge of the mantle that are used to detect chemical signals in the water. Some also have simple eyes. Bivalves have an open circulatory system.

A number of bivalve species have some commercial value, including representatives of all the groups discussed here. Bivalves are valued as food sources, and throughout history their shells have been used for many purposes. Most buttons were made from shells before the advent of plastic. In addition, shell fragments have been used for everything from roads to fertilizer. Finally, the beautiful pearls found in oysters have always been treasured.

At certain times, some mollusks can become toxic to animals and humans who consume them. Toxins produced by certain algae during a bloom (rapid population growth) bioaccumulate in the tissue of mollusks, and then may be ingested by higher-level consumers who eat the mollusks.

Clams

Clams live underground, using a muscular foot to dig down into sand or mud. They take in water for filter feeding and gas exchange through an extended part of the body called the siphon, or neck. The siphon is also used to disperse eggs or sperm. In some species, such as the geoduck clam, the siphon is extremely long, allowing the clam to remain safe deep underground. The geoduck is the world's largest burrowing clam, and can live up to 145 years.

Common predators of clams include starfish and eels. Defensive behavior involves retracting the body and closing the shell as tightly as possible.

The giant clam can grow to lengths of 1.2 meters (4 feet) and is unusual in that it harbors algae (e.g., dinoflagellates) within its tissues. The algae obtain shelter and protection from their host, while the clam obtains important nutrients that are products of algal photosynthesis .

Scallops

Scallops are unusual among bivalves in that they are capable of jet-propelled swimming. Sudden contraction of the adductor muscles quickly closes the shell, causing water to be ejected on either side of the hinge. Scallops thus swim in short spurts.

Scallops generally live on sand bottoms. They may attach themselves to rocks but are able to detach and swim to a new location. Jet propulsion also is used as an escape response to avoid potential predators, such as starfish, snails, and fish. The large, well-developed adductor muscles represent the edible part of the scallop.

Scallops are also unusual among bivalves in that their eyes are well developed. They have a series of eyes around the edge of the shell that are critical in helping to detect predators. The importance of eyes is likely related to scallops' unique locomotor capabilities among bivalves.

Scallops often occur in dense congregations known as scallop beds. These are sometimes temporary, but permanent beds do occur in areas with optimal temperature conditions and food availability. Commercial fishing of scallops has diminished many natural populations, and the use of scallop farms has helped meet consumer demand.

Mussels

Mussels are sedentary bivalves. They attach themselves to a firm substrate using secreted threads known as byssal threads, which are produced by the byssal organ of the muscular foot. Mussels frequently occur in large colonies, forming mussel beds. Like other bivalves, they are filter feeders.

Fresh-water species of mussels occur in streams and rivers. Unlike marine bivalve larvae, which are free-swimming, the larvae of fresh-water mussels are parasitic , most often on fish. Numerous species of fresh-water mussels are endangered, including roughly half the species that occur in North America.

The zebra mussel is an invasive fresh-water species that was accidentally introduced into North America in the 1980s from its original European habitat. It spread quickly through eastern North America. The zebra mussel continues to have a detrimental impact on water supplies by clogging intake pipes and other structures, and much effort continues to go into controlling its spread. Zebra mussels can be spread to new waterbodies by fishing activities and by boats, to which they often attach.

Oysters

Oysters are sessile (nonmotile) bivalves that live attached to substrates such as rocks. Like many other bivalves, they occur in beds and are filter feeders. Oyster shells tend to be rough on the outside but smooth on the inside.

Oysters are sequential hermaphrodites: that is, they are spermproducing males when young and then become female egg producers when older. The largest oysters reach sizes of up to about 38 centimeters (15 inches). Oysters grows continually throughout life and may live as long as 100 years.

Several species of oysters are valued as delicacies, and some are cultivated for food. However, oysters are perhaps best known for their pearls. (All bivalves make pearls, but those of oysters are particularly valued.) When small irritants become lodged within the shell, the oyster deposits layers of pearly material around the irritant. This material is identical to that used to line the inside of the oyster's shell.

see also Algal Blooms, Harmful; Cephalopods; Great Lakes; Mariculture; Pollution by Invasive Species.

Jennifer Yeh

Bibliography

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

Gould, James L., and William T. Keeton, with Carol Grant Gould. Biological Science, 6th ed. New York: W. W. Norton & Co., 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: Wm. C. Brown, 1994.

bivalves The phylum Mollusca is extremely diverse and contains a number of classes that at first sight can appear to be so different as to be unrelated. One of these is the bivalves (sometimes called pelecypods or lamellibranchs), an important group of bivalved molluscs familiar to all from the numerous shells that litter beaches. Bivalves are aquatic suspension-feeders, inhabiting a variety of infaunal and epifaunal habitats (i.e. living in and on the bottom) and are particularly characterized by their ability to burrow, some of them even into rock and wood. About 50 000 species have been described, of which about 10 000 are modern. It is their adaptability that has made them so successful today, although their geological history goes back into the Early Cambrian.

All bivalves have two shells or valves (Fig. 1a) that are usually virtual mirror images of each other about the commissure or edge along which the valves are in contact. The valves are connected to each other by a ligament which forms a hinge and is instrumental in opening the valves. Inside the hinge area the valves feature a set of teeth and sockets, the dentition, which is extremely variable in morphology and ensures a tight fit when the valves close. In addition the valves may show large internal scars from the attachments of the adductor muscles that operate to close the valves, and a linear scar parallel to the margin of the valves termed the pallial line. This is the line along which the muscles of the inner muscular part of the mantle are attached, the mantle being a sheet of tissue that forms the inner lining of the shell and secretes it. The mantle encloses the mantle cavity, and within this lie the gills, elongated ciliated feeding structures termed the palps, and the stomach. In burrowing forms a muscular foot projects outside the valves, and a siphon formed from the mantle projects to the surface (Fig. 1b). The presence of the siphon can be recognized by a deflection in the pallial line termed the pallial sinus.

Bivalve classification has always been a difficult problem because shell shape is often closely related to life habits, and important taxonomic characters are often present in the soft parts which do not get preserved. Virtually every organ system and hard-part structure has been used as the basis for grouping, and for many years a classification based on the gills was used. This has now been superseded by a multi-character classification in which hard-part structures, particularly the dentition, are important.

The shape and general morphology of bivalve shells directly reflects their mode of life; hence our understanding of the ways in which modern bivalves live enables us to make inferences about the way in which fossil forms lived. Many bivalves are shallow or deep burrowers, using the muscular foot to pull themselves through the sediment while rocking motions of the valves also help the digging process. Deep burrowers generally have elongate streamlined shells that enable them to move rapidly, while shallow burrowers tend not to have elongate shells. These infaunal bivalves live within the sediment for protection and remain connected to the surface by their siphons. Attached epifaunal bivalves are also common; they may be attached by a mass of threads, the byssus, as in the mussel, or be cemented down as in oysters. An extinct group of Mesozoic bivalves, the rudistids, became closely adapted to a cemented mode of life: one valve became conical while the other formed a lid. In some areas these animals were numerous enough to form reefs. In this way they parallel some Late Palaeozoic brachiopods and, remotely, even the corals. Swimming is a way of life adopted by some modern bivalves, such as the scallops, which are free-lying on the sea floor. Rapid opening and closing of the valves by the large adductor muscle allows the animal to eject water forcefully from the mantle cavity and can move it erratically just above the sea floor. This is an exhausting activity for the animal and it is used only to escape predators such as starfish. Some bivalves are adapted for life in hard substrates, boring into rock and wood by rocking and scraping actions of the valves and, in some instances, by using corrosive secretions.

Bivalves first occur in the Early Cambrian of Australia, North America, Denmark, and Siberia. These are extremely small (1 mm) and show similarities to an extinct molluscan class, the rostroconchs, which appear to have been ancestral to them. The rostroconchs were bivalve-like but possessed no hinge; the bivalves seem to have developed from them by reducing mineral deposition and increasing the deposition of ligamentous tissue in the hinge area. During the Ordovician the major groups of bivalves appeared and burrowing and bysally attached forms developed during a rapid burst of adaptive radiation. Although bivalves were less important than brachiopods (which are also bivalved benthonic suspension-feeders) during the Palaeozoic they were able to displace them in the early Mesozoic after the Permo–Triassic extinction event. This was mainly due to their ability to exploit infaunal habitats as a result of their development of a muscular foot and siphons formed by fusion of the mantle edges. At this time they appear to have first extensively colonized intertidal habitats—environments that they inhabit to the present day.

The bivalves have a long time-range and their biostratigraphic utility is limited. They are, however, used locally in Pennsylvanian (Late Carboniferous) coal measures, and in the Upper Cretaceous.

David K. Elliott

Bibliography

Boardman, R. S., Cheetham, A. H., and Rowell A. J. (eds) (1987) Fossil invertebrates. Blackwell Scientific Publications, Oxford.
Morton, J. E. (1967) Molluscs. Hutchinson, London.

bivalve 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 oyster , 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 clam , 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 mussels , have a reduced foot and are permanently attached to a substratum; some, such as the clams and freshwater mussels, burrow slowly through the sand or mud using the foot; some, such as the cockle shells, live on or near the surface of the ocean floor; still others, such as the shipworm , burrow through rocks or wood seeking protected dwellings and do damage to rock pilings and other marine installations. The scallops 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.

Classification

Bivalves are classified in the phylum Mollusca , class Pelecypoda or bivalvia.

bivalve Animal that has a shell with two halves or parts hinged together. The term most usually applies to a class of molluscs – Pelecypoda or Lamellibranchiata – with left and right shells, such as clams, cockles, mussels, and oysters. It also refers to animals of the phylum Brachiopoda with dorsal and ventral shells. Length: 2mm–1.2m (0.17in–4ft).

bivalve (by-valv) adj. consisting of or possessing two valves or sections. b. cast a plaster cast that is cut into anterior and posterior sections to monitor pressure beneath the cast. b. speculum a vaginal speculum that has two blades.

bi·valve / ˈbīˌvalv/ • n. an aquatic mollusk (class Bivalvia) that has a compressed body enclosed within two hinged shells, including oysters, clams, mussels, and scallops.

bivalve XVII. f. BI- + VALVE.

bivalves See BIVALVIA.

bivalve •multivalve, salve, valve •lipsalve • check valve • univalve •bivalve •delve, helve, shelve, twelve •absolve, devolve, evolve, exsolve, involve, revolve, solve