Class Bivalvia

Bivalvia are also known as Pelecypoda (pel-e-sip o-da), or “hatchet-footed” animals, as their name implies (Gr. pelekys, hatchet, pous, podos, foot). They are bivalved molluscs that include mussels, clams, scallops, oysters, and shipworms and they range in size from tiny seed shells 1 to 2 mm in length to giant South Pacific clams, Tridacna, which may reach more than 1 m in length and as much as 225 kg (500 pounds) in weight. Most bivalves are sedentary filter feeders that depend on currents produced by cilia on their gills to gather food materials. Unlike gastropods, they have no head, no radula, and very little cephalization. Most bivalves are marine, but many live in brackish water and in streams, ponds, and lakes.

Freshwater clams were once abundant and diverse in streams throughout the eastern United States, but they are now easily the most jeopardized group of animals in the country. Of more than 300 species once present, nearly two dozen are extinct, more than 60 are considered endangered, and as many as 100 more are threatened. A combination of causes is responsible, of which the damming and impoundment of rivers is likely the most important.

Pollution and sedimentation from mining, industry, and agriculture are other important culprits. Poaching to supply the cultured pearl industry is also a significant contributor. And in addition, introductions of exotic species make the problem worse. For example, the prolific zebra mussels attach in great numbers to the native clams, exhausting food supplies (phytoplankton) in the surrounding water.

Bivalves are laterally compressed, and their two shells (valves) are held together dorsally by a hinge ligament that causes the valves to gape ventrally. The valves are drawn together by adductor muscles that work in opposition to the hinge ligament. The umbo is the oldest part of the shell, and growth occurs in concentric lines around it.

Pearl production is a by-product of a protective device used by the animals when a foreign object (grain of sand, parasite, or other) becomes lodged between the shell and mantle. The mantle secretes many layers of nacre around the irritating object. Pearls are cultured by inserting particles of nacre, usually taken from the shells of freshwater clams, between the shell and mantle of a certain species of oyster and by keeping the oysters in enclosures for several years. Meleagrina is an oyster used extensively by the Japanese for pearl culture.

The visceral mass is suspended from the dorsal midline, and the muscular foot is attached to the visceral mass anteroventrally. The ctenidia hang down on each side, each covered by a fold of the mantle. The posterior edges of the mantle folds are modified to form dorsal excurrent and ventral incurrent openings. In some marine bivalves the mantle is drawn out into long muscular siphons that allow the clam to burrow into the mud or sand and to extend the siphons to the water above.

Locomotion Bivalves initiate movement by extending a slender muscular foot between the valve. They pump blood into their foot, causing it to swell and to act as an anchor in the mud or sand; then longitudinal muscles contract to shorten the foot and pull the animal forward. Scallops and file shells swim with a jerky motion by clapping their valves together to create a sort of jet propulsion. The mantle edges can direct the stream of expelled water, so that the animals can swim in virtually any direction.

Gills Gaseous exchange occurs through both mantle and gills. Gills of most bivalves are highly modified for filter- feeding; they are derived from primitive ctenidia by a great lengthening of filaments on each side of the central axis. As ends of long filaments became folded back toward the central axis, ctenidial filaments took the shape of a long, slender W. Filaments lying beside each other became joined by ciliary junctions or tissue fusions, forming platelike lamellae with many vertical water tubes inside. Thus water enters the incurrent siphon, propelled by ciliary action, then enters the water tubes through pores between the filaments in the lamellae, proceeds dorsally into a common suprabran chial chamber, and then out the excurrent aperture.

Most bivalves are filter feeders. Respiratory currents bring both oxygen and organic materials to the gills where ciliary tracts direct currents to the tiny pores of the gills. Gland cells on the gills and labial palps secrete copious amounts of mucus, which entangles particles suspended in water going through gill pores. These mucous masses slide down the outside of the gills toward food grooves at the lower edge of the gills. Heavier particles of sediment drop off the gills as a result of gravitational pull, but smaller particles travel along the food grooves toward the labial palps. The palps, being also grooved and ciliated, sort the particles and direct tasty ones encased in the mucous mass into the mouth.

Some bivalves, such as Nucula and Yoldia, are deposit feeders and have long proboscides attached to the labial palps. These can be protruded onto sand or mud to collect food particles, in addition to particles attracted by gill currents.

Shipworms burrow in wood and feed on particles they excavate. Symbiotic bacteria live in a special organ in the bivalve and produce cellulase to digest wood. Other bivalves such as giant clams gain much of their nutrition as adults from the photosynthetic products of symbiotic dinoflagellates living in their mantle tissue. Septibranchs, another group of bivalves, draw small crustaceans or bits of organic debris into the mantle cavity by sudden inflow of water created by the pumping action of a muscular septum in the mantle cavity.

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Useful External Links

• bivalve Brian Morton
• Class Bivalvia: The Wonders of Bivalves, Their Shells & Anatomy by Gordon Ramel
• Bivalvia by Erle G. Kauffman