Bivalve molluscks belong to the class Bivalvia (or Lamellibranchia) of the phylum Mollusca. Known by such common names as clams, mussels, cockles, oysters, and scallops, bivalves are among the most familiar aquatic invertebrates. They occur in large numbers in marine, estuarine, and freshwater habitats all over the world. More than 30,000 living species of bivalves have been described. The main divisions of the Bivalvia are the Protobranchia (the primitive nutshells), the Filibranchia (the mussels, scallops, and oysters), and the Enamellibranchia (the cockles, clams, venus shells, razor shells, and shipworms).
The name bivalve refers to the limy shell, which consists of two pieces or valves, right and left, held closed by a pair of adductor muscles, and joined together by an elastic hinge ligament. The internal, compressed body is completely enclosed by the shell valves, except for a hatchet-shaped, muscular foot, which can be extended between the lower edges of the shell so that the mollusk can burrow in soft sand or mud. The two halves of the shell are secreted by the two lobes of the body wall (the mantle), and consist of layers of calcium carbonate crystals embedded in a protein matrix.
The innermost shell layer, which is often shiny and iridescent, is called mother of pearl. If a grain of sand or other hard foreign matter gets lodged between the mantle and the shell, a layer of this pearly material is secreted around it, forming, in some species, a pearl. The space between the body wall and the mantle is known as the mantle cavity. This cavity contains a pair of large, perforated, plate-like gills that have a ciliated surface and function in both respiration and feeding. The posterior edges of the mantle lobes join to form two tubes, or siphons. The beating of the gill cilia causes water to be drawn into the mantle cavity through the lower incurrent siphon; after passing across the gills where oxygen is extracted, the water is expelled by the excurrent siphon. Bivalves lack a well-developed head, and so their sense organs (such as eyes) are located on the fringe of the mantle.
Bivalves are filter feeders, using their perforated gills as a sieve, which collects minute algae and other food particles suspended in the incoming respiratory water. These particles are trapped in strings of mucus secreted by the gills and conveyed to the mouth by cilia. Marine bivalves reproduce by releasing prodigious numbers of eggs and sperm into the water, where external fertilization occurs. The fertilized eggs then float in the surface plankton. Within 48 hours after fertilization, the embryo develops into a minute, planktonic, trochophore larvae. This stage is followed by another larval form, the veliger, which settles to the seabed and transforms into an adult. In freshwater bivalves, the eggs are retained in the gill chambers of the female, where they undergo fertilization and develop into a peculiar larval form, the glochidium. Upon its release, the larva attaches to passing fish, and lives as an ectoparasite for several weeks before settling.
Mussels and oysters do not burrow, but remain permanently fixed to a hard substrate. Mussels are attached to rocks by clumps of byssus threads. In oysters, only the left valve is cemented to the rock. Scallops are able to swim by clapping their valves and ejecting water through an opening near the hinge area to produce a jet action. Scallops also have rows of eyes on the lower edges of the mantle. Other bivalves are able to bore into limestone, clay, or wood.
Bivalves range in size from the fingernail-sized "nut shells" of the Atlantic coast of North America to the giant clam of the Indo-Pacific, which measures up to 4.9 ft (1.5 m) in length and weighs more than 495 lb (225 kg).
Bivalves are of great economic importance as a food source, and as a source of valuable products such as pearls. Some bivalves cause important economic damage. Shipworms bore into and destroy the wooden hulls of ships and wharf pilings. The zebra mussel recently colonized inland waters of North America by hitch-hiking from Eurasia in ships' ballast water. This prolific species is causing extensive damage by clogging water pipes and displacing native species of bivalve molluscs.