Bivalve

I

Introduction

Bivalve, common name for any mollusk characterized by a shell divided into two valves hinged at one side, gills specialized for feeding, and a reduced head. Bivalves first appeared in the fossil record in the late Cambrian Period, more than 500 million years ago. More than 6000 species are known, including such familiar forms as the clam, cockle, mussel, oyster, scallop, and shipworm. All are aquatic, and most are marine, but freshwater forms are also common. Bivalves typically range in length from 1 to 10 cm (0.4 to 4 in), but the tropical giant clam reaches 1.35 m (4.4 ft) and weighs up to 200 kg (up to 440 lb).

II

Structure

The typical bivalve shell is divided into a pair of right and left valves, connected dorsally by a hinge with a flexible ligament. The valves are drawn together by one or two muscles attached to the inner surface of the valves. The shell consists of an outer chitinous layer, a middle layer of calcite or aragonite, and a laminated inner layer that in some species consists of mother-of-pearl. Two lobes of tissue, collectively called the mantle, secrete the shell and also form a spacious cavity around the body.

No distinct head exists, but at one end of the mantle cavity is a mouth, and behind the mouth is a foot used in locomotion. The foot may also secrete a bundle of fibers, called a byssus, that is used to attach the animal to a particular location—a rock, for example. A pair of gills toward the rear is used in feeding, usually by taking up small particles from the water. Two tubes, called siphons, that take in and dispose of water are usually found at the posterior end of the body. The body itself contains a gut, an open circulatory system, various reproductive and excretory organs, and a fairly simple nervous system. Scallops have simple eyes at the edge of the mantle, but usually sensation is poorly developed.

This basic body plan is variously modified. Oysters become permanently attached to a substrate (the base on which an organism lives) by one shell and lose their foot as they develop. Mussels attach to a substrate with the byssus. Scallops swim by clapping their shells together. Some bivalves burrow deep into sand or mud, and these often have long siphons. Piddocks and shipworms have the shell modified into a rasping organ with which they burrow into rock or wood.

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III

Life Cycle

The usual reproductive pattern is to have separate sexes. Sperm and eggs are released into the water, where passive fertilization occurs. The young bivalve, or larva, swims until it locates an appropriate habitat in which to mature. Sometimes, however, the larval stage is omitted, and the young are cared for within the mantle cavity of the adult.

IV

Commercial Importance

Several bivalve species are important to humans as food sources and as the prey of food fish such as the flounder and cod. Their shells are also used for many purposes, including the making of buttons. The culture of oysters is a major industry in many parts of the world, not only for food but also for the pearls their mantles secrete around intruding objects or organisms. A few bivalves are pests; shipworms, in particular, burrow into wood and damage or destroy piers and vessels.

Scientific classification: Bivalves make up the class Bivalvia in the phylum Mollusca.