Bivalves are kinds of molluscs with flattened bodies contained in a shell that has two hinged parts. They include clams, cockles, oysters, scallops, mussels, razor shells and many others. Some, for example the cockles, are rounded, while razors have elongated shells and shipworms are long and thin. Most bivalves are filter feeders: they use their special gills for both feeding and breathing. To protect themselves from predators, they either bury themselves in the sand or mud on the seabed, or attach themselves to rocks. Some, such as shipworms, bore into wood. Others, including scallops, swim away from danger.

Structure

The shell of a bivalve is composed of calcium carbonate, and consists of two parts, called valves. The valves are connected along one edge by a hinge made of a flexible ligament.

A bivalve has a mouth, heart, intestine, gills, stomach and siphon, but no head. Some species, for example, scallops, have eyes. A special muscle holds the two halves of the shell together. A single, muscular foot is used either to anchor its body to a rock, as, for example, in clams, or to dig down into the sand, as in a razor shell, or to swim, as with a scallop.

Feeding and breathing

Bivalves have a pair of gills inside their valves. Water is sucked in and washes through the gills. Here, food particles—plankton—are filtered from the water and passed into the mouth. Some species have two siphons: one which reaches out of the shell to to take in water and another to expel waste.

How bivalves live

Most bivalves are live under the seabed, buried in sand, mud or coral fragments. Here they are protected both from predators and from being destroyed by the force of the waves. Many bivalves live in the intertidal zone, where the sediments remain damp while the tide is out. They extend their siphons to feed and breathe during high tide, but keep their shell tightly shut when the tide goes out, using their foot to burrow deeper into the sediments if necessary.

Other bivalves, such as mussels, attach themselves to rocks. They are much more exposed to predators than their burrowing relatives. Whelks and other gastropods feed on them by boring into their shells and sucking out their bodies. Crabs use their pincers to crack their shells open. Starfish use their tube feet to prise the valves apart and then push their stomachs inside them to digest the contents.