The Bivalves are the
mussels, clams, scallops, shipworms, piddocks and oysters. There are over 15,000 species world wide. They are
laterally compressed with a pair of shell valves hinged at the dorsal end (see drawings right). The head is greatly reduced;
they have no radula or tentacles, and most are without eyes, although some have
eyes at the margins of the mantle (see Pecten maximus below).
They are mainly sedentary filter feeders, have paired gills, and range in size
from 1 mm to over 1 m. The giant clam, Tridacna gigas, below, is the largest bivalve. It can be over 1.5 m across and weigh over 225 kg. The oldest clam ever found was estimated to be around 400 years old, and some freshwater mussels and clams can live to be over 100 years old.
Edible oysters and other bivalves have been an important food source of man since prehistoric times.
Bivalves are mainly marine, with a few freshwater species.
The sexes are separate, although some may be hermaphrodite.The two shells are held together by a dorsal hinge, the ligament can look like
glue oozing out between the two hinges (see Pecten maximus below).
The shells are held slightly open at rest, but can be kept closed by a pair of
powerful adductor muscles (see right), which work in opposition to the hinge ligament. When you eat a scallop it is the adductor muscle you are eating.