The Bivalves are the mussels, clams, scallops, shipworms, piddocks and oysters. There are over 15,000 species world wide (32 British freshwater species). They are
laterally compressed with a pair of shell valves hinged at the dorsal end (see drawings below). 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).
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, 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. The shells are held slightly open at rest, but can be kept closed by a pair of
powerful adductor muscles (see above), which work in opposition to the hinge ligament. When you eat a scallop it is the adductor muscle you are eating.
Bivalves sense gravity using microscopic sensors (see diagram below). A hollow chamber is lined with nerve cells bearing sensory hairs. These can detect when a mineral weight (statocyst, see below) is touching them, and so give information about the animal's orientation.
Locomotion is achieved by extending the foot (see below), which
then swells as blood is pumped into it and acts as an anchor in the sediment, the foot muscle is then shortened as the animal pulls itself towards it (see Yoldia limatula below). Some can
swim by clapping their valves together.
Below is Yoldia limatula. The foot has a flattened sole which can be folded up to push into mud, then opened to anchor as the rest of the body is pulled downwards.