Bivalvia - clams, mussels, oysters, scallops etc.

bivalve. It can be over 1.5 m across and weigh over 225 kg.

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.

Some freshwater mussels and clams can live to be over 100 years old.

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 left and right), 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 above left). A hollow chamber is lined with nerve cells bearing sensory hairs. These can detect when a mineral weight (statocyst, see above) is touching them, and so give information about the animal's orientation.

Bivalve locomotion

Locomotion is achieved by extending the foot (see left and below right), 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 on the left). Some can swim by clapping their valves together.

Above 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.

Scallops

Pecten maximus, the great scallop (left) can grow as large as 15 cm across. Along the edges of its mantle is has finger-like sensory processes, and between many of these are eyes.

During its early life it is attached to the substrate by a byssus (see the diagram of a mussel below), but later in life it is free living and is one of the species of bivalves which can "swim" by clapping its valves (shells) together.

Giant clam

Above is the giant clam, Tridacna gigas, the largest bivalve. Clams in the Tridacna genus usually have a symbiotic relationship with algae which live on the mantle tissue and provide an extra source of nutrition.

Mussels

Mussels attach themselves to surfaces by a thin guy rope called a byssus (see right). These are placed on the anchoring surface by the foot (see left), and enable the mussel to withstand battering from the sea.

The scar inside an empty mussel shell shows where the adductor muscle pulling the two shells together anchors.

Mussels are both freshwater (see below) and marine, and are filter feeders. Water enters through the syphon. Compare the very muscular foot of the common cockle (top) with that of the mussel. The mussel uses its foot much less. Mussels often live in large groups and can cause problems for coastal power stations by blocking the pipes of their cooling systems.

Above is Margaritifera margaritifera, the pearl mussel, a freshwater mussel. Fully grown specimens can reach 150 mm long. Most pearls are around 2 mm in diameter. It is found in large, fast rivers with sand or gravel beds in which it buries itself. Recently it has declined through pearl collecting.

On the right is Anodonta cygnea, the swan mussel. The shell when fully grown can be 15 cm long and olive brown. It is found in firm mud in canal bottoms, slow river and lakes.

The eggs are produced from June to August, as many as 500 000 per individual. They are kept in the outer gills which now become brood pouches, and are fertilised by sperm carried on a water current from a nearby male.

After 9 months they leave the brood pouch and become parasites of fish for 3 months. When a minature mussel drops off the fish and, hopefully, lands in mud.