Bivalvia - clams, mussels, oysters, scallops etc.

Bivalvia overview

The Bivalves are the mussels, clams, scallops 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 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 right). 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.

above, a statocyst (gravity detector)

Bivalve locomotion

Locomotion is achieved by extending the foot (see left), 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.

On the left 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.

Giant clam

On the right 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.

On the left is the Fluted giant clam, or Scaled clam, Tridana squamosa.

The fluted giant clam is found on sandy bottoms around coral reefs at 15 - 20 m deep in the Indo-Pacific, and has been introduced to Hawaii. The fluted edges are called scutes, and are used by crabs and other small invertebrates as shelter. It grows up to 40 cm across.

As mentioned above it gets most of its food from a symbiotic relationship with a single celled algae that colonises its mantle. The clam extends the mantle during the day for the algae to photosynthesise. This species is popular in large aquaria. It has been collected and traded for human consumption, but is now bred in captivity.

On the left is the edible American east coast oyster, Crassostrea virginica. The lower valve is usually larger than the upper valve.

The common cockle (Cerastoderma edule) shown right is found just below the surface at low tide.

When covered by water it extends two papillate siphons just above the surface of the sand or mud and filter feeds by ciliary action. Much of the material taken in is fine sand, so before cooking cockles must be kept in clean sea water so that the fine sand particles are ejected by the cilia.

On the left is an oyster shell where instead of forming a pearl round a piece of trapped grit a small fish has been trapped and preserved forever as a fish-shaped pearl. An oyster can produce as many as 50 million eggs a year.

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, and can live fore 190 years. Large pearls are rare; 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 miniature mussel drops off the fish and, hopefully, lands in mud.