The Bivalves are the
mussels, clams, scallops, shipworms, piddocks and oysters. There are over 15,000 species world wide.
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 right) 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 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.
Pecten maximus, the great
scallop (right) 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 (strong hair-like filaments), 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.
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
The Royal cloak shell, Gloripallium pallium is right. It is in the Pectinidae family, and is found in the Indo-Pacific region.
Below 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 photosynthesize. 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.
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.
Below right is the American thorny oyster, Spondylus americanus, which is found in the Caribbean, and the Atlantic coast from North Carolina down to Brazil between 9 and 45 m deep. It can reach 10 cm in diameter.
The lower shell is flat and attached to the substrate. It tends to remain in the same place. In the past it was widely traded in South America as it was used for decoration.
On the right is the Leather donax, Hecuba scortum, found in the littoral zone, usually in mud and sand in Central Indo-Pacific and South China Sea. Its length can reach 9 cm. The inner surface is a beautiful smooth, glossy violet/pink.