Clams, oysters, mussels and shipworms are members of the class Pelecypoda. There are around 15,000 living species, all are aquatic. In these mollusks the mantle cavity has been greatly enlarged, and the gills serve as a food sorting organ as well as for respiration. In some clams the gills also serve as a brood chamber for developing young. The edges of the mantle in a clam are partly fused to form a pair of siphons that pump water through the mantle cavity. The food and oxygen are removed from the water as it passes over the gill filaments. The head region has atrophied, the radula is totally absent, and the body is laterally flattened. The shell has two halves or valves that are connected by a flexible ligament. The ligament naturally opens the valves while the animals two muscles are used to shut the valves. In many clams the foot has been modified for burrowing. In mussels it secreates fibers that hold the animal to the substrate. Pelecypods have become adapted to such diverse habitats as boring into wood or rock, fastening by fibers or cementing the shell to a fixed surface, burrowing into bottom silt, or a few that swim by opening and closing their valves. They have a wide variety of shapes, sizes, color and shell sculptures.

The largest clam species, the giant clams of the genus Tridacna are found in and near shallow coral reefs of the Indo-Pacific. They have an enormously thickened and colorful mantle. The animals have a unique life style. They have colonies of algae living inside their mantle tissue. Lens like tissues with in the mantle help focus sunlight onto several different layers of algae. The largest species, Tridacna gigas, can be 54 inches long and have a shell weight of 507 pounds.


Amy Edwards,

June 13, 1997