Physiology of Finfish and Shellfish (2+1)

3.1.32. CLASS BIVALVIA (PELECYPODA) - (Bi-valv i-a) (pely-cy-poda)

The majority of bivalves are dioecious (two sexes). Their two gonads are very closely situated next to each other and they encompass the intestinal loops. The gonoducts are very simple as there is no copulation amongst bivalves.

In the Protobranchs and Filibranchs, the gonoducts opens directly into the nephridia and provide for the exit of sperm and eggs.

In the Eumellibranchs, the gonoducts opens directly into the mantle cavity very close to the nephridiopore.

A few bivalves such as the Cockles (Cardiidae), Poromyidae, a few of the oysters and scallops (Pectinidae), some of the fresh water clams Sphaeriidae and Unionidae are hermaphroditic (one sex).

In most of the bivalves, sperm and eggs are released into the surrounding water where fertilization occurs. The eggs and sperm, which were deposited into the suprabrachial chamber, are swept out along with the exhalent current.

In a few of the bivalves, such as the common oyster Ostrea edulis L., fertilization occurs within the suprabranchial chamber itself when sperm is drawn in along with the inhalent current. The fertilized eggs then develop in the gill filaments.

In some of the freshwater hermaphrodites, self - fertilization may actually occur in the genital ducts before the eggs are deposited into the suprabranchial chamber. The eggs then travel into the water tubes of the gill and there they develop into larvae.