MOLLUSCS OF BRITISH COLUMBIA

 Class Bivalvia

(Clams, Scallops and Mussels)

by

James A. Cosgrove

The Class Bivalvia (bi = two, valvia = shells or valves), (formerly known as the Class Pelecypoda), is one of the largest classes in the Phylum Mollusca.  About 15,000 species can be found worldwide in both marine and freshwater habitats.

We commonly recognize bivalves as clams, scallops and mussels.  The bivalve group is significant to humans as we harvest many bivalves for food or for their shells, which can be used for tools or for decoration.  In some cases we culture, raise and harvest specific bivalves in commercial settings.

The beauty of many bivalve shells makes them sought after by shell collectors.  The rarity of a specimen often is a factor in the monetary value of that specimen.  Some shells sell for thousands of dollars.

Most bivalves filter feed.  That means that they pull water into their bodies (often through an inhalant siphon), filter the plankton (food) out of the water and then expel the cleaned water back out the exhalant siphon.  This constant movement of water allows the bivalve to feed continuously when food is available.  The bivalves represent the only Class in the Phylum Mollusca that do not have a radula. 

Bivalves show significant variation in size.  The smallest of the bivalves is approximately half a millimeter long and is so fragile that just picking one up can cause the shell to break.  Others, such as the giant clams, can be more that a meter in length and more than 300 kilograms in weight.