Bivalves have gills
Unlike gastropods most bivalves are filter feeders, and use their gills like strainers to filter the water for plankton to eat.
The shells of bivalves grow as the animal grows larger; you can usually see the ridges, called growth lines on most shells. Typically, both valves are mirror images of each other and are joined near the top of the shell by a flexible hinge. The two valves are closed by means of powerful muscles that are attached to the inside of each valve. Although they are usually hard to see, most bivalves have eyes, usually hundreds of them! Perhaps the best known are the many bright blue eyes of scallops.
Not all bivalves look the same
Not all bivalves look like clams. Shipworms look much more like worms than clams and you have to look closely at the watering pot clam to see the remains of its bivalve shells.
Bivalves need protection
Many bivalves represent important sources of food throughout the world, and as with many resources need to be monitored to ensure that they are not over-exploited. Many freshwater bivalves in the United States are threatened by pollution, dam construction, and poor agricultural practices.
Another threat to native bivalves is the introduction of non-native species. Zebra mussels, pictured here attached to a clam, were accidentally released into the rivers and lakes of the U.S. accidentally and have spread throughout the eastern half of the country. They can become so numerous that they out compete native mussels for plankton.