The differences between clams, mussels and oysters lie mostly in the shapes of their shells. Many types of clams have the classic clam-shaped shell with ridges or concentric rings. They also come in many colors and sizes, though the ones sold at the fishmonger's are only a few inches long. Razor clams resemble straight razors.
Unlike mussels and oysters, clams don't attach themselves to a hard surface but burrow into the sand. They also eat and obtain oxygen through a syphon.
Mussel shells are slipper shaped and dark colored. They cling in large numbers to rocks and other strata with fibers known as byssus. The byssus, or beard, needs to be scrubbed off before the mussel is cooked. Like clams, mussels can be steamed or boiled.
Oysters also cling to surfaces, but their shells are rougher and more irregular in shape than those of clams or mussels. There are true oysters, which belong to the family Ostreidae, and pearl oysters, which are members of the Pteriidae family. True oysters used to be so plentiful that they were cheap and considered food for the working class. There were huge oyster beds in the harbor of New York City. However, these were destroyed by overfishing and pollution.