One of the major differences between clams and oysters is that clams are motile all their lives, pulling themselves through the sand with a foot that extends well outside their shells. After a brief larval stage at the beginning of life, oysters attach themselves to other oysters or rocks and live out their lives in one place.
The other differences between clams and oysters are subtle variations in related species of bivalves. Both clams and oysters are classed as mollusks, so they have similar anatomies. While clams and oysters have two shells, both of a clam’s shells look the same. One of an oyster’s shells is cupped while the other is flat. The smoother texture of clam shells helps them dig down through sand. While oysters have a gill, most clams filter nutrition and water through a syphon that extends up through the sand. Although both can live in brackish and salt water, many species of clam live in fresh water. Because clams usually live buried in sand and sediment and oysters live in open water, their diets are slightly different. Differences in diet account for differences in flavor when the shellfish are consumed. Also, their meats are different colors and textures. Most clams are pinkish in color and chewy, while oysters are dark gray and soft.