Oysters are soft-bodied animals that have two hard, protective shells (a bivalve). They spend their entire lives in one underwater location. The shape of the oyster's shells varies, depending mostly upon how crowded they are in the oyster bed.
Anatomy: The two hard, rough-textured shells are attached by a muscular hinge (the adductor muscles) at the narrow end. The shell is generated by the mantle, a thin layer of tissue separating the shell from the soft body. When an oyster is threatened, it closes its shells, using the very strong adductor muscle. Oysters draw in water through their gills, and extract oxygen and filter out floating algae (which they use for food). An oyster changes its sex during its life; it starts out as a male and often ends as a female. The largest oysters are up to 3 feet (1 m) long, but most are a few inches long.
Pearls: Pearls are sometimes found in oysters. When a grain of sand (or other irritating substance) gets stuck between the oyster's mantle and shell, the oyster secrets nacre. This shiny substance coats the grain of sand, and over the years, forms a lustrous pearl.
Predators of the Oyster: Many animals eat oysters, including whelks, sea stars, and people.