Adult oysters live attached to an object, either by means of tough, horny strands called byssal threads, or the byssus, or by cementing one shell, usually the right, to the object. Living permanently attached and enclosed within protective shells, oysters lack a foot or have a reduced foot, and have a reduced head and associated sense organs. Oysters are filter feeders, using their extensive gills and associated hairlike cilia to trap and move tiny food particles to the mouth. In some oysters fertilization is external, both sperm and eggs being shed into the water. In others the female retains and broods the eggs, which are fertilized by sperm drawn in from the outside.
Oysters hatch as trochophore larvae and then change to veliger larvae.
Some oysters, such as the European edible oyster, Ostrea edulis, show rhythmical consecutive hermaphroditism, in which individual oysters repeatedly change their sex from male to female and then back again. The American oyster, Crassostrea virginica, displays alternative sexuality, in which similar sex changes occur, but the change is erratic and depends on water temperature, food supply, and possibly other factors. In France, for example, tens of thousands of tons of oysters may be harvested from such beds each year. The mass of shells forms a firm bottom on which young oysters settle and attach along the sides. Thus an oyster bed, once established, tends to develop through addition of successive generations of residents.