The family Ostreidae are the true oysters, and include all the species that are commonly eaten under the title "oyster". They do not include the Pearl Oysters; these species are only distantly related to the true oysters, since although theya re also bivalves, they are members of the family Pteriidae, in the order Pterioida.
Like scallops (family Pectinidae), true oysters have a central adductor muscle, which means that the shell has a characteristic central scar, marking its point of attachment. Oysters have a larger adductor muscle than scallops. Their shell shape tends to be irregular as a result of attaching to other objects.
Members of genus Ostrea generally live continually immersed in sea water, brood their fertilized eggs for various proportions of the period from fertilization to hatching and are quite flat with roundish shells. They differ from most bivalves by having shells completely comprised of calcite but with internal muscle scars of aragonitic composition. They do best in water with a not too thick concentration of phytoplankton.
Members of genera Saccostrea and Crassostrea generally live in the intertidal zone, broadcast sperm and eggs into the sea and can thrive in water which is very rich in phytoplankton. One of the most commonly cultivated oysters is Saccostrea gigas, the Japanese oyster, which is ideally suited for oyster cultivation in seawater ponds.
Both oviparous (egg bearing) and larviparous (larvae bearing) species are known within the Ostreidae. Both types are hermaphrodites. However, the larviparous species (which include the Edible Oyster Ostreia edulis) show a pattern of alternating sex within each individual, whereas the oviparous species (such as the Eastern Oyster Crassostrea virginica) are simultaneous hermaphrodites producing either female or male gametes according to circumstances.