The Ostreidae, the true oysters, include most species of molluscs commonly consumed as oysters. Pearl oysters are not true oysters, and belong to the order Pteriida.
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scallops, true oysters have a central adductor muscle, which means the shell has a characteristic central scar marking its point of attachment. The shell tends to be irregular as a result of attaching to a substrate.
Both oviparous (egg-bearing) and larviparous (larvae-bearing) species are known within Ostreidae. Both types are hermaphrodites. However, the larviparous species show a pattern of alternating sex within each individual, whereas the oviparous species are simultaneous hermaphrodites, producing either female or male gametes according to circumstances.