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.
Like 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.
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