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• ​Oysters are mollusks and include the edible oyster. These are characterized by rough and irregularly shaped shells that are closed by one, rather than two, adductor muscles.
• Oysters are found in shallow, brackish, or tidal waters throughout all regions of the world, except the coldest.
• In the true oyster, the upper shell is large and hollowed to contain the oyster's body; the lower shell is usually flat but may be concave.
• Oysters are filter feeders, using their extensive gills and associated hairlike cilia to trap and move tiny food particles into the mouth.
• The American oyster displays alternate sexuality, in which individual oysters change their sex from male to female and then back again, but the change is erratic and depends on water temperature, food supply, and possibly other factors.