Any bivalve of two families, Ostreidae (true oysters) or Aviculidae (pearl oysters), found in temperate and warm coastal waters worldwide.
Both valves (halves) have a rough, often dirty-gray outer surface and a smooth white inner lining (nacre). The lower valve, which affixes to a surface, is nearly flat. The smaller upper valve is convex and has rougher edges. The oyster filters its food, minute organic particles, from the water. Cultivated as food, oysters are regarded as a delicacy. Pearl s are the accumulation of nacre around a piece of foreign matter.
European flat oyster ( Ostrea edulis )
G. Tomsich
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