Pearls
Twenty species of oysters are able to produce natural pearls, though most oyster farmers cultivate artificial ones for commercial purposes. Natural pearls are made when foreign matter inside the body is covered with layers of nacre, the same calcium- and protein-based material used to make shells.
Unknown Sex
Oysters are hermaphroditic and it is impossible to truly tell their sex at any given time. They appear to be born male, but contain both male and female sex organs, altering sexes periodically over a lifespan. At maturity, most oysters appear to be female.
Endangered Status
Over-fishing, industrial pollution, global warming and the acidification of the oceans have put oysters in peril. Some scientists believe oysters belong on the endangered species list. Today, most oysters used for commercial purposes are farmed.