Many oysters start their lives as males and later change their biological sex to female.

The Eastern Oyster is a common species that changes sex. Most oysters mature as males in their first year because making sperm takes very little energy. As an oyster grows, it stores more energy and can afford to make eggs instead. This switch usually happens during the winter when the oyster is resting.About 20% of oysters on a reef change their sex every year. Water temperature and the amount of food available help determine when this happens. The number of other males and females nearby also plays a role in the timing. Researchers at the Smithsonian Environmental Research Center study these patterns in the wild.A large female oyster can produce up to 100 million eggs in one season. Their bigger bodies provide the space and resources needed for this massive production. Hormones in the oyster trigger the body to reorganize its reproductive tissues. Once an oyster becomes a female, it almost never switches back to being a male.