Oysters may change their sex up to four times per year.
Most oysters do not have a fixed sex that stays with them throughout the course of their life. Instead, they are protandric animals, meaning that they can change from male to female over the course of a lifetime.
Often, their reproductive organs contain both eggs and sperm. After reaching maturation, a process that takes one year, oysters usually release sperm. A few years later, after building up the necessary energy reserves, their reproductive organs start to release eggs.