Oysters change their gender at least once in their lifetimes. All oysters start off as male, but most change permanently to female by the time they are a year old. Their reproductive organs produce both sperm and eggs, and they can change gender at will. It is technically possible, therefore, for an oyster to fertilize its own eggs.
More about gender-changing animals:
- Triggerfish change gender as well. Though all triggerfish are born female, the largest and most dominant triggerfish changes into a male in a process called "protogyny." All of the surrounding female triggerfish become his mates, and if he dies, then the next most dominant triggerfish will turn into a male.
- Several animals are born as hermaphrodites, including earthworms, garden snails and a type of fish called a hamlet.
- One type of slime mold has 29 different variants of gender-controlling genes. Each mold cell, however, can reproduce only with a cell with completely different gender-genes than it has, which means there are more than 500 possible variations of genders for this species of mold.