Oysters change gender at least once in their lives. All oysters start out as males, but most become permanently females at around one year of age. Their reproductive organs produce sperm and eggs, and they can change sex at will. It is technically possible, therefore, for an oyster to fertilize its own eggs.
More on animals that change sex:
Triggerfish also change gender. Although all triggerfish are born female, the largest and most dominant triggerfish become male in a process called “protogyny.” All surrounding female triggerfish become his mates, and if he dies, the next most dominant triggerfish will become a male.
Several animals are born as hermaphrodites, including earthworms, garden snails, and a type of fish called a villager.
One type of slime fungus has 29 different genetic variants that control the genus. However, each fungal cell can reproduce with a single cell with completely different genus genes than it has, meaning there are more than 500 possible genus variations for this fungal species.