The gender of oysters changes over time. An oyster could switch between being male or female throughout its lifetime!
In fact, oysters are usually boys, a.k.a. males, when they are born. When they are about a year old, they become females, a.k.a. girls. In addition, they can change gender several times in a year as the environment changes. An oyster could switch between being a father and a mother to facilitate the birth of the next generation.