Oysters can change sex.
Why it is true
Many species are sequential hermaphrodites, changing sex depending on water temperature and reproduction needs.
Source: Marine Biology
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.