Clownfish can change their gender
Clownfish live in schools of one female who is the dominant and largest of the group, with the rest of the group consisting of smaller males. If the female dies, then the largest male will change into a female. She will then choose a breeding partner among the available males, which is usually the largest and most aggressive male available.
Wrasses are another fish that are also capable of changing their gender, though they biologically change from female to male instead. I just learnt this today, and I find it really interesting :)