In San Francisco, I saw this fish:
and quoting the aquarium's page:
Part of the wrasse family, the California sheephead is a protogynous hermaphrodite. Simply put, all sheepheads are born as females, but eventually transform into males. Due to hormonal changes triggered by environmental and social cues, this fish can go from a reproductively-functional female to a fully-functional male.
I have never heard of something like this for any organism. Is this behavior unique?
But the real biology question is why they do that and not simply retain one gender throughout their life? In other words, what's the evolutionary advantage of such a weird approach?