A species of clownfish. Cited here as an example of hermaphroditism in Nature.
From Wikipedia:
Clownfish have a very structured society. In the Amphiprion percula species, there are zero to four individuals excluded from breeding and a breeding pair living in a sea anemone. Dominance is based on size, the female being the largest and the male being the second largest. The rest of the group is made up of progressively smaller non-breeders, which have no functioning gonads. If the female dies, the male gains weight and becomes the female for that group. The largest non-breeding fish then sexually matures and becomes the male of the group.
