Clownfish (of finding Nemo fame) can change sex.
(Seemingly Clownfish society is quite hierarchical, with a reproducing female followed by a mating male, lower down the pecking order there is a bunch of non-mating males waiting in the wings, but, if the mating female dies, the dominant male morphs into a girly fish, and chooses a partner from the various non-mating males. Now that's not something they showed in the film, is it?)