Let's just say it involves child-parent relations that would make Oedipus blush.
Father and mother clownfish are tending to their clutch of eggs at their sea anemone when the mother is eaten by a barracuda. Nemo hatches as an undifferentiated hermaphrodite (as all clownfish are born) while his father transforms into a female now that his female mate is dead. Since Nemo is the only other clownfish around, he becomes a male and mates with his father (who is now a female). Should his father die, Nemo would change into a female and mate with another male. Although a much different storyline, it still sounds like a crazy adventure!
Cooney says he won't be surprised if Pixar robs us of the scientifically accurate version of the Nemo sequel, Finding Dory – then again, the studio apparently did change the movie's ending thanks to the devastating Blackfish documentary. No small victory there.
More details (and video!) on sequential hermaphroditism at The Fisheries Blog.