OK, so, um, here's your perception warping, good times stomping news of the day: Finding Nemo is a damn lie. Not in a oh fish can talk sort of harmless lie but in an oh my god Nemo would grow up to bone his dad sort of awful lie. The Fisheries Blog broke down the science of Finding Nemo and pretty much exposed that Nemo's mum would die, Nemo's dad would switch sex and Nemo would eventually mate with his female dad.
Basically, all clownfishes are born hermaphrodites with both testicular tissue and ovarian tissue. The sex of the clownfish just depends on which tissue is encouraged to grow (and a lot of that depends on social experience). So, if a female clownfish mate died, the male clownfish mate would turn female to fill the role of female (female clownfish are dominant). With that knowledge, The Fisheries Blog describes how a scientifically accurate version of Finding Nemo would play out:
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.
Here's a video from the Beckman Institute explaining how the whole sex-change process works: