It turns out Finding Nemo had it wrong. When a barracuda ate mom in the first scene of the movie, Marlin would not become a single dad, because being a single dad clownfish is impossible. Realistically, Marlin would have become Marilyn– a single mom!
Clownfish are protandrous sequential hermaphrodites. In other more pronounceable words, clownfish start their lives as males and then change into females. Why is this necessary?
Clownfish live in sea anemones and have adapted so they are unharmed by the anemone’s stings. Because of its unique mutualistic relationship with the anemone (the clownfish eat parasites off the anemone, effectively cleaning it), the clownfish is reluctant to wander too far beyond its tentacly home. This becomes problematic when the clownfish wants to meet members of the opposite sex, so the clownfish has evolved an interesting mechanism – sequential hermaphroditism – to overcome this obstacle.
Instead of braving the dangerous open waters beyond the reef (no, neither Marlin nor Nemo would realistically breach the “drop-off” and make it back alive), all the clownfish living in one place are males except the oldest, which is always a female. After the head female dies, the next-oldest male turns into a female!
I suppose Pixar would’ve had a hard time explaining why they turned Marlin into Marilyn after one scene – I would’ve been confused too. But in reality, the animal world of romance hosts a plethora of bizarre courtship behaviors, mating rituals, and other such oddities that put human courtship to shame!
Clownfish picture 1 by ecatoncheires on Flickr: http://www.flickr.com/photos/ecatoncheires/2335044473/.
Nemo picture: from Cthomasuscg on Flickr: http://www.flickr.com/photos/cthomasuscg/3304908183/
Interesting animal romance behaviors: http://www.care2.com/greenliving/animal-romance-they-have-sex-how.html?page=1.
“Sequential hermaphroditism.” Wikipedia.org: http://en.wikipedia.org/wiki/Sequential_hermaphroditism
Clownfish information at Shedd Aquarium: http://sea.sheddaquarium.org/sea/fact_sheets.asp?id=72
Clownfish information at Visit Sea Life: http://www.visitsealife.com/explore-our-creatures/clownfish.aspx
Clownfish information at Evolution Faq: http://www.evolutionfaq.com/articles/sex-change-nature-coral-reef-fish