Finding Nemo formally canonized the adorable anemone dweller the clown fish, but the Disney film punts on the most fascinating aspect of the organisms. As sequential hermaphrodites, they lead unique home lives. All are born male, with the ability to change sex—but once a male turns into a female, she can’t turn back into he. The largest and most dominant fish among those sharing an anemone becomes the female; the next-largest develops functioning testes. She lays eggs; he fertilizes them. One of the mated pair will eventually die, to be swiftly replaced by someone down the ladder. If the matriarch dies, the fertile male who was No. 2 takes her place as No. 1, metamorphosing into a female himself. Finding Nemo painted a lifelong romance between Nemo’s parents for more than just the sake of simplicity: a real clown-fish father who loses his mate would not develop a psychologically complex system of grieving and overprotection. He would simply become Nemo’s new mother. Nemo (the only other fish remaining in the anemone) would rapidly develop mature gonads. He would become his own father while his father became his mother, and they would raise little incestuous Nemos together without a drip of sentimentality.
— Adapted from The Extreme Life of the Sea, by Stephen R. Palumbi and Anthony R. Palumbi (published by Princeton University Press in March)