"In the movie’s opening scenes we see a male and female clownfish tending to their crop of eggs before she, the mother, is suddenly eaten by a barracuda. Nemo, the only surviving egg in the batch, is reared by his father before setting out on his wild adventure. But hold on. If the mother had truly succumbed to a barracuda, things would have turned out rather differently. The father would have changed, like many male reef fish, into a female. Sequential hermaphrodism. Being an only child, Nemo, born as an undifferentiated hermaphrodite, would have grown up to be male and, in a neat twist, would quite possibly have had sex with his now-female father. But that’s not all. Should the father later die, Nemo would continue the family trend by changing into a female and having sex with their offspring, should there be no other clownfish around. And there we have it."
—From Sex on Earth, zoologist Jules Howard’s new book about the wonders and peculiarities of animal reproduction (and how better understanding animal sex can help conserve wildlife)
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