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These fish all start out male

All clownfish are born male.

There’s a good reason why both main characters in Finding Nemo are male, at least initially. All clownfish are born that way, and it’s only when a group’s dominant female dies or disappears that a male will develop into a female and become the new matriarch. All clownfish have the ability to turn female, and the change is permanent once it occurs. The transformation begins almost immediatelyafter the dominant female leaves, and starts in the brain before manifesting itself in the sex organs. Had the beloved Pixar film been devoted to scientific accuracy, Nemo’s father, Marlin, might not have been just his sole caregiver after tragedy befalls the boy’s mother — he might literally have become his mother.

Clownfish aren’t the only reef-dwellers that can change sex. The bluehead wrasse does it as well, only in reverse: When a dominant male leaves its group, the largest female transforms into a male over the course of just 21 days. Researchers have identified no fewer than 500 fish species capable of changing sex; some, like the coral-dwelling species of gobies, can even switch back and forth. The process is believed to have reproductive benefits, as it allows a single fish to reproduce as both sexes throughout its life.  

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