NEW YORK (AP) — If in the beginning there was male and female, fish seem to have forgotten the memo.

For nearly 500 fish species, including the clownfish in "Finding Nemo," the great divide between sexes is more like a murky line: If circumstances call for it, the fish can swap their sex, with females turning into males in some species and males turning into females in others.

People think of sex as being fixed, said biologist Erica Todd from the University of Otago in New Zealand, "but there are so many fish that can push it in the other direction."

Scientists have known for decades about the...