Shawdesh Desk: 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 sex trades, but they’ve had limited understanding of how the exchange happens. In a study published Wednesday in Science Advances, Todd and her colleagues detail the molecular events behind this ability, as well as what keeps mammals stuck as one sex or another. The researchers looked at the bluehead wrasse, a reef fish that swims in small groups of a dominant blue-headed male and a posse of smaller yellow females. Normally the male and females stay as they are, feeding together and occasionally mating. But if a predator happens to snatch up the lead male, the dominant female in the group will become a male.

“The sex change in this species is remarkable because it’s so quick,” Todd said. It takes only minutes to a few hours for the female’s behavior to become more territorial and aggressive like a male. In a few days, she courts other females. And after eight to 10 days, she’s fully transitioned to a male.

Todd and her team removed the lead males from several wrasse groups in the Florida Keys. As the females changed sexes, the researchers took DNA from cells in the animal’s brains and genitals so they could follow what was happening at the genetic level. They found that removing the males likely stressed females. The hormones released from that stress dial back the activity of the gene that makes the female hormone estrogen, and eventually ovary cells start to die. At the same time, those hormones increase activity in the genes that produce male hormones, and later testicles form.