Some Fish Can Change Their Sex In Adulthood – Here's Why

For most animals, sex is established in the womb. But for more than 500 species of fish, that isn't the case.

Many fish change sex in adulthood, typically as a response to changes in environmental stimuli, according to a study published in the ScienceAdvances journal. Scientists have known about sex-changing fish for quite some time, but they until recently, they weren't sure how it happened. Now, thanks to a team of New Zealand researchers including La Trobe University geneticist Jenny Graves, they have a better understanding of how the process works.

Graves focused her research on the bluehead wrasse, according to reporting from Science Daily.

"I've followed the bluehead wrasse for years because sex change is so quick and is triggered by a visual cue," Graves told Science Daily.

She added that fish are able to switch between male and female without changing their genetic makeup, "so it must be the signals that turn them off and on." Still, scientists wondered for decades how exactly the fish did it.

The process is easy enough to observe, particularly in bluehead wrasse. These fish live in groups in the Caribbean, typically on coral reefs. Dominant males sport blue heads, and each usually guards his own harem of females, who feature yellow coloring. If a male leaves or is removed from his harem, however, the largest female in the group becomes male.

The sex-switching begins immediately: Within minutes, the biggest female changes her behaviors, according to Science Daily. Her color changes to reflect male coloring patterns within hours. And within 10 days, her ovary becomes a testis and begins producing sperm.

Even though the fish's genes don't change during the sex-switching process, the genes turn on and off in response to a rearrangement of chemical tags attached to the fish's DNA, according to reporting from the Associated Press.

When a female wrasse turns into a male, the chemical tags associated with her DNA reorganize, essentially reprogramming the fish.

Biologist Erica Todd, who contributed to the study, told the Associated Press that the fish are "sort of poised and ready to go either direction," much like a seesaw.

The researchers' study, published July 10, identifies a few sex-changing stimuli in its title: "Stress, novel sex genes, and epigenetic reprogramming orchestrate socially controlled sex change." Todd, Graves and their teammates used a few methods to reach this conclusion.

The scientists utilized high-throughput RNA-sequencing and epigenetic analyses to observe how genes in the wrasse's gonads and brains turn on and off to trigger a sex change. Todd told Science Daily that their findings indicated "that sex change involves a complete genetic rewiring of the gonad," starting with genes maintaining the ovary turning off.

Understanding how bluehead wrasse change from female to male can help researchers understand how genes switch on and off in other species, as well, including humans. The study specifically provides insight into how the environment can influence that process.

Moreover, Graves added that she is studying a similar sex reversal process in Australian dragon lizards, which could add even more to these scientific efforts.

"Sex reversal in dragons and wrasse involve some of the same genes," Graves told Science Daily, "so I think we are looking at an ancient system for environmental control of gene activity."