Ricefield Eels Start Female. The Thermostat Decides What Comes Next.

The ricefield eel starts life female. Every single one. Then something in its environment starts changing the equation.

The ricefield eel (Monopterus albus), also known as the Asian swamp eel, is a protogynous hermaphroditic fish, meaning it begins life female and develops into a male through an intersex stage. What nobody had fully mapped was how the environment could push that transformation along. A team at the Chinese Academy of Sciences in Wuhan has now identified a molecular pathway connecting the outside world to the eel's sex-determination machinery.

Researchers raised 1.5-year-old females for six months at either 25 to 26 degrees Celsius (77 to 79 degrees Fahrenheit) or 33 to 34 degrees Celsius (91 to 93 degrees Fahrenheit). After 180 days, roughly 75 percent of the warm-water fish had developed ovotestes, gonads transitioning from ovaries toward testes. In the cooler group, about 20 percent had.

The pathway appears to run through Trpv4, a temperature-sensitive protein in ovarian cells. Warm conditions activate calcium signaling that ultimately switches on genes involved in male development. When researchers blocked Trpv4, that response weakened. When they activated it in cooler water, male-development genes switched on anyway.

For the ricefield eel, the weather is part of the reproductive system.

Read the full story at eLife, September 7, 2026

Hot Take: Most animals experience summer as weather. This one experiences it as instructions.