CHICAGO, July 23 (Xinhua) -- Researchers at the University of Illinois (UI) found in a study that the male-to-female sex-change for anemonefish occurs first in the fish's brain and only later involves the gonads, sometimes after a delay of months or years.
For the study, posted on UI's website on Tuesday, the researchers set up experiments in the laboratory where they paired male anemonefish together and tracked their development. In all, they followed 17 pairs of male anemonefish.
Within minutes or hours of being put together in a tank, one of the two males emerged as dominant, and began to behave as a female would.
The researchers focused on a brain structure that regulates gonad function. This structure, the preoptic area, is much bigger in females than in males, with roughly twice the number of neurons.
"We discovered that when you pair two males together, they fight, and the winner becomes female," said UI psychology professor Justin Rhodes, a behavioral neuroscientist who led the research. "But the first thing that changes is the brain, in particular the part of the brain that controls the gonads."
The findings, reported in the journal Hormones and Behavior, describe the first known example of an animal undergoing a sex change in the brain before it occurs in the sex organs, the researchers said.
The anemonefish is a gender-bending marvel. It starts out as a male, but can switch to female when circumstances allow, for example, when the only female present dies or disappears.
"We tracked changes in the preoptic area, changes in the gonads and in the blood hormones," said Rhodes, "We counted neurons in the preoptic area, took blood samples to look at the sex hormones, and examined the eggs to look at the proportion of eggs to testicular tissue in the gonads."
Within six months of being paired with another male, the dominant fish had grown its preoptic area to a size that made it indistinguishable from the same region in other female anemonefish brains.
"After six months, this part of the brain is completely changed from a male brain to a female brain," Rhodes said. "But the gonads have not yet changed, which means that they're still male gonads."
"It's a very clear demonstration that brain sex and gonadal sex can be uncoupled," Rhodes said.
Despite the researchers' expectations, the dominant fish appeared to be in no hurry to change their gonads to the fully female variety. Only three of the dominant fish in the 17 pairs actually transitioned all the way to reproductive females, with viable eggs in their gonads.
The rest of the pairs seemed to be in a holding pattern, with female brains and male gonads. The researchers followed them for three years, and the fish still hadn't made the full transition.
"We don't know exactly what they're waiting for," Rhodes said. "Maybe they're waiting to grow bigger, so they can produce more eggs. Maybe they don't have the right chemistry as a couple."
In anemonefish, females are dominant. They pair up with a single male, the largest of the males available, to mate for life. Male anemonefish will not fight with a female, but females will fight each other to the death.
If, for some reason, the female disappears, her male mate begins, almost immediately, to take on female behaviors, such as aggressively defending the nest. And the next largest male moves in to become her mate.