For many fish, changing sex is a normal part of life. For the first time, we have found out exactly how one of these species – a small cleaner fish called the bluehead wrasse – does it.
Erica Todd of the University of Otago in New Zealand and her colleagues removed some male bluehead wrasse from a few sites on reefs off Key Largo in Florida to trigger females to change sex. They then caught changing females at regular intervals after this, and looked at what was happening in their bodies down to the level of which genes were turning on or off.
They found that the loss of males makes some females stressed. They become more aggressive and to start performing male courtship behaviours.
In individuals that become dominant in a social group, the genes associated with female hormones shut down in a day or two, and their colours begin to change – females of the species are yellow and brown (see above), while the males are green and blue.
At the same time, the egg-producing tissues in their ovaries start to shrink and begin to be replaced by sperm-producing tissues. In just 8 to 10 days, the mature ovaries are transformed into testes, and the fish can mate with females and sire offspring.
After around 20 days, the fish have the full male colours and the process is complete. “The bluehead is certainly remarkable for its speed,” says Todd. “Other species do take much longer.”
However, as the fish only live around two or three years it’s a fair chunk of their lifespan – equivalent to more than 2 years in terms of human lifespan.
Around 500 species of fish can change sex, a fact long known to biologists but which got wider attention recently when the Blue Planet II documentary narrated by David Attenborough showed Asian sheepshead wrasse changing sex. It’s most common for female fish to turn into males but in some species including clownfish the males turn into females.
In at least one species, the hawkfish found around southern Japan, the females can not only turn into males but also turn back into females again if circumstances require it. For one species of shrimp, there’s no need to change back. It starts out male but becomes an hermaphrodite – a phenomenon known as protandric simultaneous hermaphroditism.
Journal reference: Science Advances, DOI: 10.1126/sciadv.aaw7006