Gender Bender of the Reef

July 1, 2007
By Mandy Calkins

In the tropical reef habitats of the Caribbean and Atlantic lives the bluehead wrasse, a small, colorful fish with an amazing ability. The bluehead wrasse is a hermaphrodite - it can change its own sex.


Bluehead wrasse: Left image, female-turned-male wrasse. Credit: Wade Pemberton. Right image, group of yellow females. Credit: Photographer, ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Australia.

Many of these small fish first mature as females, but when their population needs another male, some of these females can change into males.

Every bluehead population has two kinds of males: small ones with plain yellow coloration that do not change sex, and dominant males, which were females who underwent a sex change. These dominant males are large and flashy, with bluish-green heads and dark stripes, and they monopolize the breeding in the population.

When a dominant male in the group dies or is removed, the largest female will begin a sex change to take his place. The transformation is amazingly fast - researchers have seen female blueheads show aggressive male behavior, such as rounding up other females, within minutes after the dominant male’s removal. In as little as four to eight days, the female can take on the distinctive bluish-green coloration and begin producing sperm.

Why change sex? Experts say it’s all about breeding advantage. Dominant males are prolific breeders, mating up to 40 times a day. So when one of them disappears, the largest female in the group sees a chance to increase her mating success by becoming male.

Like any animal, the bluehead wrasse lives to pass on its genes to the next generation, and for some individuals, that means undergoing a remarkable transformation.

Copyright 2007. The University of Texas Marine Science Institute.