Is there such a thing as transsexual fish?
If you dip your head underwater in the Mediterranean (or the North Atlantic, from the Canaries to Norway), one of the first fish you'll spot is the wrasse, Coris julis! These small, slender fish are widespread. Abundant, agile, constantly active, highly colored and not at all shy... they're a sight to behold.
And their private lives are even more fascinating!
Two different names for the same species!
When they're young, blackbirds have brownish backs and lighter, yellowish-white bellies. We call them common crakes. They can measure up to 18 cm. It is estimated that only 5 to 10% of these common turnstones are males. The largest of these males are able to reproduce, but do not. The remaining individuals are females, the largest of which can reproduce. Their fecundity is proportional to their size: the larger the female, the greater the number of eggs she can produce.
Common thrush
As soon as they reach a size of 18 cm, they change color... and name: bright green back, brilliant white belly, a wide orange stripe and a black spot on the flanks. The beginning of the dorsal fin develops into a colorful pennant. These are known as kingfishers. They can reach 25 cm.
(Note that whether common or king, wrasse are well camouflaged, as are most fish: Their dark backs make them difficult to see from above, as the depths are dark. The light belly, on the other hand, makes them harder to spot from below, as the surface is much brighter)
But the amazing thing is that all kingfishers are male! All of them!
They're big males, raising their pennants in circular swims to stake out their territory, ward off other males and attract females. (And so they conquer all the females! All of them!)
Royal wrasse
But 90-95% of these royal wrens were previously reproductive females who have changed color and sex! In fact, the kingbird is a protogynous hermaphrodite, which means that females turn into males as they age! Very few males in royal livery have been males all their lives...
Most royal waxwings are therefore transsexual.
Kingfish in the early stages of transition
There are also involuntary intersex fish
While sex changes in kingfish are natural, this is not always the case.
Researchers atUGERAA, our University's Aquatic Resources Management and Aquaculture Unit, have shown that many of the fish in our rivers have ovotestis, i.e. both male and female genitalia. This hermaphroditism is clearly the consequence of the action of pollutants in their waters, such as pesticides, pharmaceutical products, industrial detergents or cosmetics, which disrupt their endocrine system.
Similar findings can be found all over the world. The Potomac River in Washington is a well-known example. Closer to home, researchers in Lyon found that a large number of male fish in the Seine and Rhône rivers were becoming female (and that the females were carrying far fewer eggs than before). Here too, they were able to link the phenomenon to river pollution...
Water temperature also has an influence. This is a technique used in aquaculture to control the sex of fish so as to produce only male or female individuals, which, depending on the species, perform better in terms of growth than the other sex.
But that's another story...
University of Liège Aquarium Museum
Photos ©Georgios Skoufas, International Hellenic University
