Love is blind for fish in murky waters 18:03 01 October 2008
NewScientist.com news service
Ewen Callaway
Some female fish have eyes for their man only. Colourful African cichlids have evolved into new species because females are partially blind to others.
But even as that discovery is made, the species are under threat because the polluted waters they live in are causing them to interbreed.
Among several closely related species of cichlids living in Lake Victoria, males come in either red or blue. Brighter males tend to get the girls, but new research suggests that both sexes have evolved to preferentially see only one colour, creating new species of fish in the process.
"Reds and blues live in the exact same spot," says Ole Seehausen, an evolutionary biologist at the University of Bern in Switzerland and the Swiss Federal Institute of Aquatic Science and Technology in Kastanienbaum. "Colour is very important in mate choice."
That is especially true in the water, where blue pigments shine brightest in shallow waters and red pigments further down. Cichlids usually heed this direction, with blue species living at shallower depths than red species.
Murky waters
To determine what happens to fish in the transition between red and blue zones, Seehausen's team studied cichlids living off the shores of five Lake Victoria islands. Because of sediment, the waters near some lakes are cloudier than others.
In clear waters, the colour that appears brighter shifts from red to blue gradually with depth, and red and blue fish stick to their zones, cementing their genetic differences. In murky waters, however, the transition from red to blue happens much quicker and blue and red fish sometimes interbreed, destroying species differences.
Mutations in the genes for light-detecting proteins called opsins explain why, Seehausen says. His team found that these genes had changed faster than the rest of the fish's genes, a sign of evolution in action. Red fish evolved red-sensing opsins, while blue fish developed blue-sensing proteins.
Tests in laboratory tanks confirmed that females with red-sensing eyes went for red males, while blue-eyed females followed suit. Hybrid females, just like those in the murky waters, showed no preference at all.
Mass extinction
Evolution by vision could explain why Lake Victoria is home to hundreds of species of cichlids, which are popular aquarium fish. However, as urban growth pumps more sediment and algae-feeding fertilisers into Lake Victoria unchecked, many species are vanishing.
"Species diversity in this lake has imploded in the last 30 years," Seehausen says. "It is the largest human-witnessed mass extinction of vertebrates."
Rather than die out, species facing radically changed optical environments interbreed out of existence, he says.
Cichlids might not be the only animals that evolve new species by adjusting their vision, says Mark Kirkpatrick, an evolutionary biologist at the University of Texas in Austin. Guppies and perch might be branching off because of who they see.
"It's not just restricted to fishes," he says. Some species of primates see in full colour, while others only in black and white. Perhaps our ancestors split off from other monkeys because they saw the light.