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Fish species: Color is more than skin deep

The love life of a freshwater African fish has just helped answer an enduring question: How do species form? The fish in question, called the cichlid, has diversified into 500 species in Lake Victoria.

Due to this profusion of species, the lake attracts evolutionary biologists like roadkill attracts crows.

Biologists have long known that species can form through geographic isolation: If a mountain range prevents two groups of animals from inter-breeding, they may evolve into separate species.

No duh.

So how could species form in a lake, which lacks such geographic barriers? One possible mechanism rests on the senses. To mate, animals must recognize their own species, and because a large element in the senses is genetic, genes influence the senses, which influence mating choice, which influences the genes, and so on. And because the senses are also necessary for finding food and avoiding predators, individuals with superior senses get the ultimate evolutionary payoff -- more offspring.

Courtesy Ole Seehausen.

A darker cichlid fish would be more likely to live in clearer water; the fish with the red accents is more at home in deep, cloudy water. A new study found a genetic parallel: the eyes of a red fish are more suited to seeing red than the eyes of a darker fish.

But sensory cues may themselves vary with the environment: A blue fish would be easy to spot in the white light of clear, shallow water, while a red fish would stand out in deeper water. And thus the optimal senses for detecting the correct mate could vary according to location and conditions.

Getting deep, getting murky

In a new study, Ole Seehausen, of the University of Bern/Swiss Federal Institute of Aquatic Science and Technology, and colleagues looked at closely related cichlid fishes around five islands in Lake Victoria. At some islands, the water was murky, and the light color changed quickly with depth as the blue light was blocked by particles in the water. Where the water was clearer, the change in light color was more gradual.

The researchers found a strong correlation between water quality, fish color and eye genetics.

In areas where the light changed suddenly from white to red, the cichlids had mainly an intermediate color. "But in clear water, there is a very strong association between fish color and depth," Seehausen says. The species that lived near the surface had blue males, while males were red in the deeper-living fish, where red is more visible.

Color's only skin deep

In water where light color changed gradually, the color difference was echoed by a difference in genes making a light-detecting protein in the retina. The eyes of blue fish were "tuned" to recognize blue, while the eyes of red fish were more suited to recognizing red. Furthermore, DNA that does not make proteins, and therefore can track genetic exchanges between the fish, also paralleled the variations in fish color and retinal genetics.

These clear genetic distinctions between the blue and red fish bolstered the idea that two species lived in the clear water. In the cloudy water, however, the cichlids mated randomly, and there was no association between fish color and gene flow.

So what?

The study provided evidence for the "sensory drive hypothesis," which holds that sensory effects (in this case visual ability in different waters) can foster the creation of individual species by changing mating behavior.

In a commentary in Nature, biologists Mark Kirkpatrick and Trevor Price wrote that the mating choices of cichlids could also depend on memories of mom, who houses and protects the young in her mouth: "Learning at a young age (sexual imprinting) apparently contributes to reproductive isolation in these cichlids, as it does in other groups such as birds. ... It remains to be seen if imprinting, vision and perhaps other mechanisms have been sufficient to generate new species without geographical isolation."

Courtesy Ole Seehausen.

The waters range from clear to cloudy around Zinza Island in Lake Victoria.

Seehausen suggests that his results help explain the disappearance of half of the 500 cichlid species that occupied Lake Victoria 30 years ago. Over the past century, due to pollution from farming, logging and cities, he says, "The clear water has largely disappeared. More of the aquatic communities have moved from a regime with many light environments, conducive to many species, to a community with fewer generalist populations, where the light environment cannot support multiple species."

- David J. Tenenbaum