Bright fish + dim light = diversity lost.

Thirteen thousand years ago, Lake Victoria, now one of East Africa's great lakes, didn't exist. Since then, the basin has flooded first with water and then with an evolutionary flourish: the cichlids, a brightly colored family of perchlike fish that diversified to fill every nook in the Ireland-size pond.

As recently as 1978, researchers estimate, 500 cichlid species lived in Lake Victoria. About half are now extinct, the victims of overfishing, habitat loss, and annihilation by the Nile perch, an introduced fish whose numbers exploded in the 1980s.

A group of researchers from the University of Leiden in the Netherlands now has evidence of another toll on the cichlids' famed diversity Turbidity in the lake is interfering with the ability of these strongly visual fish to find their preferred, bright mates. The result may be interbreeding of closely related species and loss of the more spectacularly colored forms.

The analysis by Ole Seehausen and his colleagues in the Sept. 19 Science is a "beautiful piece of work," says Les Kaufman of the Boston University Marine Program.

The study grew from the observation that even cichlids in protected, rocky areas of the lake were disappearing. "Species disappeared not only in habitats that were affected by the Nile perch but ... also in areas where we knew that Nile perch has basically zero impact," says Seehausen.

He and his colleagues measured the light and other environmental features in these areas, along with the brightness of the cichlids. They found the strongest correlation between bright mate colors and clear, well-lit water. In recent laboratory experiments, females of two closely related species chose males of their own species in strong light but mated indiscriminately in limited light.

Lake Victoria--which is slated for ecological restoration-has been clouding up since at least the turn of the century, when forests were cut, researchers say. As Nile perch have preyed on algae-eating cichlids, the cloudiness has increased even more, aggravating the cichlids' perilous state. "It's as if they were painted into the last corner, and then that corner gets painted," says Kaufman.

For biologists, these extinctions have flipped the evolutionary value of the lake, he adds. For vertebrate evolution, "it's the keystone model for both sides of the coin: the origin of new forms and the destruction of forms."