Cichlid Fish Diversity Threatened by Eutrophication That Curbs Sexual Selection

Abstract

Cichlid fish species of Lake Victoria can interbreed without loss of fertility but are sexually isolated by mate choice. Mate choice is determined on the basis of coloration, and strong assortative mating can quickly lead to sexual isolation of color morphs. Dull fish coloration, few color morphs, and low species diversity are found in areas that have become turbid as a result of recent eutrophication. By constraining color vision, turbidity interferes with mate choice, relaxes sexual selection, and blocks the mechanism of reproductive isolation. In this way, human activities that increase turbidity destroy both the mechanism of diversification and that which maintains diversity.In the Great Lakes of Africa, large and diverse species flocks of cichlid fish have evolved rapidly (1, 2). Lake Victoria, the largest of these lakes, had until recently at least 500 species of haplochromine cichlids (3). They were ecologically so diverse that thev utilized almost all resources available to freshwater fishes in general (2), despite having evolved in perhaps as little as 12,400 years (I ) and from a single ancestral species (4). This s~ecies flock is the most notable . . example of vertebrate explosive evolution known today. Many of its species have vanished within two decades (5, 6 ) , which can only partly be explained by predation by the introduced Nile perch (Lates spp.). Stenotopic rock-dwelling cichlids, of which there are more than 200 species (7), are rarely eaten by Nile perch (8). Yet, many such species have disappeared in the past 10 years (6, 7). Because gene flow between island ~o~ulations of these cichlids is effectivelv iiGited by stretches of sand and mud botto4 (9), populations underlie local selection regimes. Here we demonstrate that increasing turbidity, by curbing the impact of sexual selection on sexual isolation, is responsible for the decline in cichlid diversity.The seven Great Lake basins of tro~ical Africa in which haplochromine cichlids formed endemic s~ecies flocks have distinctly clearer waters than the five in which they did not (300 to 2200 versus 20 to 130 cm maximum Secchi disc readings; n = 12, t = 2.99, P = 0.015). Significance increases when the three very large lakes Victoria, 'Malawi, and Tanganyika are excluded (n = 9, t = 3.83, P = 0.009), ruling out the alternative hypothesis that lake size explains the difference. Lake Victoria has rapidly eutrophied (10) and become turbid. Water transparency decreased in deep open waters from 5.5 to 8 m in the 1920s to 1.3 to 3 m in the 1990s and decreased in the littoral zone from 3 to 1.5 m within the past decade (I I). We have investigated the effect of these changes on the cichlids. Postmating reproductive barriers have not been found among Lake Victoria cichlids (12), presumably because of their phylogenetic youth. Reproductive isolation among sympatric species is maintained only by mate choice. Haplochromines have vibrantly colored males and usually cryptically colored females (2,7). Their eyes are equipped with three retinal cone pigments ...

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