How female fish perceive colour may be an important role in the evolution of some cichlid fishes, according to research published in a recent issue of the journal Nature.
The study, by Ole Seehausen and coauthors, studied the mating preferences of the closely related Lake Victorian cichlids Pundamilia pundamilia and P. nyererei, and correlated these preferences with the differential sensitivities of the eyes in female fishes to blue and red.
The authors were able to determine the sensitivities of the visual pigments in the fishes by sequencing visual pigment (opsin) genes (red and blue fish tended to possess different alleles at some of the opsin gene loci).
Because colour sensitivity of the fish varies with the depth at which the female fishes live (those living in shallower water are more sensitive to blue and those in deeper water more sensitive to red), there is a selective advantage for males to utilize nuptial colours closer to the visual sensitivities of the females.
The authors found that this was indeed the case, with female cichlids whose eyes are more sensitive to blue tending to prefer blue-coloured males, while females with photoreceptors better able to detect red light choose males with red nuptial coloration .
However, this role that colour sensitivity in female fishes plays in speciation is only made possible in relatively clear waters, where the ambient light changes gradually with depth (because this gives enough room for different genetic variants to exploit the competitive advantages conferred by their differential sensitivities to light).
In cases where the ambient light changes quickly due to turbidity, the authors found only a single intermediate form whose females were neither predominantly sensitive to red nor blue light and whose breeding males were neither predominantly red or blue (instead of separate red and blue species).
This has implications for the maintenance of cichlid diversity and, according to the authors, provide a mechanistic explanation for the collapse of cichlid fish species diversity during the anthropogenic eutrophication of Lake Victoria.
Eutrophication changes the slope of environmental light gradients, and, by steepening them, potentially moves sites from the region in parameter space that is permissive of species coexistence into the region that is not.
For more information, see the paper: Seehausen, O, Y Terai, IS Magalhaes, KL Carleton, HDJ Mrosso, R Miyagi, I van der Sluijs, MV Schneider, ME Maan, H Tachida, H Imai and N Okada (2008) Speciation through sensory drive in cichlid fish. Nature 455, pp. 620"626.