Light-induced modulation of retinal development in the cichlid fish Haplochromis sauvagei (PfefFer, 1896)

Phenotypic modulation is a common feature in haplochromine cichlids. External changes influence the phenotype within the limits set by the genetic program. Retinal development in the cichlid Haplochromis sauvagei has been observed in specimens reared in various chromatic environments and in darkness. Rearing in darkness led to the reduction of photoreceptor densities, the apparent loss of single cones and the enlargement of double cones. To a lesser degree, similar effects were observed in specimens deprived from short-wave light. Ganglion cell densities were not affected by light deprivation. The light-induced alterations could be the result of changes in developmental rates within the retina or of early degeneration. The modulation seems to cause a reduction of blue sensitivity, possibly for the benefit of luminosity or long-wavelength detection. This would agree with the combination of decreased light intensity and a spectral shift towards the longer wavelengths as is the case in the natural environment of the Lake Victorian haplochromines. In that sense, the modulation is probably adaptive.