Photopic adaptations to a changing environment in two Lake Victoria cichlids

Abstract

During the past 30 years, Lake Victoria cichlid fishes have encountered severe environmental and ecological changes including an introduced predator and new prey types. Furthermore, increased eutrophication has led to reduced water transparency and shifted the spectral composition of underwater light to longer wavelengths. In the present study, collections of two cichlid species, Haplochromis pyrrhocephalus and Haplochromis tanaos, from before and after the environmental changes, were compared with respect to their photopic resolution and sensitivity. Eyes of both species were dissected and retinal features were measured from tangential sections. In both species, the eyes became smaller, independently of body size. This decrease possibly occurred to make space for other structures that increased in size. In H. pyrrhocephalus, a significantly lower resolution was found. However, the size, and thus photon-catching ability of the double cones, remained unchanged, despite the smaller eyes. In the modern populations of H. tanaos, the double cone size increased in relation to eye size, so that the photon-catching ability of the smaller fish remained the same. However, no significant decrease in resolution was found. Shortwave sensitivity was found to be lower in both modern populations because of a reduction or the complete absence of single cones. The results obtained in the present study imply that these resurgent zooplanktivores are capable of adapting their eye morphology to the changed environmental conditions without losing crucial aspects used for survival and reproduction. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106, 328‐341. ADDITIONAL KEYWORDS: adaptive response ‐ angular density ‐ diet change ‐ eutrophication ‐ hypoxia ‐ introgressive hybridization ‐ microevolution ‐ plasticity ‐ spatial constraints ‐ water transparency.