Influence of dark-rearing on the ontogenetic development of Sarotherodon mossambicus (Cichlidae, Teleostei): I. Effects on body weight, body growth pattern, swimming activity and visual acuity.

The ontogenesis of the cichlid fish, Sarotherodon mossambicus was investigated under normal- and dark-rearing conditions from hatching to 100 days. Light deprivation did not change normal increase in body weight. The increase in body length was slightly retarded in dark-reared animals (DR), whereas the increase in body height was more pronounced as compared to normal-reared animals (NR). DR failed to swim up after resorption of the yolk sack. Visual acuity was severely impaired. The first deficits in visual discrimination were observed after 20-30 days in the dark. After 50 days DR failed to show any optokinetic nystagmus.