Ontogenetic change of visual thresholds in the cichlid fish Haplochromis sauvagei

The ontogenetic alterations of visual sensitivity thresholds and acuity of the cichlid Haplochromis sauvagei were quantitatively predicted from the developmental change of size and convergence of the photoreceptors and measured behaviourally using optomotor response. The observed enlargement of the cone-photoreceptors was considered to improve their photon catching ability. Accordingly, the ontogenetic change of the photopic threshold was accurately predicted by the increase of the cone size during growth. The measured scotopic threshold decreased to a greater degree during growth than was predicted by the increasing number of rods connected to a ganglion cell. Additionally, the resolution and the flicker fusion frequency were derived from histological and behavioural measurements, respectively. The behaviourally measured resolution was much higher than expected, which was probably due to the method used. In conclusion, the ontogenetic addition and enlargement of photoreceptors in the eyes of fishes lead to improved visual properties in general, which enable a fish to perform new visual tasks.