Visual resolution during growth in a cichlid fish: a morphological and behavioural case study.

The spatial resolution of the visual system during growth of the cichlid fish Haplochromis argens was deduced from the cone density according to two models of possible connectivity patterns. The models include a convergency type of 5 cones per visual unit and a divergency type of 1.25 cones per visual unit. The minimum separable angle in this species was measured during ontogeny using operant conditioning in a two choice discrete trial situation. As a consequence of the moderate performances of the juveniles, the behavioural study showed a greater change of visual resolution than was predicted by the morphology. The minimum separable angle of the adult fishes was accurately predicted by the 'divergency model', which led to rejection of the other morphological model. It is argued that the high resolution as found in some other fish species by authors using the same technique may be due to imperfect calculations.

Interspecific variation of photoreceptors in four co-existing haplochromine cichlid fishes.

The photoreceptors of four co-existing haplochromine cichlids have been studied morphometrically and microspectrophotometrically, and the results were interpreted in terms of resolution and sensitivity. Among the three species, Haplochromis argens, H. piceatus and H. ishmaeli, the cone dimensions and the visual pigments differed only slightly, in spite of differences in vertical distribution and feeding behaviour between these species. The fourth species H. pyrrhocephalus showed distinctly different visual properties, with very large double cones at low densities, and its spectral sensitivity shifted to significantly longer wavelengths. To some degree, the photoreceptor morphometrics and visual pigments appeared to be attuned to the photic conditions of the specific habitats. The two species H. piceatus and H. pyrrhocephalus, occupying a dimmer habitat with less penetrating blue light, had reduced blue sensitive single cones, whereas the surface dwelling species H. argens had relatively small red/green sensitive double cones. Also an attempt has been made to relate the visual capacity of the cichlids to their behavioural patterns, focussing on prey detection. The greatest visual differences were observed in the two species H. argens and H. pyrrhocephalus, with resources coincidence and with the highest encounter frequency, which may indicate that visual segregation contributes towards reducing interference competition among co-existing species with a similar diet.

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.