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 morphometries 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 resource 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.

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The frequently occurring photoreceptor patterns in fish are explained using functional and environmental demands in a geometric model. The shape of the double cone provides a number of constructional properties leading to a limited number of appropriate configurations. The probability of their occurrence is estimated from the degree to which the combination of properties of each eonfiguration meets specific environmental light conditions. A row pattern of merely double cones is especially suitable for vision in a dim homochromatic environment; a triangular pattern is quite appropriate for high resolution and accurate movement detection, whereas the known square pattern has a high adaptive capacity to varying spectral distributions. In this context the transforming capacities of both square and row patterns can be understood.

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.

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