Vision is a complex process in the teleost fishes inhabiting different habitats especially those of both nocturnal and diurnal living. In the present study, four species of fresh water teleost were collected from the River Nile at Dakahlea Governorate, Egypt. These are, three nocturnal species including Clarias greipinus (Clariidae), Malopterus electricus (Malapteruridae) and Anguilla anguilla (Anguillidae) and one diurnal fish Orechromus niloticus. The ocular regions of fishes were dissected separating lens, cornea and retina for histological, morphometric, scanning and transmission electron microscopy investigations. Morphologically, the gross structure of the ocular region varied between the species. The lens exhibited organized lens fibers with characteristic ball and socket structure densely grouped in Clarias gariepinus and Orechromis niloticus compared to less compacted fibers in Anguilla anguilla and become widened at both peripheral lens angles in Malopterus electricus. The cornea was pigmented in the Anguilla anguilla and clear in the other examined fishes. The retinal thickness and visual acuity were markedly increased in the nocturnal fish compared to the diurnal species. There was an increase in the pigmented epithelium and photoreceptors of the nocturnal fish such as C. gareipinus and M. electricus however this was less dense in both A. anguilla and O. niloticus. Also, the photoreceptors of the nocturnal fishes were composed mainly of rods and few single and double cones. The neural circuit of retinal cells showed comparatively increased distribution of amacrine and Muller cells in between the ganglion and inner nuclear cells in both the C. gareipinus and M. electricus. These data suggest that nocturnal fishes C.garepipinus, M.electricus and A. anguilla and the diurnal fish O. niloticus exhibited differences in corneal, lenticular and retinal structures accommodated to their survival.

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