Micro-morphology of the retina of the light-adapted African catfish (Clarias gariepinus).

The current study aimed to investigate the ultrastructure of the retinal photoreceptors of the African catfish and to demonstrate their adaptation to nocturnal or diurnal visions or by the two ways. The eyes of eight adult catfish were collected during the daytime, and the retinae were separated and examined by light and transmission electron microscopy. The photoreceptors' layer appeared in contact with the retina's pigmented epithelium. Two photoreceptors were detected in cones and hidden rods. Cones predominate in light-adapted retinae. The outer segments of cones appeared between the retinal pigmented epithelium protrusions, which indicates the movement of melanosomes away from the photoreceptors as a retinomotor response of the catfish. The two types of retinal tapetum were in between cones. The first type, the cored granules, were large, spherical, and had black peripheral parts and central lucent parts, and contained some granules. The second type was Guanine crystallites of tapetum lucidum, which were small electron-lucent, and their shape varied from spherical to rectangular. Melanosomes vary in shape from spherical to elliptical. The Müller cells were darkly stained elongated cells that measured about 5.5-8.5 μm in length and 2.2-2.5 μm in width, and their microvilli appeared between the inner segments of the rods and cones. Müller cell processes were extended from the photoreceptor layer to the inner limiting membrane. Zonula occludentes appeared between the Müller cell processes and the internal segment of the rods and cones. African catfish have eyes which are adapted not only for nocturnal but also for daytime light.