Studies on the eye of some nile teleosts III- a comparative study on the histology of the retina of two nile teleostean fishes

ABSTRACT

In this study, the retinae of two Nile teleostean fishes; a superficial living fish [Gambusia affinis]and a deep living one [Clarias lazera] were examined mainly by light microscopy and found to be duplex i.e. to contain both rods and cones. The different layers of the two retinae were described with particular regard to the visual cell layer. In the retina of Gambusia, the cones predominated over the rods and vice versa in the bottom fish, Clarias lazera. This form of structure formed an adaptation to the habitat of each fish, where Gambusia lived in nearly dark medium near the bottom of the river. Foveal regions were found only in Gambusia and in contrast to mammals, all the layers of the retina were well represented in the foveal regions. In Gambusia, there was only one blind spot because the optic nerve had one exit, while the optic nerve in retina of Clarias exited in about eight to ten points that constitutes eight or ten blind spots in this fish. Retinal blood vessels were more numerous in the inner surface of the retina of Clarias. It was concluded that the foregoing findings pointed to the great efficiency of the retina in Gambusia [which lives in a well illuminated habitat] compared with the retina of Clarias, the bottom living fish