![]() More rhodopsin is present in the retina of lamprey migrating to the sea while porphyropsin predominates in the eye of spawners in freshwater. Migratory species like Atlantic salmon (Salmo salar), eels (Anguilla) and lamprey (Petromyzon marinus) possess both rhodopsin and porphyropsin. Rhodopsin is purple coloured (marine Actinopterygii) while porphyropsin is rose-coloured (freshwater Actinopterygii). The retina of fishes contains two types of light sensitive pigments, rhodopsin and porphyropsin. ![]() In sight feeders which are active during the day, the cones are more in number than rods, and the revenue is found in many crepuscular species which are more active in twilight. The relative number of rods and cones vary considerably in different species. Such photomechanical or retinomotor reaction is much pronounced in fishes than other vertebrates. ![]() Simultaneously the contractile myoid element in the base of the rods and cones move the cell tips in such a manner that rods are moved away from the lens in the bright light to be covered by the epithelium melanin, whereas in dark the rod myoids contracts and brings the cells towards the lumen of the eyeball.Ĭones migrate in the opposite direction towards the lens in the bright light and towards the outer epithelium in dim light (Fig. Melanin is photosensitive and in bright light it spreads and shades the sensitive rods in weak light, it aggregates near the choroid border, thus photosensitive cells are fully exposed to the amount of light available (Fig. The epithelium of retina contains melanin and borders on the choroid coat. Therefore, the vascularization patterns in the eye of Plecoglossus altivelis and carp is further classified into type E-a and type E-b respectively. In Plecoglossus altivelis, embryonic fissure is present in the retina. This type of intra-molecular vascularization is found in carp and eel. Type E blood vessels are present in the vitereo retinal boundary and are radially arranged from the centre, but they are absent in the retina. The vessels are restricted to the falciform processes and are absent in the retina. The man vessel enters the eye from beneath the optic nerve papilla. Type A can be seen in the eye of rainbow trout which possesses falciform processes, containing the small blood vessels branches from the main vessel. Thus these fishes have capacity to focus simultaneously the distant and nearby objects. Other is for the light rays reflected from close object and focus on the posterior retina. In trouts (Salmoninae) the lens has two focal lengths, : one is for the light rays reflected from a distant object lying lateral to the fish and focus on the central retina. However, some sharks and rays (Elasmobranchii) accommodate by small changes in lens convexity. The lens is moved with the help of muscular papillae present on the ciliary body and adjustment of the vision occurs. In fishes, accommodation is usually achieved by altering the position of lens rather than changing lenticular shape. It is not homogenous and has an actual refractive index from 1.53 at the centre to 1.33 near the periphery. Fish lens has a very high effective index of refraction. ![]() In deep sea fishes, the eyes are projected with large lenses. Sometimes the lens is pyriform (Anablepidae) to provide for aquatic and aerial vision. However, in sharks and rays the lens is horizontally compressed. ![]() The bony fishes have roughly spherical lens. The lens fibres are highly modified epithelial cells. The lens substance is composed of lens fibres which are arranged in flat hexagonal prism. ![]()
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