[Retinal adaptations to habitat].

Abstract

Vertebrates have, through the process of evolution, adapted to their photic environment. This is well manifested in the retinal adaptations to various habitats. Although all vertebrates are considered, emphasis is placed on fishes because they form about 50% of the vertebrate species. In addition, they occupy a wide range of habitats, thus retinal modifications of fishes serve as models for all other vertebrates. The present article reviews morphological, physiological and biochemical retinal adaptations. The quality and quantity of light reaching the aquatic organism are functions of the incident light as well as the aquatic environment. Thus, in well lit, clear waters fishes are arhythmic and possess almost equal populations of rods and cones; whereas fishes in dimly lit environments (due to turbidity or depth) have retinas that are more specialised for high sensitivity-multi-banked retinas, long outer segments, grouped photoreceptors, hypertrophied ellipsoid mitochondria, reflecting tapetum. Similarly, the ratio and distribution of visual pigments (rhodopsin and porphyropsin) and S-potential change with respect to fresh/sea water, clear/turbid water and air/aquatic environments. Thus, in fresh waters, where the photic environment shifts to longer wavelengths, porphyropsin predominates; while in land vertebrates and almost all marine fishes the dominant pigment is rhodopsin. With respect to the latter, fishes in turbid, greenish or yellowish coastal waters have 'rhodopsins' with lambda mas above 500 nm; fishes in clear coastal habitats have 'rhodopsins' with lambda max near 500 nm; while those in the blue-lit environment of deep seas have lambda max below 500 nm. The retinal pigment composition is also associated with habitat changes during diadromous migrations in fishes or during amphibian metamorphosis. It is interesting to note that the dorsal and ventral retinas of Rana catesbeiana and Anableps microlepis which view aquatic and aerial environments, respectively, show a predominately porphyropsin-rich dorsal retina compared to a rhodopsin-rich ventral retina. Similar shifts in the S-potential are observed with change in habitats. Fresh water fishes exhibit L-responses with lambda max in longer wavelengths compared to marine fishes where the maximum of the L-response shifted towards shorter wavelengths.