Retinal cytoarchitecture in Some Mountain-stream Teleosts of India

In this study, the cytoarchitecture of retina in 10 mountain-stream teleosts (seven cyprinid and three loach species) of India was examined by conventional light microscopy. The mountain-streams are shallow, cold water bodies having high turbidity in the Monsoon and clear water in the colder months. The incumbent fishes are, thus, periodically exposed to low and bright light of the streams, respectively. It was of particular interest to see whether their retina would reflect adaptations to the changing photic environments of the mountain streams. The retina of the cyprinids possess multiple cone types, a moderate cone density, high convergence ratios and prominent retinomotor responses. Triple cones occur only in the danio, Danio aequipinnatus; other cyprinids possess miniature, short and long single cones, and double or twin cones. In general, the cones are bulky in appearance. Cone droplets (ellipsosomes) are found in Garra lamta and G. gotyla gotyla. All cone types, including the miniature single cones, undergo elongation during dark adaptation. The common retinal features in the loaches are the bulky cones, ellipsosomes, low rod density and low convergence ratios. There is no area-like specialization of the retina in any of the species examined. No relationship is obvious between cone pattern and ambient light or food habit in any of those teleosts studied; row pattern is present throughout the retina of the surface-dwelling, total insectivores (cyprinids: Danio aequipinnatus and Barilius spp.), whereas square mosaic pattern is present in the bottom-dwelling total herbiovores (Garra spp.) as well as in the insectivores (loaches: Nemacheilus spp.). The retinal features have suggested that the cyprinids are well-adapted to both clear and turbid water (ellipsosomes, multiple cone types, bulky cones, high rod count and high convergence ratios); the loaches, on the other hand, are better adapted to clear water of the mountain streams.