Retinal organisation in a hill-stream cyprinid, Crossocheilus latius latius Hamilton.

The retinal organisation of a cyprinid fish, Crossocheilus latius latius Hamilton, which inhabits the sub-Himalayan torrents of Sikkim, India, has been studied by light microscopy. The fish are small and cylindrical having relatively large eyes with a large, oval pupil. They are adapted to the low-light level of the hill-torrents, which is caused by the heavily silted monsoon water. The retina possesses numerous rods and bulky cones, the latter containing multiple oil droplets, and located mostly sclerally and some both sclerally and vitreally. There are three rows of large horizontal cells and a few bipolar and sparsely distributed ganglion cells. Retinomotor response, as evidenced by the scleral migrations of photoreceptors and pigment epithelium during dark-adaptation, is prominent in the species. The bulky cones with their prominent, multiple oil droplets have been suggested as the devices which help the fish to negotiate in the hill-stream's turbid environment.