Visual pigments in the four-eyed fish, Anableps anableps

Abstract

The retinae of terrestrial and marine vertebrates generally contain the visual pigment, rhodopsin, whereas those of freshwater vertebrates contain porphyropsin^1,2. The latter pigment absorbs light at longer wavelengths than rhodopsin and therefore has increased red sensitivity, which is possibly beneficial to animals living in water containing suspended particles since these may cause image degradation due to light scattering at short wavelengths. Many fish and amphibians have both types of pigment, and in some fish, the proportion of rhodopsin to porphyropsin is greater in the dorsal half of the retina³. In the bullfrog, Rana catesbeiana, the ventral retina contains only rhodopsin, whereas the dorsal retina has appreciable amounts of porphyropsin⁴. This may be attributable to the fact that the bullfrog often has only its eyes and nose protruding above the water surface, thus the dorsal part of the retina is used for aquatic vision and the ventral part for aerial vision. The ‘four-eyed’ fish, Anableps, is able to see simultaneously in air and water; it normally swims so that the eye is bisected by the water meniscus. Each eye has a single retina, but the dorsal part receives images from the water, and the ventral region receives aerial images^5,6. Thus it has been suggested that the dorsal region should show more red dominance⁷, with porphyropsins being restricted to this area. We have measured the absorbance spectra of the photoreceptors, and report here that the same visual pigments are present in the dorsal and ventral regions of Anableps retina.