In the four-eyed fish (Anableps anableps), the regions of the retina exposed to aquatic and aerial light do not express the same set of opsin genes.

The four-eyed fish, Anableps anableps, has eyes with unusual morphological adaptations for simultaneous vision above and below water. The retina, for example, is divided such that one region receives light from the aerial field and the other from the aquatic field. To understand better the adaptive value of this partitioned retina, we characterized photoreceptor distribution using in situ hybridization. Cones expressing sws1, sws2b and rh2-2 (i.e. UV, and short wavelength-sensitive) opsins were found throughout the retina, whereas cones expressing rh2-1 (middle wavelength-sensitive) were largely limited to the ventral retina and those expressing lws (long wavelength-sensitive) opsins were only expressed in the dorsal retina. We next asked when this pattern evolved relative to the 'four-eyed' morphology. We characterized opsin expression in Jenynsia onca, a member of the sister genus to Anableps with typical teleost eye morphology. In J. onca, sws1, sws2b, rh2-2 and rh2-1 opsins were expressed throughout the retina; while lws opsins were not expressed in the ventral retina. Thus, the change that coincides with the evolution of unusual anablepid eye morphology is the loss of rh2-1 expression in the dorsal retina, probably to accommodate increased lws opsin expression. The retinal area that samples aerial light appears not to have changed with respect to photoreceptor transcription.