New study into how Four-eyed fish see

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New research is helping to explain how the unique capability of the Four-eyed fish (Anableps anableps) to see both above and below water at the same time has evolved.

The Four-eyed fish is so named because it has eyes with pupils divided into two, which function like bifocal lenses, with the top half for seeing out of the water and the bottom half for seeing in the water. The fish spends all of its time swimming at the surface and exposing the top half of its eyes out of the water line. 

The study by Gregory Owens and coauthors, to be published in a forthcoming issue of the journal Biology Letters, helps explain how the Four-eyed fish can see in and out of the water, as well as demonstrating the role of gene expression in the evolution of this unusual visual system.

The authors studied the opsin genes in the fish, which code for light receptors in the eyes.  By developing molecular probes that, when applied to the retina, would bind to specific opsin gene messenger RNA (mRNA translates DNA into proteins) and studying the type and distribution of mRNA in the retina, they were able to assess the light sensitivity of the eye.

The authors found that the part of the retina exposed to aerial light, has cones (photoreceptor cells) bearing opsins that are sensitive to the green wavelengths that predominate in the air, while the portion of the retina covering the submerged portion of the eye has cones with opsins more attuned to the yellow wavelengths of muddy water.

The entire retina also has cones bearing opsins sensitive to blue, violet and ultraviolet wavelengths.

By comparing opsin gene expression in the retina of the Four-eyed fish with a closely-related species with normal eyes (Jenynsia onca), the authors surmised that the lower half of the eye in the Four-eyed fish lost green sensitivity and gained yellow sensitivity in order to gain better aquatic vision in the turbid water of its natural habitat.

For more information, see the paper: Owens, GL, DJ Rennison, WT Allison and JS Taylor (2011) 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. Biology Letters, doi:10.1098/rsbl.2011.0582

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