As if swimming upstream and dodging bears weren’t difficult enough, salmon must also switch their vision systems to navigate the long journey from open ocean to inland waters. Scientists have long wondered how freshwater fish and amphibians shift their vision from marine or terrestrial environments — where the light environment is blue-green — to the waters of inland streams where organic particles filter out light from the blue end of the visual spectrum, creating a red and IR light environment. The Cyp27c1 enzyme converts vitamin A1 to vitamin A2, enhancing the ability of freshwater fish and amphibians to see red and IR light. Photo courtesy of Corbo Laboratory. Now, scientists at Washington University School of Medicine in St. Louis report in the journal Current Biology that they have discovered an enzyme that switches the visual systems of some fish and amphibians, and supercharges their ability to see IR light. The enzyme — called Cyp27c1 — is closely linked to vitamin A, long known for promoting good vision, especially in low light. Cyp27c1 converts vitamin A1 to vitamin A2, which enhances the ability to see longer-wavelength light, such as red and IR. The findings could lead to advances in biomedical research, particularly in optogenetics, where applications are currently limited to visible light that penetrates only the top layer of neural tissue. If scientists are able to incorporate the newly discovered enzyme, they may be able to activate photosensitive neurons with IR light, which penetrates much deeper. “Just as the enzyme helps fish peer into murky water, it could help us peer deeper into the brain,” said Joseph Corbo, professor of pathology and immunology at Washington. Corbo and his team made the enzyme discovery in zebrafish and confirmed their findings in bullfrogs, whose eyes are uniquely designed for the light environments of both air and freshwater. Bullfrogs sit with their eyes at the water’s surface, allowing them to look up into the air and down into the water at the same time. The researchers found vitamin A2 and Cyp27c1 in the upper half of the bullfrogs’ eyes, but not in the lower half. Salmon and other freshwater fish and amphibians supercharge their ability to see red and IR light. Photo courtesy of National Park Service. The researchers also showed that zebrafish with normal copies of the cyp27c1 gene moved toward IR light shined into a dark aquarium. Fish with disabled cyp27c1 genes continued to behave like they were in the dark, whether or not the IR light was on. The human genome does contain a form of the cyp27c1 gene, but it’s not active in the eye. Thus humans can’t enhance their IR vision in the same way fish can. How exactly the gene is utilized in the human body is not yet clear. “But just because our eyes don’t make vitamin A2 doesn’t mean we can’t use it,” Corbo said. Research on medical students in the 1940s showed that people who consume vitamin A2 have an enhanced ability to detect red and IR light. And in 2013, a group of biohackers successfully crowdfunded an experiment to extend their vision into the near-infrared spectrum by eating a diet supplemented with vitamin A2. “I wouldn’t necessarily recommend following their dietary advice, but the concept is sound,” Corbo said.