Salmon have developed a clever way to navigate through murky waters that come in handy when they're looking for a place to spawn upstream, where vision gets tricky due to an abundance of sediments clouding the water. To see more clearly,  salmon and other freshwater fish have evolved an ability to detect infrared light. 

"We've discovered an enzyme that switches the visual systems of some fish and amphibians and supercharges their ability to see infrared light," Joseph Corbo, senior author of the study and an associate professor of pathology and immunology at Washington University School of Medicine in St. Louis, explained in a news release. "For example, when salmon migrate from the ocean to inland streams, they turn on this enzyme, activating a chemical reaction that shifts the visual system, helping the fish peer more deeply into murky water."

This study sheds light on how freshwater fish and amphibians are able to easily shift their vision between marine and inland environments. Essentially, marine water is blue-green, whereas inland  streams shifts to red and the infrared end of the spectrum, becuase mud, algae and other particles filter out blue light.

To adapt to changing environments, fish have evolved with an enzyme researchers refer to as Cyp27c1, which is closely linked to vitamin A and known to promote good vision in low light, according to the release.

Researchers have found that this enzyme is also present in freshwater zebrafish and – surprisingly – bullfrogs who sit with their eyes at the water's surface so that they can easily look up into the air for flies, while still being able to perceive what's going on in the water below. In bullfrogs, the enzyme is only present in the part of the eye that looks down into the water. 

To further test that Cyp27c1 is the enzyme responsible for the animals' night vision abilities, researchers examined its presence in zebrafish, who were able to detect an infrared light aimed into a dark aquarium and swim towards it. Other fish with disabled Cyp27c1 genes became confused and were unable to detect the infrared light through the darkness.

This study, recently published in the journal Current Biology, has broader applications for biomedical research, particularly in optogenetics, researchers noted.

"Just as the enzyme helps fish peer into murky water, it could help us peer deeper into the brain," Corbo added.

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