Extreme adaptation of a fish’s vision beneath the ice | Polarjournal
This is Chionodraco hamatus which belongs to the group of ice fish, the Notothenioides. A group of animals that make up 95% of the biomass of Antarctica between 0 and 3,000 meters depth. Image: Marrabbio2 / Wikimedia

The ice fish Chionodraco hamatus has acquired the extreme ability to see beneath the ice with a photosensitive retina that captures the near infrared despite the surrounding cold: -1,9 °C.

Underneath the ice it is very dark and very cold. Gianni Castiglione of Vanderbilt University and Belinda Chang of the University of Toronto studied the ability of icefish to capture light under such conditions and published their study this month in Molecular Biological Evolution displaying some surprising results about the physiology of these cold-blooded animals. Their eyes are equipped with receptors called rhodopsin which are sensitive to infrared too. They are active even while their metabolism is slowed down by the extremely low surrounding temperatures.

Already equipped with antifreeze proteins, the cells of ice fish are protected. But this does not explain the ability of the receptors to respond to light. Belinda Chang usually works on “all kinds of different vertebrates, like whales, bats, but also lizards, snakes, and birds… we’re working on evolutionary questions to try to understand how these animals improve their eyesight in the different environments they live in. Gianni, one of the brightest Ph.D. students I’ve ever worked with came up with this great project of looking at icefish.” This idea came to them “after having studied in the lakes of the Peruvian Andes the visual capacities of fish living in conditions comparable to those of Antarctica”, adds Gianni Castiglione.

Images: Andrei Lomize / GM. Woodward

“Rhodopsin is quite common, it is found in many animals and in many organs. It is a family of proteins that groups together and forms receptors. They cross cell membranes and are immersed in a mixture of lipids”, says Julien Bous, structural biologist of proteins, from the Karolinska Institute in Sweden. “Depending on its function, rhodopsin can receive different signals such as hormones, small molecules or light. It serves as an intermediary between the outside and inside of the cell.”

For ice fish, rhodopsin covers the back of the eye. ” The light-sensitive cells are completely filled with these receptors, almost to the limit. The fish try to capture as much light as possible,” adds Belinda Chang. “This rhodopsin is activated in light when a vitamin A derivative changes position on contact with photons, much like a light switch. This is one of the most sensitive responses in the animal world.”

The mutations that allowed these fish to see in the cold darkness are due to “a structural change that reduced the stability of the protein and made it more sensitive in this slow-moving environment,” concludes Gianni Castiglione.

Camille Lin, PolarJournal

Link to study: Castiglione, G.M., Hauser, F.E., Van Nynatten, A., Chang, B.S.W., 2023. Adaptation of Antarctic Icefish Vision to Extreme Environments. Molecular Biology and Evolution 40, msad030. https://doi.org/10.1093/molbev/msad030.

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