How Fish May See Color in the Deep Ocean’s Darkness

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Fish that have never known sunshine could be able to see the world in shades of blue and green we can’t even imagine.

The silver spinyfin, or little dori, inhabits a layer of the deep sea, where the Twilight Zone’s blue fades to black, often half a mile below the surface. Down there, they may see the world like no other animal known to science.

Scientists have generally understood that color vision wasn’t necessary in the deep sea. It’s too far for sunbeams to penetrate, and so there’s no light to give way to color. But in a study published Thursday in Science, researchers interested in the evolution of color vision analyzed the genomes of 101 different fishes. They discovered that one, the silver spinyfin, has more genes for discriminating dull light than any other vertebrate on the planet. These genes make it possible to see the whole range of residual daylight and the full spectrum of bioluminescence in the deep sea. Other fishes may have this ability to detect color in the deep sea, too.

“In vertebrate fishes, nothing has been seen like this before,” said Megan Porter, who studies how vision evolved at the University of Hawaii at Mānoa and was not involved in the research. “This goes against what we understood as how visual systems evolved in the deep sea, which means we have to question how visual systems work and function in the dim light.”

The basics of vision start when light hits our retinas, which contain photoreceptor cells called rods and cones that are sensitive to particular wavelengths. Inside these cells, photopigments, or proteins called visual opsins help translate light into signals our bodies can understand.

Typically vertebrates have up to four cone photoreceptors, and one rod photoreceptor. Most humans, for instance, see color input from three cones — red, green and blue. Cones help us see colors in bright light, but in dim light, we’re generally colorblind, and see only intensity based on input from a single rod.

Zuzana Musilová, an evolutionary biologist at Charles University in Prague who led the study, and her team first noticed these fish had lost the genes other fish had for making cone cells and opsins that could detect red and ultraviolet parts of the spectrum. This wasn’t surprising: These wavelengths don’t penetrate the deep sea. But then they found some deep sea fishes had extra copies of genes to make rods.

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