Color Vision Found in Fish that Live in Near Darkness

The tube-eye fish, Stylephorus chordatus, was found to use five different rod opsins within its eyes. The long cylindrical shape of its eyes increases light capture and also enables the fish to move the eyes from a horizontal to a vertical position. (Source: W.-S. Chung, U. Queensland)

An international team of researchers discovered a previously unknown visual system that may allow color vision in deep, dark waters where animals were presumed to be color­blind. “This is the first examination of a diverse set of fishes and finds how versatile and variable their visual systems can be,” said Karen Carleton, a biology professor at the Uni­versity of Maryland. “The genes that determine the spectrum of light our eyes are sensitive to turn out to be a much more variable set of genes, causing greater visual system evolution much more quickly than we anti­cipated.”

Vertebrate eyes use two types of photo­receptor cells to see – rods and cones. Both rods and cones contain light-sensitive pigments – opsins, which absorb specific wavelengths of light and convert them into electro­chemical signals that the brain interprets as color. The number and type of opsins expressed in a photo­receptor cell determine the colors an animal perceives. Before this new study, it was accepted that cones are responsible for color vision, and rods are responsible for detecting bright­ness in dim conditions. This new work indicates that is not strictly the case. By analyzing the genomes of 101 fish, the researchers dis­covered that some fish contained multiple rod opsins raising the possi­bility they have rod-based color vision.

Cones typically contain genes for expressing multiple opsins, which is why they are used for color vision. But they are not as sensitive as rods, which can detect a single photon and are used for low-light vision. In 99% of all verte­brates, rods express just one type of light-sensitive opsin, which means the vast majority of verte­brates are colorblind in low-light condi­tions. Vision in most deep-sea fish follows this same pattern, but the new research revealed some remarkable exceptions. By analyzing the genes for expressing opsins in rods and cones of fish living from the shallow surface waters down to 6,500 feet of depth, the researchers found 13 fish with rods that contained more than one opsin gene. Four of those, all deep-sea fish, contained more than three rod opsin genes.

Most remarkable was the silver spinyfin fish, which had a surprising 38 rod opsin genes. That is more opsins than the researchers found in the cones of any other fish and the highest number of opsins found in any known verte­brate. Human vision by comparison uses four opsins. In addition, the rod opsins found in silver spinyfin fish are sensitive to different wave­lengths. “This was very surprising,” Carleton said. “It means the silver spinyfin fish have very different visual capa­bilities than we thought. So, the question then is, what good is that? What could these fish use these spectrally different opsins for?”

Carleton believes the answer may have to do with detecting the right prey. It has long been presumed that animals living in very deep water have no need for color vision, because only blue light pene­trates deeper than 600 feet. But despite the lack of sunlight, the deep sea is not devoid of color. Many animals that live in darkness generate their own light through bio­luminescence. The new study found that in fish with multiple rod opsins, the specific wavelength of light their opsins are tuned to overlap with the spectrum of light emitted by the bio­luminescent creatures that share their habitat.

“It may be that their vision is highly tuned to the different colors of light emitted from the different species they prey on,” Carleton said. It’s important to note that the four species of fish found to have more than three rod opsins are unrelated species. This suggests that rod-based color vision, which can be thought of as deep-water color vision, evolved inde­pendently multiple times and must confer some benefit to survival. The researchers say their next steps are to broaden the study to other deep-sea fish and to look for shallow-water relatives of silver spinyfin fish that may have evolved a large number of rod opsins. (Source: U. Maryland)

Reference: Z. Musilova et al.: Vision using multiple distinct rod opsins in deep-sea fishes, Science 364, 588 (2019); DOI: 10.1126/science.aav4632

Link: Zoological Institute, Dept. of Environmental Sciences, University of Basel, Basel, Switzerland

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