Deep Ocean Fish Can Distinguish Colors In Darkness

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Scientists have always wondered what life looks like for fish living in the depths of the ocean where it’s extremely difficult to distinguish different colors. However, a group of researchers found that some deep ocean fish may be able to distinguish between colors, despite the popular belief that they are color blind.

An international team of scientists studied the visual system which enables deep ocean fish to see different colors in near total darkness. Their findings appear in a paper published on May 10 in the journal Science.

“This is the first paper that examines a diverse set of fishes and finds how versatile and variable their visual systems can be,” co-author and University of Maryland biology professor Karen Carleton said in a statement. “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.”

The eyes of vertebrates use two types of photoreceptor cells. Rods and cones are equipped with pigments sensitive to light called opsins, which can absorb wavelengths of light and then convert them into a signal that the brain uses to distinguish differences between colors. However, the number and type of opsins in photoreceptor cells is responsible for how animals perceive differences in color.

Before the most recent research was conducted, scientists believed cones enable color vision while rods perceive brightness and dimmer areas. However, after testing the genomes of 101 fish, this study found that some deep ocean fish had multiple rod opsins, which could suggest that they have color vision.

One of the most significant deep ocean fish used in the research was the silver spinyfin fish, which carries 38 rod opsin genes. This number is greater than the number found in other fish. In fact, it’s the highest number of opsin genes found in all known vertebrates. Humans have only four opsins.

“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 that perceiving different colors enables deep ocean fish to detect the right prey. This is a new hypothesis because scientists have long thought that deep ocean fish didn’t have a need for the color vision, given that other light frequencies can’t penetrate as deep as blue light, beneath 600 feet.

The researchers have further plans regarding their research, and that includes broadening their study to other deep-sea fish and looking for other relatives of the silver spinyfin fish, which may have evolved accidentally to have a larger number of rod opsins.

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