Deep Color

Fish living in deep, dark waters where animals were presumed to be colorblind may actually have the ability to experience life in living color after all.

An international team of researchers that includes a Maryland biology professor report this previously unknown visual system in a paper featured on the cover of today’s issue of 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,” said biology Professor Karen Carleton, a co-author of the paper. “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, fishes and humans included, use two types of photoreceptor cells to see—rods and cones. They contain light-sensitive pigments called 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, scientists accepted that cones are responsible for color vision, and rods are responsible for detecting brightness in dim conditions of the sort where deep-sea fish live. But 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 single photons in low-light conditions. In 99% of all vertebrates, rods express just one type of light-sensitive opsin, which means the vast majority of vertebrates are color-blind 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—the highest number 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.

“It means the silver spinyfin fish have very different visual capabilities than we thought,” Carleton said. “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.

This research was conducted in collaboration with researchers from the University of Queensland in Australia, Charles University in the Czech Republic and the University of Basel in Switzerland.