Remarkable fish see color in deep, dark water

Fish living up to 1500 metres below the surface have developed surprisingly diverse vision that could help them determine predator from prey in the dimly-lit depths of their fish-eats-fish world.

An international research team involving University of Queensland scientists believes the deep-sea discovery which suggests fish may see colour in the dark, shines new light on the evolution of vision in vertebrates, including humans.

UQ Queensland Brain Institute scientist Dr Fabio Cortesi said vertebrates used two types of photoreceptor cells - rods and cones - in order to see.

"Cones are used in bright-light conditions, while rods are generally used in dim-light."

Both rods and cones contain light-sensitive proteins called opsins that absorb light at specific wavelengths.

Colour vision in vertebrates is due to the fact that cones use around four different opsins, said Dr Cortesi.

"This variety allows sensitivity to a broad range of colours."

"Ninety-nine per cent of all vertebrates have just one opsin protein in their rods, so most are colour-blind in dim-light conditions because they rely only on that single rod opsin," he said.

Deep sea fish that live at 200 to 1500 meters below the surface tend to be no exception.

UQ deep-sea visual ecology specialist Dr Fanny de Busserolles said water at that depth filtered most light out.

"Down there it's very monochromatic, and most fish just perceive blue light," she said.

"But we have discovered some spectacular exceptions."

Dr Cortesi said researchers examined the genomes of 101 species of fish.

"We found 13 species had more than one rod opsin gene, and one - the silver spinyfin fish - had a remarkable 38 of these opsins.

He said gene sequence analysis and experiments on how those rod opsins function suggested silver spinyfins are able to pick up a wide range of wavelengths of light, meaning they probably see many colours.

Dr Cortesi said this ability could have evolved as a survival weapon.

"There are many colours of bioluminescence - light produced and emitted by living organisms - down there, and it mainly appears in flashes coming from other fish.

"If you want to survive down there you need to quickly decide if you are seeing a potential predator or potential prey," he said.

University of Basel

New type of highly sensitive vision discovered in deep-sea fish
The deep sea is home to fish species that can detect various wavelengths of light in near-total darkness. Unlike other vertebrates, they have several genes for the light-sensitive photopigment rhodopsin, which likely enables these fish to detect bioluminescent signals from light-emitting organs. The findings were published in the journal Science by an international team of researchers led by evolutionary biologists from the University of Basel.

Color vision in vertebrates is usually achieved through the interaction of various photopigments in the cone cells found in the retina. Each of these photopigments reacts to a certain wavelength of light. In humans, for example, these wavelengths are the red, green and blue range of the light spectrum.

Color vision is only possible in daylight, however. In darkness, vertebrates detect the few available light particles with their light-sensitive rod cells, which contain only a single type of the photopigment rhodopsin - explaining why nearly all vertebrates are color-blind at night.

A genetic record for the silver spinyfin
An international team of researchers lead by Professor Walter Salzburger from the University of Basel recently analyzed more than 100 fish genomes, including those of fish living in deep-sea habitats. The zoologists discovered that certain deep-sea fish have expanded their repertoire of rhodopsin genes.

In the case of the silver spinyfin (Diretmus argenteus), they found no less than 38 copies of the rhodopsin gene, in addition to two other opsins of a different type. "This makes the darkness-dwelling silver spinyfin the vertebrate with the most photopigment genes by far," explains Salzburger.

The deep-sea fish's many different rhodopsin gene copies have each adapted to detect a certain wavelength of light, the researchers further reported. They demonstrated this through computer simulations and functional experiments on rhodopsin proteins regenerated in the lab.

The genes cover exactly the wavelength range of light "produced" by light-emitting organs of deep-sea organisms. This is known as bioluminescence, which is the ability of an organism to produce light on its own or with the help of other organisms. For example, anglerfish attract prey with their bioluminescent organs.

Detecting signals in the dark
The deep sea is the largest habitat on Earth and yet one of the least explored due to its inaccessibility. Many organisms have adapted to life in the near-total darkness of this inhospitable environment. For example, many fish have developed highly sensitive telescope eyes that allow them to detect the tiny amount of residual light that makes it to the depths of the ocean.

In vertebrates, 27 key spectral tuning sites have been identified in the protein for rhodopsin. These sites directly affect which wavelengths are detected. The researchers discovered that in the various gene copies of the deep-sea silver spinyfin, 24 of these positions exhibited mutations.

"It appears that deep-sea fish have developed this multiple rhodopsin-based vision several times independently of each other, and that this is specifically used to detect bioluminescent signals," says Salzburger. He adds that this may give deep-sea fish an evolutionary advantage by allowing them to much better see potential prey or predators.

"In any case, our findings help redefine the current paradigm of vertebrate vision in terms of the role of rod photoreceptors," the zoologists write. This presents yet another instance in which analyzing whole genomes led to new biological discoveries.

University of Maryland

Color vision found in fish that live in near darkness
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. The research appears on the cover of the May 10, 2019, 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 Karen Carleton, a biology professor at the University of Maryland and 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 use two types of photoreceptor cells to see--rods and cones. Both rods and cones 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, 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.

The research paper "Vision using multiple distinct rod opsins in deep-sea fishes," Zuzana Musilova, Fabio Cortesi, Michael Matschiner, Wayne I. L. Davies, Jagdish Suresh Patel, Sara M. Stieb, Fanny de Busserolles, Martin Malmstrom, Ole K. Torresen, Celeste J. Brown11, Jessica K. Mountford, Reinhold Hanel, Deborah L. Stenkamp, Kjetill S. Jakobsen, Karen L. Carleton, Sissel Jentoft, Justin Marshall, Walter Salzburger, was published in the journal Science on May 10, 2019.

Related Links
University of Queensland
Water News - Science, Technology and Politics

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