Some Deep-Sea Fish Can See Colour In Near Total Darkness

Illustration of the silver spinyfin (Diretmus argenteus)—the vertebrate with the highest number of opsin genes. (Image: Alexandra Viertler, University of Basel)

A newly discovered visual system in deep-sea fish could allow them to discern predators from prey in the low-light conditions found at the bottom of the ocean, new research suggests.

A team led by Walter Salzburger and Zuzana Musilova from the University of Basel in Switzerland has identified a previously undocumented visual system in deep-sea fish, according to a new paper published today in Science Advances.

The fish endowed with this highly sensitive form of vision, of which 13 species are now known, may discern certain wavelengths of colour at depths reaching 1,500 meters, where light from the surface is practically non-existent. Armed with this capacity, the deep sea fish may be able to detect bioluminescent organisms, though future research will have to explore this possibility further.

To understand how this newly detected visual system works, we’ll need to review the basics behind vision.

Vertebrates with the capacity for vision, whether fish or human, use specialised light-sensitive photopigments to convert incoming light into a signal the brain can interpret and relay as vision. Those photopigments involve proteins called opsins that are capable of absorbing light. Cone opsins are used when the lighting is bright, while rod opsins tend to be used when it’s dark. The opsins of rods and cones absorb light at specific wavelengths, but colour vision arises from the interplay of multiple cone opsins. The vast majority of vertebrates have a single rod opsin, and they’re virtually colour-blind in low-light conditions. Marine biologists figured deep-sea fish were no exception, but the new research suggests otherwise.

Lanternfish have both bioluminescent organs and an increased number of rhodopsin genes. (Image: Zuzana Musilová, Charles University, Prague)

Rather than a single rod opsin, some deep-sea fish are equipped with a series of distinct rod opsin photopigments, according to the new study. Working together, these rod opsins allow the fish to hone in on a wide range of visual wavelengths — namely the wavelengths produced by bioluminescent deep-sea organisms.

“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,” said Salzburger in a statement.

The ability could allow the fish to detect prey, predators, or even members of their own species at the bottom of the sea.

For the study, the researchers analysed over 100 different fish genomes. This resulted in the discovery of at least 38 different rhodopsin genes that encode for light-sensitive photopigments. These rhodopsin genes, as it turns out, are fairly widespread among deep-sea fish.

The researchers managed to identify 13 different species with multiple rod opsin genes, including the silver spinyfin which had a whopping 38 rod opsins — the most yet discovered in a vertebrate species, according to the new research. This fish can likely see a variety of colours in low-light conditions. Other fish with extra rod opsins included lanternfish, tub-eye fish, and viperfish.

By using computer simulations, and by growing the rhodopsin-related proteins in the lab, the researchers were able to conduct tests in which the photopigments were shown to capture different wavelengths of light, including those associated with bioluminescence. That said, the researchers still need to test this presumed ability on actual living fish, which will now be the focus of future work.

The viperfish has a prominent bioluminescent organ under its eye, along with multiple rhodopsin genes. (Image: Wen-Sung Chung, University of Queensland)

“This study provides convincing evidence for a novel and unexpected mechanism that deep-sea fishes might use for optimising detection of the scarce photons of light, transmitted from the Sun or emitted by bioluminescence from other organisms, deep below the water surface,” Vladimir J. Kefalov, a professor of ophthalmology and visual sciences at Washington University in St. Louis who wasn’t affiliated with the new study, wrote in an email to Gizmodo.

“[T]he authors of the study demonstrate that the different variants of the rhodopsin gene expressed in these deep-sea fish have different absorption spectra, spanning a range of about 70 [nanometres], dramatically broader than the 5-10 nm range found in most other animals. This could allow deep-sea fish to detect very dim light over an unusually wide spectral range, giving them a competitive edge for survival and procreation.”

Kefalov said the study was “elegant,” and that it demonstrated the “power of adaptive evolution.” In this case, the new research showed a unique mechanism by which a species living in extreme conditions can evolve “ingenious mechanisms for survival,” he said.

We will end with a poem, sung to the melody of, and inspired by, the not-so-classic track, “The Fish Head Song.”

Fish eyes, fish eyes

Extra-opsin fish eyes

Fish eyes, fish eyes

Seeing with no Sun

Fish eyes, fish eyes

Deep-sea suited fish eyes

Fish eyes, fish eyes

Colour vision’s fun!

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