Deep-Sea Fish Have Evolved Highly Sensitive Vision To See Color In Near-Total Darkness

The viperfish (Chauliodus) has a prominent bioluminescent organ under its eye. Wen-Sung Chung, University of Queensland

Ever wonder how the anglerfish is able to reel its victims in with its bioluminescent fishing lure? Now, you may not have to. Scientists believe they have worked out how certain deep-fish species have evolved biological adaptations to see – and do so in color – in the darkest depths where bioluminescence is often the only source of light.

The whole-genome sequencing of more than 100 fish genomes found a previously undescribed visual system that allows fish to see in near-total darkness between depths of 200 to 1,500 meters (650 to nearly 5,000 feet). Most animals are able to see by converting the light that enters the eye into electrochemical signals that our brains interpret. This light is picked up by cones (that work better in daylight) and rods (which are more sensitive to dark). Both of these contain light-sensitive proteins called opsins, which absorb light and different wavelengths.

Most cones use around four different opsins that allow vertebrates to see a broad range of color – but deep-sea fishes go far beyond that.

"Ninety-nine percent of all vertebrates have just one opsin protein in their rods, so most are color-blind in dim-light conditions because they rely only on that single rod opsin," said Fabio Cortesi, co-author of the study published in Science.

Rather than using a single rod protein to see in the dark, deep-sea fish have multiple distinct rhodopsin (RH1) photopigments that can pick up on bioluminescence given off by other deep-sea organisms. To determine this, researchers analyzed 101 fish genomes, including those found in deep-sea habitats. Thirteen deep-sea fish species, in particular, have an expanded set of rhodopsin genes capable of tuning into different wavelengths of light. In particular, the spinyfin (Diretmus argenteus) had at least 38, suggesting that it can probably see a wide variety of colors

"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 study co-author Watler Salzburger. 

Computer simulations and functional experiments on lab-regenerated rhodopsin proteins allowed researchers to determine that these many gene copies are more than likely an adaptation to detect the certain wavelength of light and may be used as a way to find prey or avoid predators.

"There are many colors of bioluminescence – light produced and emitted by living organisms – down there, and it mainly appears in flashes coming from other fish,” said Cortesi. “If you want to survive down there you need to quickly decide if you are seeing a potential predator or potential prey.”

Fish in the deepest depths of the ocean have adapted in a number of ways to see. Some fish, for example, have developed highly sensitive telescope eyes that allow them to pick out the tiniest amounts of residual light that make it toward the bottom of the ocean.

The lanternfish has bioluminescent organs and an increased number of rhodopsin genes. Zuzana Musilová, Charles University, Prague
The tub-eye 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. Dr. Wen-Sung Chung, University of Queensland, Australia



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