The silver spinyfin, or little dori, inhabits a layer of the deep sea, where the Twilight Zone’s blue fades to black, often half a mile below the surface. Down there, they may see the world like no other animal known to science.
Scientists have generally understood that color vision wasn’t necessary in the deep sea. It’s too far for sunbeams to penetrate, and so there’s no light to give way to color. But in a study published Thursday in Science, researchers interested in the evolution of color vision analyzed the genomes of 101 different fishes. They discovered that one, the silver spinyfin, has more genes for discriminating dull light than any other vertebrate on the planet. These genes make it possible to see the whole range of residual daylight and the full spectrum of bioluminescence in the deep sea. Other fishes may have this ability to detect color in the deep sea, too.
“In vertebrate fishes, nothing has been seen like this before,” said Megan Porter, who studies how vision evolved at the University of Hawaii at Mānoa and was not involved in the research. “This goes against what we understood as how visual systems evolved in the deep sea, which means we have to question how visual systems work and function in the dim light.”
The basics of vision start when light hits our retinas, which contain photoreceptor cells called rods and cones that are sensitive to particular wavelengths. Inside these cells, photopigments, or proteins called visual opsins help translate light into signals our bodies can understand.
A member of the lanternfish family.CreditDr. Wen-Sung Chung, University of Queensland
Typically vertebrates have up to four cone photoreceptors, and one rod photoreceptor. Most humans, for instance, see color input from three cones — red, green and blue. Cones help us see colors in bright light, but in dim light, we’re generally colorblind, and see only intensity based on input from a single rod.
But some deep sea fish appear to see their world in a very different way.
Zuzana Musilová, an evolutionary biologist at Charles University in Prague who led the study, and her team first noticed these fish had lost the genes other fish had for making cone cells and opsins that could detect red and ultraviolet parts of the spectrum. This wasn’t surprising: These wavelengths don’t penetrate the deep sea. But then they found some deep sea fishes had extra copies of genes to make rods.
Looking more closely at 101 genomes, they found that three different lineages of deep sea fish had extra copies of genes for seeing in dim light. Among these, the silver spinyfin took the cake. It had genes for two cone opsins and 38 rod opsins — more abundant and sensitive to more blue than any known vertebrate.
“We were quite, let’s say, astonished by this finding itself because that’s very unique,” said Dr. Musilová.
But more astonishing was that silver spinyfins still used up to 14 of these rod-opsin-making genes (adults in deeper water expressed more rod genes than their larvae living in shallower water). And each opsin is tuned to a slightly different wavelength of green or blue.
These fish may use their extra genes for detecting subtle differences and see the world in shades of blue and green we can’t even imagine.
It’s also possible that these fish may not utilize all this extra visual power. Scientists previously studied mantis shrimp, which have complex eyes with more than a dozen color-receptive cones and a visual processing system like no other animal on the planet. Sure, they can detect more colors, but further research showed that mantis shrimp are no better than humans at discerning subtle differences in colors when they had to associate them with treats.
“The mantis shrimp is a case study about how wrong we were about how they were using all their different receptors,” Dr. Porter said. It’s possible these fish use rods to see color, but what would that world look like? “I’m not even willing to hazard a guess, just because it’s so unlike anything else we ever have seen,” she added.
Most scientists thought that as creatures moved into and adapted to the deep sea, they shed expensive, unnecessary features, like vision, particularly genes used for seeing color. So what’s the benefit of copying and keeping rod opsin-making genes?
Silver spinyfins, which aren’t bioluminescent, may be able to better detect bioluminescent signals from predators and prey that use them in the deep. Their vision could also help them see better as they migrate between different depths during development.
“Either there’s some difference ecologically that’s driving this, or have we just not looked at enough things yet to see the same pattern in other marine species?” Dr. Porter said. “There’s still a lot we don’t know about the deep sea.”