You can’t ask fish what they see. But you can study their genomes to get clues about how their eyes work. When scientists did that with some freaky deep-sea fish, they saw something surprising: Four species seem to have evolved four distinct and previously unknown systems for spotting color amid utter darkness more than a mile down.
In fish eyes, just like yours, cones and rods detect light. Cones handle the color. Pigments in the cones called opsins absorb specific wavelengths and send signals to the brain that interprets colors. Rods are more light-sensitive—they can detect a single photon. Rods help with brightness, especially in dim light, but they don’t detect colors. This setup renders nearly all vertebrates, including you and me, colorblind when the lights are down.
That’s what was long thought, anyway.
The new study found four deep-sea fish that have more than three opsin genes for rods, “raising the possibility” that the fish “have rod-based color vision,” the researchers write.
The deep-sea silver spinyfin fish has a “surprising” 38 rod opsin genes, the researchers said. That’s more than in the cones of any other fish or any known vertebrate.
“This was very surprising,” said Karen Carleton, a biology professor at the University of Maryland and co-author of a paper on the research, published May 10 in the journal Science. “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?”
She figures it may have to do with detecting prey. Here’s why:
In fish with multiple rod opsins, the wavelengths they’re tuned to overlap with the spectrum of light emitted by some bioluminescent creatures that live down there. So while no sunlight reaches down to 6,500 feet, where some of these fish live…
“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.
Since four different species of fish have rod opsins, the researchers say deep-water color vision might’ve evolved independently multiple times, a hint that it must confer some advantage.
Many deep-sea creatures are thought to be blind. Some have developed gargantuan eyes. Others detect subtle movements in the water by changes in pressure. And you may be familiar with the anglerfish, which uses a fishing pole atop its head to dangle a bioluminescent “lure” that other sea creatures see, at their peril.
The new research adds to a growing understanding of different ways creatures survive and thrive in constant darkness.
Another surprising fish-eye discovery was made back in 2017, this time with actual cells spotted under a microscope. Fish called pearlsides, which live in the deep ocean but hunt near the surface mostly at dawn and dusk, have developed a completely different solution to low-light vision.
“Instead of using a combination of rods and cones, they combine aspects of both cells into a single and more efficient photoreceptor type,” said Fanny de Busserolles, a University of Queensland scientist involved in that study, which was reported in the journal Science Advances.
Based on the structure and appearance of the newfound cells, researchers called them “rod-like cones.”
The pearlside study’s lead researcher, Queensland professor Justin Marshall, captured the essence of the scientific process:
“Humans love to classify everything into being either black or white,” he said. “However our study shows the truth might be very different from previous theories.”