How do deep-sea fish see in dark waters? This new study could hold the key

Three species of Red Sea fish appear to rely on special ‘hybrid’ retinal cells to see in dark environments

By Jackie Flynn Mogensen edited by Claire Cameron

Some deep-sea fish can see light differently than most other vertebrates. according to a new study. The fish, found in the Red Sea, have what the scientists behind the new study describe as “hybrid” photoreceptors: light-detecting cells in the retina that combine elements of two distinct types of photoreceptors, cones and rods.

In the human retina, cone cells allow us to see in bright environments, detecting colors and fine details, while rod cells are sensitive to dim light, allowing us to see in the dark. But not all animals’ eyes work this way.

Scientists discovered the hybrid photoreceptors in the larvae of three species of fish found in the Red Sea: members of the hatchetfish, lanternfish and lightfish groups, all of which live in predominantly dark, deep waters. One of the fish, a hatchetfish, maintains these hybrid cells into adulthood.

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The one from the ocean twilight zone is not an ideal environment for rod or cone cells, says Lily Fogg, a postdoctoral researcher at the University of Basel in Switzerland and lead author of the study. “From a visual point of view, it’s a bit of a nightmare,” she says.

Yet many deep-sea fish often begin their lives there, raising the question: “How do these tiny larvae see well enough to feed, avoid predators, and survive in the murky depths of mid-water?” » said Fogg.

The answer was deep in their eyes. By examining the retinas of fish larvae, Fogg and his colleagues discovered cells with both rods and cones. While hatchetfish retain these cells as adults, lanternfish and lightfish appear to lose them, developing only rods. The results suggest that photoreceptors do not exist in two rigid categories – rods and cones –, says Fogg, “but rather along a spectrum.”

The findings, published Wednesday at Scientific advancescould enrich researchers’ understanding of the development of vision in vertebrates.

Similar photoreceptors have been found in other species, including jawless fish and some reptiles and amphibians, Fogg says. Taken together, the evidence “suggests that this flexibility may be a deeply ingrained feature of vertebrate vision rather than a strange exception.”

“It reminds us that biology is rarely as simple as we think,” she says.