Some deep-sea fish may be able to see light in a different way from 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-sensing cells in the retina that combine elements of two distinct kinds of photoreceptors, cones and rods.
In human retinas, cone cells enable us to see in bright environments, detecting color and fine detail, while rods are sensitive to low light, enabling us to see in the dark. But not all animals’ eyes work that way.
Scientists found the hybrid photoreceptors in larvae from three species of fish found in the Red Sea—members of the hatchetfish, lanternfish and lightfish groups, all of which live in mostly dark, deep water. One of the fish, a hatchetfish, maintains these hybrid cells into adulthood.
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The ocean’s twilight zone isn’t an ideal environment for either rod or cone cells, explains Lily Fogg, a postdoctoral researcher at the University of Basel in Switzerland and the lead author of the study. “From a visual perspective, that’s a bit of a nightmare,” she says.
Yet many deep-sea fish often start their lives there, which raised the question: “How do these tiny larvae see well enough to feed, avoid predators, and survive in the murky midwater depths?” Fogg says.
The answer lied in the back of their eyes. By examining the retinas of the fish larvae, Fogg and her colleagues found cells with features of both rods and cones. While hatchetfish hold on to these cells as adults, lanternfish and lightfish seem to lose them, developing only rods. The findings suggest photoreceptors don’t exist as two rigid categories—rods and cones—Fogg says, “but rather along a spectrum.”
The findings, published Wednesday in Science Advances, could add to researchers’ understanding about how sight developed in vertebrates.
Similar photoreceptors have been found in other species, including jawless fishes and some reptiles and amphibians, Fogg says. Taken together, the evidence “hints that this flexibility may be a deeply rooted feature of vertebrate vision rather than an odd exception.”
“It’s a reminder that biology is rarely as simple as we think it is,” she says.