Light-sensitive cells in the eyes of some fish do double-duty. In pearlsides, cells that look like rods — the stars of low-light vision — actually act more like cones, which only respond to brighter light, researchers report November 8 in Science Advances. It’s probably an adaptation to give the deep-sea fish acute vision at dawn and dusk, when they come to the surface of the water to feed.
Rods and cones studding the retina can work in tandem to give an animal good vision in a wide variety of light conditions. Some species that live in dark environments, like many deep-sea fish, have dropped cones entirely. But pearlside eyes have confused scientists: The shimmery fish snack at the water’s surface at dusk and dawn, catching more sun than fish that feed at night. Most animals active at these times of day use a mixture of rods and cones to see, but pearlside eyes appear to contain only rods.
“That’s actually not the case when you look at it in more detail,” says study coauthor Fanny de Busserolles, a sensory biologist at the University of Queensland in Australia.
She and her colleagues investigated which light-responsive genes those rod-shaped cells were turning on. The cells were making light-sensitive proteins usually found in cones, the researchers found, rather than the rod-specific versions of those proteins.
These rodlike cones still have the more elongated shape of a rod. And like regular rods, they are sensitive to even small amounts of light. But the light-absorbing proteins inside match those found in cones, and are specifically tuned to respond to the blue wavelengths of light that dominate at dawn and dusk, the researchers found. The fish don’t have color vision, though, which relies on having different cones sensitive to different wavelengths of light.
“Pearlsides found a more economical and efficient way of seeing in these particular light conditions by combining the best characteristics of both cell types into a single cell,” de Busserolles says.
A few other animals have also been found to have photoreceptors that fall somewhere between traditional rods and cones, says Belinda Chang, an evolutionary biologist at the University of Toronto who wasn’t involved in the study. Chang’s lab recently identified similar cells in the eyes of garter snakes. “These are thought to be really cool and unusual receptors,” she says.
Together, finds like these begin to challenge the idea that rods and cones are two separate visual systems, de Busserolles says. “We usually classify photoreceptors into rods or cones only,” she says. “Our results clearly show that the reality is more complex than that.”