For vertebrates, vision requires a compromise between signals from two types of light-detecting cells: rods and cones. Humans see sharply during the day thanks to a cluster of color-detecting cones in the back of our eyes. At night, those cones turn off and light-sensitive rods turn on. Extra rods in cats, for example, help them see best in the dark.

And then there are elephantnose fish, whose retinal arrangement of rods and cones seems to doom them to poor vision at night and during the day. In these fish, cone cells cluster into cups along the retina, which reduces the spatial resolution of their vision. These cups shield the rod cells buried underneath, preventing these cells from picking up dim light as well. Now Andreas Reichenbach of the University of Leipzig and his colleagues have figured out how those cups help the fish (Gnathonemus petersii) see in murky African waters.

Here’s how it works. Layers of thin crystals made from guanine, one of the bases in DNA, line the outside of each cup in the retina. This photonic crystal reflects red light that penetrates muddy water and funnels those photons to the bottom of the cup where the light-sensitive bits of the cone cells reside. A simulation revealed that this reflection helps the cone cells capture five times more light than enters the cup. Very little light passes through to the super-sensitive rod cells below the cup.

This arrangement compensates for the low light sensitivity of the cones, while tempering the activity of the rods. Measuring electrical signals from the fish’s retina, the scientists found that both rods and cones fire during the day. Shifting these sensitivities so the two cells work together is the most ingenious trick of the elephantnose fish’s retina, Reichenbach says. In humans, rods and cones only work simultaneously for a few minutes at sunrise or sunset each day.

Humans can see sharp details because each cone cell in that cluster, or fovea, sends a signal to our brain like pixels in a picture, he says. But the elephantnose fish sees the world in cup-sized pixels rather than cell-sized ones, because all the cone cells in one cup register the same image. The fish’s vision is so poor that it can only see an object larger than six times the diameter of the full moon in our eyes, Reichenbach adds.

But this poor resolution can be helpful, too. A movie of an expanding circle startled goldfish, often studied for their good vision, and elephantnose fish. But when the circle was covered with gray splotches, the goldfish swam away fewer times than the elephantnose fish. It seems the goldfish couldn’t see the circle through the gray splotches, but the elephantnose fish could, Reichenbach says. Since the elephantnose fish can’t see small particles, it might be able to see past the mud and bubbles in the water and only detect large predators in the water, he adds.

Filtering out details to leave the most important visual information extends the principles of retinal function beyond working well in the daylight or dim light, Reichenbach said in the Science podcast. This natural design might help engineers build light detectors for machines working in turbid fluids, he adds.

Science, 2012. DOI: 10.1126/science.1218072 (About DOIs).