Photonic crystals help fish see in the murk

Ashley N. Paddock, ashley.paddock@photonics.com

The elephantnose fish has light-reflecting cups lined with photonic crystals in its retinas that help it navigate its dark, murky environment, scientists have found. This unusual eye structure might aid future sensors that filter signal noise or peform detection in turbid fluids.

These freshwater fish, found in the cloudy depths of African rivers, use electrosensing to navigate their dark environment, but they also depend somewhat on vision. Until recently, these weakly electric fish were thought to be blind, or almost blind, University of Leipzig neurophysiologist Andreas Reichenbach said in a podcast.

“The visual capabilities of this fish are pretty poor,” he said. “It’s color-blind. It cannot see anything that is not bigger than six times the size of a full moon. But surprisingly, it’s optimal for its environment.”

Scientists had long thought the elephantnose fish (Gnathonemus petersii) to be blind, but it turns out to have light-reflecting cups lined with photonic crystals in its retinas that help it navigate in dark waters. Courtesy of Gerhard von der Emde.

Vertebrate eyes have rod photoreceptors, which are very sensitive to light but do not detect color or fine details, and cone photoreceptors, which are less sensitive to light but can distinguish color and details. Most vertebrate eyes optimize primarily either rods or cones, but an international team of scientists has discovered that the retina of the elephantnose fish is structured so that both types of receptors work simultaneously.

The researchers observed that the cones are grouped together within larger, light-reflecting cups lined with photonic crystals. The rods are positioned behind these reflectors. This unusual arrangement works because the mirrorlike surfaces of the cups propagate the light in a way that delivers just the right wavelength to both the rods and cones.

“Behind the eye in the so-called retina pigment epithelium, there are huge cells, which form kind of parabola mirrors reflecting the light,” Reichenbach said. “From outside, you can see that light is reflected, like in the cat’s eye, but it is focused on a certain level in this case. The astonishing thing is that within such parabola mirrors, there are about 30 cone photoreceptors, which are responsible for high-acuity vision in our case, but not in this case, and a couple hundred rod photoreceptors.”

Light amplification by photonic-crystal light collector onto photoreceptor cells in the retina of the elephantnose fish helps it to see. Courtesy of Moritz Kreysing.

Each photoreceptor sees the same part of an image because all the rods and cones are illuminated by the same structure, meaning that the visual acuity is very bad, he said. The fish are at an advantage because they are not able to see or get distracted by the small particles, such as dead worms and bubbles or air moving around them, but they can see the big predators moving.

“The special structure of the retina of the fish enables the fish to see large moving objects more reliably than any other fish, and this makes him thrive under these [turbid water] conditions,” he said.

While this vision proves advantageous for the fish in dark, dim, muddy waters, it’s detrimental in aquarium settings with other fish. “If the fish swims in an aquarium together with other fish under normal laboratory conditions, it’s almost blind,” Reichenbach said. “This was the reason why the fish had been considered blind for many years.”

The study appeared in Science (doi: 10.1126/science.336.6089.1617-c).