For Fish, Invisibility is Only Skin Deep

CREDIT: Fish image via Shutterstock View full size image

Most people who open a can of sardines don’t see the fish’s glossy shimmer as a marvel of nanotechnology. But looks can be deceiving. The skin of sardines and other fish actually helps them disappear in water, a mystery that seemingly defies physics.

Scientists at the University of Bristol recently unraveled the mystery of the optical properties of silvery fish skin, and their findings could have implications for the electronics industry.

Their research revealed that the fish’s skin layer, located beneath the scales, reflect all of the light hitting them, regardless of the angle it’s coming from, without polarizing it. This makes them highly reflective, and allows some fish to appear to blend into their environment. Polarized light is oriented in one direction, and is easier for predators to detect.

The fish’s skin “reflect as much light as would be coming through that point in space normally. Effectively, [the fish] just disappear into the background,” said Nicholas Roberts, the study’s lead researcher.

[Electron ‘Invisibility Cloak’ Could Lead to New Kinds of Electronics]

But how they did this was unknown. Normally, when light hits a reflective surface, part of it gets refracted; the rest gets reflected but is polarized, meaning all of the light waves are oriented in the same direction. Polarized reflection would spell trouble for a fish, since a predator who happens to be in the right position could see through the fish’s camouflage.

When the researchers examined sardines, Atlantic herring and sprat more closely, they found that their skins contained a layer that was actually made of two different crystallized forms of the biomolecule guanine, each with different optical properties, that sandwich a substance found in cells called cytoplasm. It was previously thought this layer was made from a single type of guanine crystal.

“We’ve known for some time that the reason fish are silvery is due to this structure that they have within their skin,” said Roberts. “But some of the measurements that are in the literature really didn’t quite add up. They had these unusual properties and no one ever explained it.”

Having two different guanine types means they can be oriented within the scales to reflect light from all angles.

The new findings could have implications for electronics, the team says. Light emitting diodes (LEDs), for example, need reflectors to maximize the light in their systems. While nonpolarizing reflectors do already exist, their quality doesn’t even come close to the fish’s solution, experts say.

Reflectors made with the fish-skin model are much more stable, easier to make, and can be used in a wider range of scenarios.  

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