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Fish find way around polarization problem

Gavin Parsons/ARDEA

There’s more to fish skin than meets the eye.

Herring, sardine and sprat will grace scores of dinner plates across the world tonight, but an ingenious combination of crystals in their skin ensures that their still-living relatives have a better chance of avoiding predators. And in finding a way around a physical law governing light, these silvery fish may have pointed researchers towards a way to make better reflectors.

When unpolarized light strikes surfaces at a particular angle, called the Brewster angle, the light reflected back from the surface is polarized. Fishermen exploit this knowledge by wearing sunglasses that screen out the polarized light reflected off water surfaces so they can see better into the water.

But the skin of Atlantic herring (Clupea harengus), European sardine (Sardina pilchardus) and sprat (Spratus spratus) are made up of alternating layers of guanine crystals and cytoplasm that avoid polarizing light even at the Brewster angle. This not only means that unpolarized light is reflected — potentially useful when many predators can see the polarization of light — but also ensures that more light is reflected, a useful adaptation for an animal that uses its silvery shimmer to hide in light filtering down from the surface.

Nicholas Roberts, who works on light perception in nature at the University of Bristol, UK, and his colleagues discovered that there are actually two types of guanine crystal in the skin of these fish. These two crystals have different Brewster angles; by mixing the two types, the fish’s skin avoids polarizing the light and maintains a high reflectivity, they report in Nature Photonics.

“Some light is always reflected,” says Roberts. “The overall effect is it comes out unpolarized.”

This fish trick could even make its way into artificial reflectors, such as those used to increase the output of light-emitting diode (LED) sources. At present, non-polarizing mirrors are generally made by exploiting the differences between materials in the mirror and the environment. But using the fish trick could allow the reflector to be made of the same stuff as their surrounding environment, which is potentially useful in technologies, such as bulbs, in which heat conduction is very important.


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