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.