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In the new study, Lia Addadi and colleagues note that researchers have known for years that guanine crystals in the skin underneath the scales of fish reflect light to produce a mirror-like sheen.

This silvery reflectance acts as a form of camouflage that helps protect fish from predators as fish swim near the water's surface. However, the exact shape of these guanine crystals and how they work remained a mystery.

The researchers extracted guanine crystals from the skin of the Japanese Koi fish and analyzed the crystals using X-ray diffraction and an electron microscope. They compared the results to guanine crystals made in the laboratory.