The shimmery, metallic sheen of a fish in shallow water may confuse predators or dazzle mates. Now scientists have uncovered clues to how the fish build their bling. Somehow fish alter the growth of the light-reflecting crystals layered in their skin’s cellular matrix, bestowing superior shine. The findings hint at a way to create super-reflective crystals in the lab for use in products like cosmetics and paints.

SCALY SECRET. Fish alter the growth of crystals in their skin, giving it a shiny, metallic luster, Israeli scientists report. Florida Keys NMS, B. Care

“We were amazed,” says Avital Levy-Lior, a materials scientist at the Weizmann Institute of Science in Rehovot, Israel. “The crystals’ growth is completely opposite than what we expected.”

To investigate, Levy-Lior and her colleagues extracted guanine crystals from the skin of the Japanese koi fish and red-eye tetra and analyzed them with X-ray diffraction and an electron microscope. The researchers then compared the fish-made crystals to crystals grown in the lab. While similar in shape, the fish-made crystals were much thinner and smoother than the lab-grown guanine, which appeared chunky and stepped. Even more surprising, the fish seem to be limiting the growth of their crystals in the direction that usually grows the fastest, the researchers report in an upcoming issue of Crystal Growth & Design. This inhibition likely enhances their reflectivity, says Levy-Lior.

The mechanism by which the fish limit their crystals’ growth isn’t clear. Living creatures usually create crystals in enclosed chambers. Since completely dry conditions are required for the growth of guanine crystals in the lab, it’s likely that the fish also sequester their guanine in a dry compartment, ensuring super shimmery skin when wet.

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