Researchers have discovered unique nanostructures responsible for the electric blue spots on the blue-spotted ribbontail ray (Taeniura lymma), opening possibilities for developing chemical-free coloration methods. This discovery, detailed in Advanced Optical Materials, was presented at the Society for Experimental Biology’s Annual Conference in Prague from July 2 to 5, 2024. The team is also exploring the similarly mysterious blue coloration of the blue shark (Prionace glauca). Skin coloration is crucial for communication among organisms, providing essential visual cues for survival, such as warning, attraction, or camouflage. While these rays feature electric blue spots on their skin, the biological processes behind them remained unknown until now.

“If you see blue in nature, it’s almost always produced by tissue nanostructures, not pigments,” said Mason Dean, Associate Professor of Comparative Anatomy at City University of Hong Kong (CityU). “Understanding structural color in animals involves not only optical physics but also the materials involved, their intricate organization in tissue, and how the color appears in the animal’s environment. To piece all this together, we assembled a diverse team of scientists from various countries, leading to a surprising and exciting solution to the ray coloration puzzle,” Dean remarked.

These colors are generated by minute structures that manipulate light rather than chemical pigments. “Blue colors are particularly fascinating because blue pigments are extremely rare, and nature often relies on nanometer-scale structures to produce blue,” explained Viktoriia Kamska, a postdoctoral researcher studying natural coloration mechanisms at CityU. “We are particularly interested in the ribbon-tailed rays because, unlike most other structural colors, their blue color remains consistent regardless of the viewing angle,” Kamska continued.

The research team utilized a variety of techniques to examine the skin’s architecture under natural conditions. “To understand the fine-scale structure of the skin, we used micro-computed tomography (micro-CT), scanning electron microscopy (SEM), and transmission electron microscopy (TEM),” Dean elaborated. “We found that the blue color is produced by unique skin cells with a stable 3D arrangement of nanometer-scale spheres containing reflective nanocrystals, akin to pearls suspended in bubble tea,” stated Amar Surapaneni, a former postdoctoral researcher in Mason Dean’s group and now a visiting scholar at Trinity College Dublin.

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