Bluespot ribbontail rays produce blue in a unique way, researchers report in a new study. The novel nanostructures that give stingrays their namesake spots, the study shows, complexly compress light on the rays’ skin, creating a vivid blue color that doesn’t change with viewing angle.
Blue is a tricky color for creatures to imagine. Most shades in the natural world are produced by compounds designed to absorb sunlight and emit a specific color. In cases where red light is emitted, the chemical must absorb higher energy wavelengths such as greens, blues and violets.
Produces blue light Nature rarely solves the difficult task of absorbing low-energy red wavelengths and emitting more powerful ones.
A simple solution seems to be using another approach – scattering unwanted wavelengths using light-bending nanostructures referred to as structural coloration.
„If you see blue in nature, you can be sure it’s made of tissue nanostructures, not pigment.” He says Mason Dean, associate professor of comparative anatomy at the City University of Hong Kong and co-author of the new study.
These structural blues usually arise from a highly ordered nano-structure, although the researchers’ examination of the blue dots of these rays revealed something different.
The structural blue is „unusually bright and angle-independent,” they report, „arising from a highly disordered array of scattering components with a previously undescribed core-shell ultrastructure.”
That alone is impressive, but the discovery may also have practical applications for humanity, inspiring new types of chemical-free colors.
Dean points out that structural colors like these are complex natural phenomena, influenced by variables beyond how they bend light.
„Understanding structural color in animals is not only about optical physics, but also about the materials involved, how they are finely organized in tissues, and how color appears in the animal’s environment,” Dean He says.
In this case, bluespot ribbontail rays have skin cells with special properties that help them produce this almost ethereal blue, explains lead author Amar Surapaneni.
„We discovered that the blue color is produced by distinct skin cells, with a stable 3D arrangement of nanoscale spheres that reflect nanocrystals (like pearls suspended in a bubble tea),” Soorapaneni said. He says.
„Because the size of the nanostructures and their spacing are effective multiples of the wavelength of blue light, they specifically reflect blue wavelengths.”
Besides those blue-producing cells, rays have another trick that gives them an extra boost of blue.
„To clean up any extraneous pigments, a thick layer of melanin beneath the pigment-producing cells absorbs all the other pigments, creating a very bright blue skin,” says Dean. He says.
„Ultimately, the two cell types are a perfect cooperation: the structural pigment cells enhance the blue color, while the melanin pigment cells suppress the other wavelengths, resulting in a very bright blue skin.”
Bluespot ribbontail ray (Danny Career Limma) lives mainly in shallow waters around coral reefs in the tropical Indian and western Pacific oceans, extending to southern Africa and northern Australia.
It has powerful venom, and its eye-catching color may have evolved as a warning. Nevertheless, it is generally fearful around humans, usually fleeing rather than stinging if possible. Given the colors of its native habitat, the blue may aid in camouflage.
„In water, blue penetrates deeper than any other color, helping animals blend in with their surroundings,” Dean He says. „The bright blue skin spots of the stingray do not change with viewing angle; therefore, they may have specific advantages in camouflage as the animal swims or maneuvers rapidly with undulating wings.”
Borrowing some ideas from these rays could also benefit our species, Dean adds.
„We are collaborating with fellow researchers to develop flexible biomimetic structural color systems inspired by the softness of stingray skin for safe, chemical-free colors in textiles, flexible displays, screens and sensors,” he said. He says.
The study was published in AAdvanced optical materials.