Blue-spotted stingrays create blue in a unique way, researchers report in a new study. The new nanostructures give the spikes their signature points, the study shows, by intricately bending the light on the skin of the rays to form a vivid blue that doesn’t change with viewing angle.
Blue is a tricky color for living things to conjure up. Most shades in the natural world are produced by compounds that evolved to absorb sunlight and emit a particular color. In cases where red light is emitted, the chemical must absorb the more energetic wavelengths, green, blue and violet.
Producing blue light usually requires absorbing low-energy red wavelengths and emitting more powerful ones, a more difficult task for which nature rarely has a solution.
A simpler solution, it seems, is to use another approach – scattering unwanted wavelengths using light-bending nanostructures in what is called structural dye.
“If you see blue in nature, you can be pretty sure it’s made from tissue nanostructures, not pigment,” says Mason Dean, associate professor of comparative anatomy at the City University of Hong Kong and co-author of the study. young.
These structural blues usually arise from highly ordered nano-architectures, however the researchers’ examination of the blue spots of these rays revealed something different.
The structural blue is “extraordinarily bright and angle-independent,” they report, “arising from a more disordered array of scattering elements with a previously undescribed core-shell ultrastructure.”
That alone would be interesting, but this discovery could also have practical applications for humanity, potentially inspiring new types of chemical-free dyes.
Structural colors like this are complex natural phenomena, Dean points out, influenced by variables beyond the way they bend light.
“Understanding the structural color of animals is not only about the optical physics, but also about the materials involved, how well they are organized in the tissue, and what the color looks like in the animal’s environment,” says Dean.
In this case, blue-spotted striped rays have areas of skin cells with special properties that help them produce this almost unusual blue, explains lead author Amar Surapaneni, formerly a postdoc with Dean’s group and now an academic visitor at Trinity College Dublin.
“We discovered that the blue color is produced by unique skin cells, with a stable 3D arrangement of nanoscale spheres containing reflective nanocrystals (like pearls suspended in a bubble tea),” says Surapaneni.
“Because the size of the nanostructures and their spacing is a useful multiple of the wavelength of blue light, they tend to specifically reflect blue wavelengths.”
In addition to those blue-producing cells, rays have another trick that gives their blue color an extra boost.
“To clear any extraneous color, a thick layer of melanin under the color-producing cells absorbs all other colors, resulting in extremely bright blue skin,” says Dean.
“Ultimately, the two types of cells are a great collaboration: structural color cells are affected by the blue color, while melanin pigment cells suppress other wavelengths, resulting in extremely bright blue skin.”
The radius of the blue dotted bar (Taeniura lymma) lives mainly in shallow waters around coral reefs in the tropical Indian and western Pacific Oceans, with a range extending to South Africa and Northern Australia.
It has powerful venom and its attractive coloration may have evolved as a warning. However, it is generally shy around humans, usually running away rather than biting when possible. And given the colors of its native habitat, blue may also help with camouflage.
“In water, blue penetrates deeper than any other color, helping animals blend in with their surroundings,” says Dean. “The bright blue skin patches of stingrays do not change with viewing angle; therefore, they may have specific advantages in camouflage as the animal is swimming or maneuvering quickly with flapping wings.”
Borrowing some ideas from these rays could also benefit our species, Dean adds.
“We are pursuing collaborations with other researchers to develop flexible biomimetic systems with structural dyes inspired by the soft nature of echinoderms for safe, chemical-free dyes in textiles, flexible screens, displays and sensors,” he says.
The study was published in AAdvanced optical materials.