The Disney-esque hues on bluespotted ribbontail rays come from a unique arrangement of nanostructures, say researchers. The findings, along with those gleaned from studying blue sharks, could help lead to new chemical-free color techniques.
Go snorkeling in the shallow waters of the Indo-West Pacific region of our oceans, and one of the more striking things you're likely to see are bluespotted ribbontail rays. These roundish rays only grow to be about 12-14 inches in diameter, but their electric blue showy spots make a big impression.
Researchers at City University of Hong Kong (CityU) say that the source of these spots has long been a bit of a marine mystery, especially because their blue color remains the same no matter what direction they are viewed in. Still, scientists had an inkling that the spots would have a structural component to them.
Color puzzle
"If you see blue in nature, you can almost be sure that it's made by tissue nanostructures, not pigment," says study co-author Mason Dean. "Understanding animal structural color is not just about optical physics but also the materials involved, how they're finely organized in the tissue, and how the color looks in the animal's environment. To draw all those pieces together, we assembled a great team of disciplines from multiple countries, ending up with a surprising and fun solution to the stingray color puzzle."
Sure enough, thanks to a detailed analysis carried out by the team, the puzzle has been solved. It turns out that the coloration is, in fact, due to the arrangement of tiny balls in the structure of the ray's skin rather than from pigmentation.
"We discovered that the blue color is produced by unique skin cells, with a stable 3D arrangement of nanoscale spheres containing reflecting nanocrystals (like pearls suspended in a bubble tea)," says study lead author Amar Surapaneni. "Because the size of the nanostructures and their spacing are a useful multiple of the wavelength of blue light, they tend to reflect blue wavelengths specifically."
In studying the rays, the team used microcomputed tomography (micro-CT), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Not only did the analysis reveal the blue-reflecting nanostructures, but it also showed that a layer of melanin beneath the spots absorbed other colors of light, making the blue "pop" even more. The researchers theorize that the spots might help with camouflaging the rays.
"In water, blue penetrates deeper than any other color, helping animals blend with their surroundings," says Dean. "Bright blue skin spots of stingrays do not change with viewing angle; therefore, they might have specific advantages in camouflage as the animal is swimming or quickly maneuvering with undulating wings."
Because the rays' spots are produced through a mechanical rather than chemical process, the research team feels the findings could help produce chemical-free coloration techniques in the future.
"We are pursuing collaborations with fellow researchers to develop flexible biomimetic structurally colored systems inspired by the soft nature of stingray skin for safe, chemical-free colors in textiles, flexible displays, screens, and sensors," says Dean.
As part of the ongoing research, the team is also beginning to study blue coloration in other marine animals, including the blue shark.
"Despite the name 'blue shark' and its ecological aspects being well studied, no one still knows how the blue color is produced on its skin," says Viktoriia Kamska, a postdoc studying natural coloration mechanisms at CityU. "Preliminary results demonstrate that this coloration mechanism is different from the stingray's – but just like the stingray, we need to try different combinations of fine imaging tools and address multiple related disciplines in optics, material, and biological science."
The current study has been published in the journal Advanced Optical Materials and has just been presented at the Society for Experimental Biology Annual Conference in Prague.
Source: Society for Experimental Biology via PhysOrg
