Its trademarks shine bright blue from all angles: Researchers have now discovered what underlies the amazing color brilliance of the blue-spotted stingray. According to them, sophisticated nanostructures and contrast elements in the skin areas of the spots ensure the intense color effect. The concept may be suitable for the development of new structural colors, say the researchers.
They are intended to attract, warn or camouflage: colors fulfill important functions for many animals that serve their survival or visual communication. In many cases they are based on pigments – dyes such as melanin that are stored in the skin and other body structures. However, there is another coloring system in nature. It is based on the sophisticated manipulation of light by tiny structural elements. They determine which wavelengths are emitted by wings, feathers or skin. This allows particularly spectacular color effects to be created. In addition, animals can color themselves “cost-effectively” thanks to the structural concept. This is because the production of pigments can be energetically expensive. This is particularly true for blue dyes.
Questioning look at the blue-spotted stingray
“When you see blue markings in nature, you can be almost certain that they are created by nanostructures and not by pigments,” says senior author Mason Dean from the City University of Hong Kong. He and his colleagues are interested in the concepts of structural color production in nature because, in addition to their biological significance, they could also serve as models for the development of new coloration systems. The international research team’s focus was on an animal whose splendor has already impressed many a snorkeler in coral reefs: the blue-spotted stingray (Taeniura lymma).
“We are particularly interested in this marine animal because, unlike most other structural colors, its blue color does not change when viewed from different angles,” says co-author Viktoriia Kamska from the City University of Hong Kong. To clarify what the color effects of the blue dots are based on, the research team analyzed the fine structures of the relevant skin areas using micro-computed tomography, scanning electron microscopy and transmission electron microscopy. “Understanding the structural color of animals is not just about optical physics, but also about the materials involved and how they are organized in the tissue,” says Dean.
Beads with nanocrystals
The investigations have now revealed that the color effect comes from special cells in the blue areas of the skin, in which special structural elements are formed. These are tiny beads that in turn contain nanometer-sized crystals made of the substance guanine. “Because the size of the nanostructures and their spacing are specially adapted, they reflect primarily blue wavelengths,” explains lead author Venkata Surapaneni from the City University of Hong Kong. The results also explained why the color remains unchanged depending on the viewing angle: Since the beads with the light-scattering crystals are not aligned in the same way, but are arranged in a disordered manner in the tissue, the color effect is transmitted in all directions.
As the team discovered, there is another component that plays a role in color brilliance in addition to structural color, and this has to do with pigments: “To eliminate all foreign colors, a thick layer of melanin under the color-producing cells absorbs all other colors, resulting in a very bright blue color effect,” says Dean. “Ultimately, the two cell types form an ideal team: the structural color cells focus on the blue color, while the melanin-pigment-producing cells suppress other wavelengths,” says the scientist.
Potential for applications
But what purpose do the blue spots actually serve for the ray? Although they initially appear particularly conspicuous, the researchers assume that they serve as camouflage: “In the water, blue penetrates deeper than any other color and helps the animals to blend in with their surroundings. Since the skin spots do not change at different viewing angles, they could have a particularly positive visual effect when the animal swims with its undulating wing structures,” explains Dean.
The team now sees potential for applications in the insights into the color secrets of the blue dots. “We are currently working with other researchers to develop flexible biomimetic, structurally colored systems inspired by ray skin,” says Dean. In concrete terms, such concepts could be used in textiles or flexible displays, for example. Since there are certainly many other sophisticated coloring systems in marine life, the team is now expanding its research. The blue shark (Prionace glauca) is in their sights. “The preliminary results show that the mechanism of blue coloration in this fish is different from that of the ray,” says Kamska.
Source: Society for Experimental Biology, scientific article: Advanced Optical Materials, doi: 10.1002/adom.202301909