Mysterious origin of ribbon ray electric blue spots revealed

Researchers have discovered the unique nanostructures responsible for the electric blue spots of the blue-spotted tapeworm (Taeniura lymma), with potential applications for the development of chemical-free dyeing. The team is also conducting ongoing research into the equally enigmatic blue color of the blue shark (Prionace glauca).

This research, titled “Stingray Ribbontail Skin Employs a Core-Shelf Photonic Glass Ultrastructure to Make Blue Structural Color,” is published in Advanced optical materials .

Skin coloration plays a key role in organismal communication, providing life-critical visual cues that can warn, attract, or camouflage. Blue-spotted striped rays possess striking electric blue spots on their skin. However, the biological processes that produced these electric blue spots were a mystery until now.

“If you see blue in nature, you can almost be sure it’s made from tissue nanostructures, not pigment,” says Mason Dean, Associate Professor of Comparative Anatomy at the City University of Hong Kong (CityU). “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 tissues, and how the color appears in the animal’s environment. To draw all of them pieces together, we brought together a large cross-disciplinary team from many countries, ending up with a surprising and fun solution to the stingray color puzzle.”

Structural colors are produced by extremely small structures that manipulate light, and not as a product of chemical pigments.

“Blue colors are particularly interesting because blue pigments are extremely rare, and nature often uses nanoscale structures to make blue,” says Viktoriia Kamska, a postdoc who studies natural coloring mechanisms at CityU. “We’re particularly interested in striped stingrays because unlike most other structural colors, their blue color doesn’t change when you look at them from different angles.”

The research team combined a variety of techniques to understand the architecture of skin under different natural conditions.

“To understand the fine-scale architecture of the skin, we used computed tomography (micro-CT), scanning electron microscopy (SEM) and transmission electron microscopy (TEM),” says Dr. Dean.

“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 Amar Surapaneni, a postdoc with Dean’s group until recently and now. a visiting academic at Trinity College Dublin. “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.”

Interestingly, the team found that the unique “quasi-ordered” arrangement of the spheres helped ensure that color remained unchanged across the viewing angle.

“And to clear any extraneous color, a thick layer of melanin beneath the color-producing cells absorbs all other colors, resulting in extremely bright blue skin,” says Dr. 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 team believes that this striking blue coloration is likely to provide camouflage benefits for the breasts.

“In water, blue penetrates deeper than any other color, helping animals blend in with their environment,” says Dr. Dean. “The bright blue patches of stingrays’ skin 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.”

Applications for this research currently being explored include bio-inspired pigment-free color materials.

“We are pursuing collaborations with other researchers to develop flexible biomimetic systems with structural dyes inspired by the soft nature of skin fibers for safe, chemical-free dyes in textiles, flexible displays, screens and sensors,” says Dr. Dean.

In addition to their work on spikes, Dr. Kamska and her team are also investigating the blue color of rays and other sharks, including the blue shark.

“Despite the fact that the name “blue fish” and its ecological aspects are well studied, no one yet knows how the blue color is produced in its skin,” says Dr. Kamska. “Preliminary results show that this coloring mechanism is different from that of the hoopoe – but like the oyster, we need to try different combinations of excellent imaging tools and address multiple related disciplines in optics, material and biological science. “

There is also an upcoming article on Frontiers in Cell and Developmental Biologytitled “Intermediate filaments spatially organize intracellular nanostructures to produce the bright blue structural color of ribbon stripes throughout ontogeny.”

This research is being presented at the Society for Experimental Biology Annual Conference in Prague from July 2 to 5, 2024.