Mysterious origin of ribbon beam electric blue spots revealed

Scientists have discovered the unique nanostructures responsible for the electric blue spots of the blue-spotted ray cannon (Taeniura lymma) with possible applications for the development of chemical-free coloration. The team is also conducting ongoing research into the equally mysterious blue coloration of the blue shark (Prionace glauca).

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

Skin coloration plays a key role in organismal communication, providing vital visual cues that can warn, attract or camouflage. Blue-spotted stingrays have striking electric blue spots on their skin. However, the biological processes that produced these electric blue spots have been a mystery until now.

“If you see blue in nature, you can almost be sure that it is made up of tissue nanostructures, not pigment,” says Mason Dean, associate professor of comparative anatomy at City University of Hong Kong (CityU). “Understanding the structural colors of animals is not only about the optical physics, but also about the materials used, how they are finely organized in the tissue, and how the color looks in the animal’s environment. To put all these pieces together, we have assembled a great cross-disciplinary team, which ended up being a surprising and fun solution to a colorful stingray puzzle.”

Structural colors are created by extremely small structures that manipulate light, rather than as a product of chemical pigments.

“Blue colors are particularly interesting because blue pigments are extremely rare and often used by nature to create blue structures at the nanoscale,” says Viktoriia Kamska, a postdoctoral researcher studying natural coloration mechanisms at CityU. “We are particularly interested in stingrays because, unlike most other structural colors, their blue color does not change when you look at them from different angles.”

The research team combined different techniques to understand the architecture of the skin in different natural conditions.

“To understand the fine architecture of the skin, we used micro-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 nanospheres containing reflective nanocrystals (like pearls suspended in bubble tea),” says Amar Surapaneni, until recently a postdoctoral fellow in Dean’s group 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 the color remained unchanged across the viewing angle.

“And to remove foreign colors, the thick layer of melanin beneath the color-producing cells absorbs all other colors, resulting in extremely bright blue skin,” says Dr. Dean. “Ultimately, these two cell types are a great collaboration: the structural pigment cells enhance the blue color, while the melanin pigment cells suppress other wavelengths, resulting in extremely bright blue skin.”

The team believes that this fascinating blue coloration is likely to provide the stingrays with camouflage advantages.

“In water, blue penetrates deeper than any other color, helping animals blend in with their surroundings,” says Dr Dean. “The bright blue patches on stingray skin do not change with viewing angle; therefore, they may have specific advantages in camouflage as the animal swims or maneuvers rapidly with undulating wings.”

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

“We continue to collaborate with other researchers to develop flexible biomimetic structurally colored systems inspired by the delicate nature of stingray skin for safe, chemical-free colors in textiles, flexible displays, screens and sensors,” says Dr. Dean.

In addition to working on stingrays, Dr. Kamska and her team also investigate the blue coloration of other stingrays and sharks, including the blue shark.

“Despite the name ‘blue shark’ and its ecological aspects being well researched, no one still 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 stingray – but like the stingray, we need to try different combinations of fine imaging tools and look at more related fields in optics, materials and biological science.”

It is also in the upcoming article Frontiers in Cell and Developmental Biologytitled “Intermediate filaments spatially organize intracellular nanostructures to produce the bright structural blue of stingrays stringy across ontogeny.”

This research is presented at the annual conference of the Society for Experimental Biology in Prague from July 2 to 5, 2024.