A new blue: Mysterious origin of stingray’s electric blue spots revealed

Researchers have discovered the unique nanostructures responsible for the electric blue spots of the blue-spotted ribbontail ray (Taeniura lymma), with potential applications for developing chemical-free coloration. The team also has ongoing research into the equally enigmatic blue coloration of the blue shark (Prionace glauca).

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

Skin color plays an important role in organismic communication, providing vital visual cues that can warn, attract, or camouflage. Bluespotted ribbontail rays possess striking electric blue spots on their skin. However, the biological processes that produce these electric blue spots have remained a mystery until now.

“When you see blue in nature, you can be almost certain that it’s made by tissue nanostructures, not pigment,” says Mason Dean, associate professor of comparative anatomy at City University of Hong Kong (CityU). “Understanding the structural color of animals isn’t just about optical physics, it’s also about the materials involved, how they’re precisely organized in the tissue, and how the color looks in the animal’s environment. To put all those pieces together, we assembled a great team of disciplines from multiple countries, resulting in 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 especially interesting because blue pigments are extremely rare and nature often uses nanoscale structures to create blue,” says Viktoriia Kamska, a postdoc 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 skin architecture under different natural conditions.

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

“We found that the blue colour is produced by unique skin cells, with a stable 3D arrangement of nanoscale spheres containing reflective nanocrystals (like pearls floating in a bubble tea),” says Amar Surapaneni, until recently a postdoc in Dean’s group and now a visiting professor at Trinity College Dublin. “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.”

Interestingly, the team found that the unique ‘quasi-ordered’ arrangement of the spheres ensured that the color remained unchanged regardless of viewing angle.

“And to clean up all the excess color, a thick layer of melanin beneath the color-producing cells absorbs all other colors, resulting in an extremely bright blue skin,” says Dr. Dean. “Ultimately, the two cell types work in tandem: the structural color cells focus on the blue color, while the melanin pigment cells suppress other wavelengths, resulting in an extremely bright blue skin.”

The team believes this captivating blue color provides the stingrays with good camouflage.

“Blue penetrates more deeply in water than any other color, allowing animals to 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 camouflage advantages when the animal is swimming or maneuvering quickly with undulating wings.”

Applications currently being explored for this research include bio-inspired, pigmentless colored materials.

“We are working with other researchers to develop flexible biomimetic structurally colored systems inspired by the soft nature of stingray skin. These systems are suitable for safe, chemical-free coloring in textiles, flexible displays, screens and sensors,” said Dr. Dean.

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

“Despite the name ‘blue shark’ and the well-studied ecological aspects, no one yet knows how the blue color on its skin occurs,” says Dr. Kamska. “Preliminary results show that this coloration mechanism is different from that of the stingray, but as with the stingray, we need to try different combinations of fine imaging tools and address multiple related disciplines in optics, materials science, and biological science.”

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

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