Scientists uncover the secret behind the electric blue spots of the Ribbontail Ray

Bluespotted ribbontail ray. Credit: Morgan Bennet Smith.

Researchers have unveiled the mystery behind the electric blue spots of the bluespotted ribbontail ray (Taeniura lymma).

These discoveries could lead to developing chemical-free colors for various applications. The team is also investigating the blue coloration of the blue shark (Prionace glauca).

The study, titled “Ribbontail Stingray Skin Employs a Core–Shelf Photonic Glass Ultrastructure to Make Blue Structural Color,” is published in Advanced Optical Materials.

Skin color is vital for animal communication, providing visual signals that can warn, attract, or camouflage.

Bluespotted ribbontail rays have striking electric blue spots on their skin. The biological process behind these spots was a mystery until now.

“If you see blue in nature, it’s often made by tiny structures in the tissue, not pigment,” says Mason Dean, Associate Professor of Comparative Anatomy at City University of Hong Kong (CityU). “Understanding animal structural color involves optical physics, materials, and how these colors appear in the environment. We assembled a great team from multiple countries and found a surprising solution to the stingray color puzzle.”

Structural colors are created by small structures that manipulate light, not by chemical pigments.

“Blue colors are especially interesting because blue pigments are rare, and nature often uses nanoscale structures to make blue,” says Viktoriia Kamska, a postdoc at CityU. “We’re interested in ribbontail stingrays because their blue color doesn’t change when viewed from different angles.”

The research team used various techniques to study the skin’s structure under different conditions.

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

“We found that the blue color is produced by unique skin cells with a stable 3D arrangement of nanoscale spheres containing reflecting nanocrystals, like pearls in bubble tea,” says Amar Surapaneni, a former postdoc with Dean’s group and now at Trinity College Dublin. “The size and spacing of these nanostructures reflect blue light specifically.”

Interestingly, the team discovered that the unique “quasi-ordered” arrangement of the spheres ensured the color remained unchanged at different angles.

“A thick layer of melanin under the color-producing cells absorbs all other colors, resulting in extremely bright blue skin,” says Dr. Dean. “The structural color cells focus on the blue color, while the melanin pigment cells suppress other wavelengths, creating bright blue skin.”

This blue coloration likely provides camouflage benefits for the stingrays.

“In water, blue penetrates deeper than any other color, helping animals blend with their surroundings,” says Dr. Dean. “The bright blue skin spots of stingrays don’t change with viewing angle, which might help in camouflage as the animal swims or maneuvers.”

The research team is exploring bio-inspired, pigment-less colored materials.

“We are collaborating with researchers to develop flexible, structurally-colored systems inspired by stingray skin for safe, chemical-free colors in textiles, displays, screens, and sensors,” says Dr. Dean.

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

“Despite its name, no one knows how the blue color is produced in blue shark skin,” says Dr. Kamska. “Preliminary results show this coloration mechanism is different from the stingray’s, but we need to use various imaging tools and explore multiple related fields in optics, materials, and biological science.”

A forthcoming article in Frontiers in Cell and Developmental Biology, titled “Intermediate filaments spatially organize intracellular nanostructures to produce the bright structural blue of ribbontail stingrays across ontogeny,” will provide more insights.

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