How did the ribbontail ray get its electric blue spots?

A team of international researchers has recently identified the unique nanostructures responsible for the electric blue spots on the bluespotted ribbontail ray (Taeniura lymma), which could inspire chemical-free color production. 

Additionally, they are also investigating the blue coloration of the blue shark (Prionace glauca).

Mysterious colors of the ribbontail ray

Skin coloration is crucial for communication in organisms, offering visual cues for warning, attraction, or camouflage. While the bluespotted ribbontail ray is known for its vivid blue spots, the biological processes behind these spots were previously unknown.

“Understanding animal structural color is not just about optical physics but also the materials involved, how they’re finely organized in the tissue, and how the color looks in the animal’s environment,” said senior author Mason Dean, an associate professor of comparative anatomy at the City University of Hong Kong (CityU).

“To draw all those pieces together, we assembled a great team of disciplines from multiple countries, ending up with a surprising and fun solution to the stingray color puzzle.”

Extremely rare blue coloration 

Dean noted that if you see blue in nature, you can almost be sure that it’s made by tissue nanostructures, not pigment. These structural colors are created by nanoscale structures that manipulate light. 

“Blue colors are especially interesting because blue pigments are extremely rare, and nature often uses nanoscale structures to make blue,” said Viktoriia Kamska, a postdoc studying natural coloration mechanisms at CityU.

Fine-scale architecture of the skin 

The scientists utilized various techniques to analyze skin architecture under natural conditions. “To understand the fine-scale architecture of the skin, we used microcomputed tomography (micro-CT), scanning electron microscopy (SEM), and transmission electron microscopy (TEM),” Dean said. 

The analyses revealed that the blue color arises from unique skin cells with a stable 3D arrangement of nanoscale spheres containing reflecting nanocrystals. 

“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,” said Amar Surapaneni, a postdoc in Dean’s group.

Bright blue skin of the ribbontail ray 

The “quasi-ordered” arrangement of these spheres ensures the color remains unchanged when viewed from different angles. 

“To clean up any extraneous colors, a thick layer of melanin underneath the color-producing cells absorbs all other colors, resulting in extremely bright blue skin. In the end, the two cell types are a great collaboration: the structural color cells hone in on the blue color, while the melanin pigment cells suppress other wavelengths, resulting in extremely bright blue skin,” Dean explained.

This coloration likely offers camouflage benefits. “In water, blue penetrates deeper than any other color, helping animals blend with their surroundings. Bright blue skin spots of stingrays do not change with viewing angle; therefore, they might have specific advantages in camouflage as the animal is swimming or quickly maneuvering with undulating wings,” he added.

Future research directions

The researchers are exploring applications for bio-inspired, pigment-less colored materials. 

“We are pursuing collaborations with fellow researchers to develop flexible biomimetic structurally-coloured systems inspired by the soft nature of stingray skin for safe, chemical-free colors in textiles, flexible displays, screens, and sensors,” Dean reported.

The experts are also investigating blue coloration in other rays and sharks, including the blue shark. “Despite the name ‘blue shark’ and its ecological aspects being well studied, no one still knows how the blue color is produced on its skin,” Kamska said. 

“Preliminary results demonstrate that this coloration mechanism is different from the stingray’s – but just like the stingray, we need to try different combinations of fine imaging tools and address multiple related disciplines in optics, material, and biological science,” she concluded.

The study is published in the journal Advanced Optical Materials.

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