Bluespotted ribbontail rays make blue with unusual nanostructures

Bluespotted ribbontail rays generate blue in an unusual manner, according to a new study. The research finds that novel nanostructures create the stingrays' namesake spots by intricately manipulating light in their skin, producing an intense blue that remains unchanged from different viewing angles.

Why blue is difficult to produce in nature

For living organisms, creating blue is particularly challenging. Most colours found in nature come from compounds evolved to absorb sunlight and produce a specific colour. To produce red light, for instance, a chemical compound must absorb the more energetic green, blue and violet wavelengths.

Creating blue light generally means absorbing lower-energy red wavelengths and producing more powerful wavelengths instead - a far more difficult process for which nature has rarely developed solutions.

A more straightforward alternative appears to be the use of structural colour: light-bending nanostructures scatter unwanted wavelengths away.

"If you see blue in nature, you can almost be sure that it's made by tissue nanostructures, not pigment," says Mason Dean, associate professor of comparative anatomy at City University of Hong Kong and co-author of the new study.

Structural blue usually results from highly ordered nano-architectures. However, when the researchers investigated the rays' blue spots, they found a different arrangement.

The structural blue is "unusually bright and angle-independent," they report, "arising from a more disordered array of scattering elements with a previously undescribed core-shell ultrastructure."

The finding is notable in itself, but it could also offer practical benefits for humans by inspiring new types of chemical-free colour.

Bluespotted ribbontail ray skin cells and structural colour

Dean notes that structural colours are complex phenomena in nature, shaped by more than the way they bend light.

"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," Dean says.

For bluespotted ribbontail rays, areas of skin contain cells with specialised characteristics that enable the animals to create this almost otherworldly blue, says lead author Amar Surapaneni. Surapaneni was previously a postdoctoral researcher in Dean's group and is now a visiting academic at Trinity College Dublin.

"We discovered that the blue color is produced by unique skin cells, with a stable 3D arrangement of nanoscale spheres containing reflecting nanocrystals (like pearls suspended in a bubble tea)," Surapaneni says.

"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."

The rays possess a second mechanism that further intensifies their blue, alongside these colour-producing cells.

"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," Dean says.

"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."

Camouflage and chemical-free colour applications

The bluespotted ribbontail ray (Taeniura lymma) primarily inhabits shallow tropical waters around coral reefs in the Indian Ocean and western Pacific. Its range reaches as far as South Africa and Northern Australia.

Although it carries potent venom, its striking colouring may have evolved as a warning signal. Even so, the ray is generally shy around people and will usually flee rather than sting if it can. Given the colours in its natural environment, blue may also serve as camouflage.

"In water, blue penetrates deeper than any other color, helping animals blend with their surroundings," Dean says. "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."

Dean adds that adapting some principles from these rays could also benefit humans.

"We are pursuing collaborations with fellow researchers to develop flexible biomimetic structurally colored systems inspired by the soft nature of stingray skin for safe, chemical-free colors in textiles, flexible displays, screens, and sensors," he says.

The study was published in Advanced Optical Materials.