From the iridescence of butterfly wings to the shocking red of a poison dart frog and the shifting patterns of octopus skin, animals make a riot of colors that are more than just decorations; they are signals to mates, warnings to predators, and life-saving camouflage.

Colors can be made by pigments or by infinitesimally small structures that reflect light in a particular way. Although many animals’ color-making mechanisms have been studied, no one had ever looked at how sharks and rays make colors.

Researchers studying the ribbontail stingray have found that it makes its electric blue spots in a new way, one that combines pigments and structural colors.

To find something new, look somewhere new

Sharks and rays’ skin had been studied before, but for its texture, not its color — remember the super-fast swimsuits based on shark skin?

Mason Dean, a professor of biology at City University of Hong Kong who has been studying the biology of sharks and rays was inspired to investigate their colors by a serendipitous comment.

In a talk about animal coloration, Matthias Kolle from MIT commented that blue colors in nature are almost always structural. Dean had assumed that structural colors had to be iridescent, meaning they change color depending on the viewer’s angle, like the shine of a beetle’s back or the glitter of a butterfly’s wings.

But Dean says he could think of many beautiful blue patterns on sharks and rays that appeared blue no matter which way he looked at them. “This meant […] that there was likely a whole undiscovered world of new color-making mechanisms hiding in plain sight in these animals, overlooked in favor of sharks’ flashier reef fish cousins (the ‘bony fish’),” he said.

To explore the interesting ways sharks and rays produce color, Dean worked with Michael Blumer at the Medical University of Innsbruck in Austria and Venkata Amar Surapaneni, a postdoctoral researcher at City University of Hong Kong. Surapaneni had long been fascinated by tiny tissue structures in nature and how they influence animals’ interactions with their environment.

“The design or arrangement of these tiny structures [that produce color] is unique to every plant or animal, and understanding nature’s solutions to bright colors is quite fascinating,” said Surapenini.

Paint or shape?

Colors are produced in two main ways: Pigments absorb some wavelengths of light and reflect others; a pigment that reflects red wavelengths, absorbing all others, appears red. In contrast, structural colors are created by a material’s shape, like nanoscale ridges or barbs that reflect light in a particular way, reflecting particular wavelengths more strongly than others.

Surapaneni, Blumer, and Dean chose to investigate the ribbontail stingray, a small species that lives near coral reefs from South Africa to Papua New Guinea. Its back is dotted with blue spots and its tail sports two stripes of the same color. The spots are electric blue, no matter the viewing angle.

Under the microscope, the cells of the blue spots revealed the secret of their color. “Inside these cells there is a stable suspension of nanoscale spheres [called vesicles] — like the pearls in bubble tea — which in turn contain tiny light-reflecting crystals,” said Surapaneni. The crystals are made of guanine, one of the bases that makes up DNA, and they are perfectly sized and spaced to reflect blue light.

However, the guanine crystals alone are not enough to explain the color — their arrangement is critical. The crystal-containing vesicles are identically sized and evenly spaced, tethered together in a soft scaffolding that keeps them equidistant.

In contrast to this quasi-ordered arrangement of vesicles, the crystals they hold are randomly oriented, reflecting blue light in all directions. This mix of orderly arrangement and disorderly reflectors makes the cells — and their skin — appear blue, no matter how you look at them.

Layer upon layer

There is a final, critical layer to how the color is produced. During their microscope work, the scientists noticed that the spots had two cell layers: a pale upper layer, and a jet black layer underneath. When they looked at the top layer of skin alone, it appeared white instead of blue. Replacing the black background restored its blue hue.

The black cells contain a dark pigment called melanin that acts like a color sponge, absorbing light of any other wavelengths that is bouncing around between the cells. “In the end, the two cell types are a great collaboration: the structural color cells hone in on blue color wavelengths, while the pigment cells suppress other wavelengths, resulting in an extremely bright blue skin,” said Surapaneni.

Discovering new ways that animals make color may help engineers find solutions to make colorful materials without dyes that are more durable and environmentally friendly. But current methods require absolutely precise nano-structuring, which can be expensive to produce.

The ribbontail stingray’s blue may provide inspiration to simplify manufacturing of structural colors because it is similar to a photonic glass — a suspension of optical particles.

“Unlike precisely printed nanomaterials, photonic glasses can be prepared from liquid solutions of nanoparticles and therefore, can be manufactured at relatively low cost and on a large scale, to produce colors which do not change with different viewing angles,” said Surapaneni. “Just by controlling the size, spacing and material of their nanostructures, like simply changing the size/shape/materials of the floating pearls of a bubble tea, the colors can also be tuned.”

This discovery may also be a gateway to discovering other ways that animals produce colors that we may be able to mimic.

“The many ‘solutions’ nature has found for making blue — without chemistry and at low (body) temperatures — then give a big playground for discovering new sustainable solutions for really vibrant colors,” said Dean. “There are more than 1,000 shark and ray species, many with lovely colors, patterns and mottling, but really no one has stopped to look at how these designs are made or controlled.”

Reference: Venkata A. Surapaneni, Mason N. Dean, et al., Ribbontail stingray skin employs a core-shell photonic glass ultrastructure to make blue structural color, Advanced Optical Materials (2024). DOI: 10.1002/adom.202301909

Feature image credit: Ribbontail Stingray (Taeniura lymma) by Bernard Dupont, licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license