A new study shows how unique cells full of crystals give this ray one of the brightest blue colors in nature
Photo of Dr. Shahrouz Amini, MPI Potsdam, photographing a ray’s blue spots
Coral reef fishes come in all the colors of the rainbow (and perhaps even in some colors that we can’t see). But such color is often not what it seems- green moray eels, for example, don’t actually have green skin.
These bright colors are used for lots of ecological and behavioral purposes. “Many male birds, insects, and fish use their flashy colors and patterns to attract female mates,” Dr. Amar Surapaneni of the City University of Hong Kong and the lead author of the new study told me. “The differences in skin or plumage colors between males and females may also help in identifying different sexes during mating. The quality of the skin color is also an important attribute – it indicates animal or plant health and might affect their success in communicating with mates, prey or predators.”
Dr. Suprapaneni and his team became fascinated by the unusually ultra-bright blue of the bluespotted ribbontail stingray, and the biomechanics of how such a color can be made in nature. “Pigments that produce blue are very rare in nature, and most blues—whether on butterfly wings, peacock feathers, or flowers—are not produced by pigments but by exceedingly ornamented tissue architectures,” Dr. Surapaneni told me. “These architectures are made of extremely small light-reflecting structures (ranging from 10s to 100s of nanometers), and the colors we see —so-called ‘structural colors’— depend on how these tiny structures are arranged inside the animal or plant skin. When these nanostructures are arranged in perfect or almost-perfect order (which is usually the case), the way light interacts with them produces rainbow-like colors that change with the angle of observation. But when the structures are not in perfect order, but rather a somewhat intermediate order, like the nanostructures on ribbontail stingray skin, the same color is amplified in all directions. Therefore, the blue color remains the same regardless of the direction of observation.”
Image credit: Morgan Bennett Smith, courtesy of Dr. Suprapaneni
The biological structures that underly this vibrant coloration were as fascinating as the research team hoped. “What is unique about the stingray blue is the blue color is achieved by unique cells that consist of a stable suspension of nanoscale spheres—like the pearls in bubble tea—which in turn contain tiny light-reflecting crystals,” Dr. Surapaneni told me. “The size of these crystals and their spacing are a multiple of the wavelength of blue light, therefore producing the blue color. In addition, their arrangement being in an intermediate state between perfect order and absolute disorder—quasi-order—further contributes to the skin color. Resembling man-made photonic glass, this partially ordered arrangement helps reflect the blue color strongly, while its disorder helps deflect blue wavelengths in all directions. These somewhat ordered, somewhat messy nanostructures contribute to the blue hue, but a black melanin backing layer is a key addition, absorbing any other colors that might be bouncing around. In the end, the two cell types are a great collaboration: the structural color cells hone in on the blue color, while the pigment cells suppress other wavelengths, resulting in extremely bright blue skin.”
Shark and ray biomechanics expert Dr. Mason Dean, Dr. Surapaneni’s Postdoc advisor and a coauthor on this new paper, is excited about these results. After hearing a fascinating talk on the biomechanics of coloration by a colleague, he told me, “I realized I knew several sharks and rays with beautiful blue skin patterns This meant, although most people think of sharks and rays as drab compared to reef fish, 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’). We are really excited about our findings on stingray blue, because it showcases a unique natural design for bright structural colors in elasmobranchs! 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. Body colorations shape how these fish interact with mates and predators and their surroundings, so there are a ton of new angles to explore!”