Nudibranchs are the jewels of the underwater world.
That's not just a nice metaphor: Scientists have now discovered that the vibrant colors of sartorial sea slugs really are made up of thousands of tiny crystals embedded in their skin.
Biologists had long assumed that the eye-catching colorways sported by nudibranchs arise from pigment, the same way a toucan's beak packs in its many hues, for instance.
But with a background in materials science rather than biology, researcher Samuel Humphrey of the Max Planck Institute of Colloids and Interfaces suspected that there was a lot more to these multi-colored molluscs than meets the eye.
By taking a closer look at six different species of nudibranchs, Humphrey and his collaborators have confirmed that pigments are not the only 'paints' in a nudibranchs' pallet.
"We were surprised to find that nudibranchs use structural colors," Humphrey says.
"Using this elegant color generation mechanism, these beautiful animals are able to generate an astounding array of colors from a single material."
Structural color is an effect of light reflecting off microscopic structures of a material. It's seen in insects, chameleons, plants, seaweeds, and even oil slicks and bubbles.
Pigments, on the other hand, produce color by absorbing certain wavelengths of light and reflecting others, based on the chemical composition of the material, rather than its structure.
Yet, many colorful effects are achieved by combining pigment with structural color.
A male peacock's tail, for instance, is pigmented brown, but because of the way microscopic structures in the feathers interfere with light, we see iridescent blues, greens, and purples, made all the more vibrant by their dull brown base.
In nudibranchs, it turns out, the structural color is formed mainly by nanocrystals of guanine molecules. The way these crystals are arranged, their length and angles, determines the particular color seen on a nudibranch's skin.
But usually, structural color is associated with iridescence, like the glitter of a butterfly's wing or the sparkle of a jellyfish's tentacle.
Part of the reason it's so surprising that nudibranchs use structural color at all is that their markings often appear matte, flat, and bold – the qualities we usually associate with pigment-based color.
Humphrey and team found an answer to this as well.
In a nudibranch's skin, the guanine nanocrystals are stacked in layers within individual 'pixels' dotted across the surface.
If all the crystals within those pixels were neatly ordered and exactly identical, we would see an iridescent effect, but a degree of randomness in each pixel's nanostructure helps to 'flatten' the color.
"They therefore reflect light of the same colors in very different directions, so that the colors do not shimmer like those of butterflies, but appear matte," says Humphrey.
The guanine crystals allow nudibranchs to display bold, bright colors across the entire visible spectrum with just a few 'tweaks' of that structure between species.
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The discovery could explain how this family of animals have managed to evolve such a dazzling diversity of colors and patterns among themselves – and it could inspire new materials for human use, too.
"We often draw inspiration from nature when developing new materials and techniques," says physicist Silvia Vignolini, also from Max Planck.
"It might be possible to develop sustainable colors based on the same principles which are used by nudibranchs."
As if we could ever be as well-dressed as a nudibranch.
The research was published in Proceedings of the National Academy of Sciences.