Nudibranchs are often regarded as the gems of the ocean. This is not merely a metaphor; scientists have found that the brilliant colors of these fashionable sea slugs are indeed due to a multitude of tiny crystals in their skin.
Initially, biologists believed the vivid hues of nudibranchs came from pigments, similar to the colors in a toucan’s beak. However, Samuel Humphrey from the Max Planck Institute of Colloids and Interfaces, coming from a materials science background, suspected there was more to these colorful mollusks.
Upon examining six different nudibranch species, Humphrey and his team discovered that pigments are not the sole contributors to the color palette of these creatures.
“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 results from light interacting with microscopic structures in a material, observed in insects, chameleons, plants, seaweeds, oil slicks, and bubbles.
Pigments, in contrast, create color by absorbing specific wavelengths of light and reflecting others, based on their chemical composition rather than structure.
Many vibrant effects emerge from the combination of pigment and structural color.
For example, a peacock’s tail is pigmented brown, yet the microscopic structures in the feathers reflect light to create iridescent blues, greens, and purples, enhanced by the brown base.
In nudibranchs, structural color primarily comes from guanine nanocrystals. The arrangement, length, and angles of these crystals define the color visible on a nudibranch’s skin.
Typically, structural color is linked to iridescence, such as the sheen of a butterfly’s wing or the glow of a jellyfish’s tentacle.
The surprising aspect of nudibranchs using structural color lies in their often matte, flat, and bold markings, traits typically linked to pigment-based color.
Humphrey and his team uncovered an explanation for this phenomenon.
In the skin of nudibranchs, guanine nanocrystals are arranged in layers within individual ‘pixels’ scattered across the surface.
If these crystals were uniformly ordered and identical, they would create an iridescent effect. However, the randomness in each pixel’s nanostructure contributes to the matte appearance.
“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 enable nudibranchs to showcase vivid colors across the visible spectrum with minor structural adjustments between species.
Related: Scientists Recreate Rare Pigment Behind Octopus ‘Superpowers’
This discovery might clarify how these animals have developed such an impressive variety of colors and patterns and could also lead to innovations in materials for human use.
“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.
