Impressionist sea slugs create their patterns by arranging colorful photonic crystals

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Nudibranchs are often referred to as the butterflies of the sea. Nudibranchs live worldwide, primarily in warm, shallow marine regions, and stand out for their flamboyant colors and diverse shapes. A team from the Max Planck Institute of Colloids and Interfaces in Potsdam and the University of Cambridge has now discovered how they create their colorful patterns. According to their findings, published in the Proceedings of the National Academy of Sciences, the color is produced by nanostructures, each of which creates a specific color impression.

"We were surprised to find that nudibranchs use structural colors," says Samuel Humphrey, who conducted the research at the Max Planck Institute of Colloids and Interfaces. "Biologists had previously assumed that the colors were produced by pigments." Pigments are chemical compounds and differently colored pigments have different chemical compositions.

In contrast, in structural colors, color is not a chemical property of the material, but it depends on the length scale of nanostructures composing the material. Such nanostructures, also called photonic crystals, are responsible for the coloration of chameleons, as well as many birds and butterflies. In such structures, color is produced by the regular arrangement of materials with different refractive indices.

In the case of nudibranchs, as well as for chameleons, one of the components is the DNA building block guanine. The guanine molecules can form nanocrystals, and the organization of such nanocrystals determines which color the crystal reflects. "Using this elegant color generation mechanism, these beautiful animals are able to generate an astounding array of colors from a single material," says Humphrey.

Matte color impression due to the speckled arrangement of the structural colors

The colors of nudibranchs have also been attributed to pigments until now, as their patterns appear matte. In contrast, the structural colors of photonic crystals are usually shimmery. This is not the case with nudibranchs because the structures that produce their colors act as individual microscopic pixels. In other words, the application of color resembles that of Impressionist painters, who placed dots of different colors side by side in their paintings.

Humphrey and his colleagues discovered this by examining six species of nudibranchs using a combination of optical and electron microscopy. Within the microscopic color dabs of nudibranchs, the guanine crystals are oriented differently in each case. "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 pixel-like application of color also allows nudibranchs to produce different colors in two distinct ways. On the one hand, they can vary the properties of individual crystals; on the other hand, much like in an Impressionist painting or a television, they can mix dots or pixels of different colors to create, for example, violet from red and blue.

"We often draw inspiration from nature when developing new materials and techniques," says Silvia Vignolini, in whose department at the Max Planck Institute of Colloids and Interfaces the study took place. "It might be possible to develop sustainable colors based on the same principles which are used by nudibranchs."

Publication details

Samuel Humphrey et al, Nudibranch color diversity shares a common physical basis in guanine photonic structure 'pixels,' Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2525419123

Journal information: Proceedings of the National Academy of Sciences

Key concepts

PigmentationCrystalline systemsNanostructures

Provided by Max Planck Society