Impressionist Sea Slugs

Nudibranchs create their colourful patterns using structural colours, which they arrange in microscopic dots similar to Impressionist paintings

To the point:

• Surprising coloration: Nudibranchs, a type of marine slug, create their colourful patterns using structural colours rather than pigments, as previously thought.
• Structure creates colour: The structural colours of these sea slugs are created by photonic crystals of the DNA building block guanine.
• Pixel-like patterns: In nudibranchs, the structural colours are organized in pixels, similar to that found in Impressionist paintings or televisions, thereby creating a wide range of colour impressions.

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 colours 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 colourful patterns. According to their findings, the colour is produced by nanostructures, each of which creates a specific colour impression. "We were surprised to find that nudibranchs use structural colours," says Samuel Humphrey, who conducted the research at the Max Planck Institute of Colloids and Interfaces. "Biologists had previously assumed that the colours were produced by pigments." Pigments are chemical compounds and differently coloured pigments have a different chemical compositions.

In contrast, in structural colours, colour 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, colour 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 nano-crystals, and the organization of such nanocrystals determines which colour the crystal reflects. "Using this elegant colour generation mechanism, these beautiful animals are able to generate an astounding array of colours from a single material," says Humphrey.

Matte colour impression due to the speckled arrangement of the structural colours

The colours of nudibranchs have also been attributed to pigments until now, as their patterns appear matte. In contrast, the structural colours of photonic crystals are usually shimmery. This is not the case with nudibranchs because the structures that produce their colours act as individual microscopic pixels. In other words, the application of colour resembles that of Impressionist painters, who placed dots of different colours side by side in their paintings. Samuel Humphrey and his colleagues discovered this by examining six species of nudibranchs using a combination of optical and electron microscopy. Within the microscopic colour dabs of nudibranchs, the guanine crystals are oriented differently in each case. "They therefore reflect light of the same colours in very different directions, so that the colours do not shimmer like those of butterflies, but appear matte," says Samuel Humphrey.

The pixel-like application of colour also allows nudibranchs to produce different colours 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 colours 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 colours based on the same principles which are used by nudibranchs."