Unveiling the Secret Behind Nudibranch's Vibrant Hues: Tiny Crystals, Not Just Pigment

Scientists have determined that the vibrant coloration of nudibranchs, a type of sea slug, is primarily produced by thousands of tiny crystals embedded in their skin, rather than solely by pigment. This discovery challenges prior assumptions regarding the origin of their colors and offers insights into biological structural coloration. The research also suggests potential applications for the development of new materials.

The Discovery: Crystals Over Pigment

The study, led by Samuel Humphrey of the Max Planck Institute of Colloids and Interfaces, involved examining six different species of nudibranchs. Humphrey, who has a background in materials science, and his collaborators confirmed that structural color is the main mechanism behind the sea slugs' appearance. The findings were published in the Proceedings of the National Academy of Sciences.

Understanding Structural Coloration

Structural color results from light reflecting off microscopic structures within a material. This phenomenon contrasts with pigment-based color, which arises from the absorption and reflection of light wavelengths determined by a material's chemical composition. Structural color is observed in various organisms, including insects, chameleons, and plants.

In nudibranchs, this structural color is primarily formed by nanocrystals composed of guanine molecules. The specific color displayed on the nudibranch's skin is determined by the arrangement, length, and angles of these crystals.

The Mystery of Matte Colors

A notable characteristic of nudibranch markings is their frequently matte and flat appearance, which differs from the iridescence often associated with structural color, such as that seen in butterflies or jellyfish.

Researchers found that this matte effect is achieved because the guanine nanocrystals are stacked in layers within individual "pixels" on the nudibranch's skin. A degree of randomness in each pixel's nanostructure helps to 'flatten' the reflected color by causing light to be reflected in various directions, thereby preventing a shimmering effect.

Broader Implications and Future Potential

This mechanism enables nudibranchs to display a wide range of bold, bright colors across the visible spectrum through subtle structural variations between species. The discovery provides insights into how these animals evolved such diverse color patterns. According to physicist Silvia Vignolini, the research could also inspire the development of new, sustainable color materials for various applications.