The Prettiest Sea Slugs Use Crystals in Their Skin as 'Pixels' of Color

The Prettiest Sea Slugs Use Crystals in Their Skin as 'Pixels' of Color Nature 05 April 2026 By Jess Cockerill Nudibranchs such as Chromodoris annae stand out for their vibrant colors and varied shapes. (SirWat/iStock/Getty Images) 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." Hypselodoris bullockii and a highly-magnified view of the 'pixels' that make up its color. (Randi Ang/Moment Open/Getty Images. Inlay: Samuel Humphrey/MPI of Colloids and Interfaces) 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. Digital microscope images showing structurally colored granules in H. tryoni , skirt (B), H. bullockii mantle (C), C. annae skirt (D), C. willani mantle (E), S. neapolitana ceras (F), and B. stephanieae ceras (G). (Humphrey et al., PNAS , 2026) 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