True Colours

Sea slugs or nudibranchs are often called the butterflies of the sea. Like butterflies, their striking colours result from are a combination of pigment-based chemicals, and structural colour – where light is separated into colours as it bends around tiny contours on their bodies. Stacks of tiny refractive crystals, a bit like pixels on a screen, allow this Chromodoris annae nudibranch to produce a bright silvery blue colour. But the vibrant hues are a warning – or aposematism – to the sea slug’s deadly trick. Nudibranchs are chemical thieves – wonderfully named 'kleptochemists' – reusing and modifying chemicals ripped from their prey for their own defences. In other research, human chemists find that some of these bioactive chemicals are toxic to cancer cells, suggesting nudibranchs could be living factories for future life-saving drugs.

Written by John Ankers

John Ankers

Writer

A former researcher in systems biology, John is now an online biology tutor and professional coach supporting the wellbeing of scientists and NHS professionals. He's writing, too, of course.

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• Video from work by Samuel Humphrey and colleagues; Cancer drug research by Servillera & Peña et al, in Marine Drugs, Aug 2025
• Department of Sustainable and Bio-inspired Materials, Max Planck Institute of Colloids and Interfaces, Potsdam, Germany
• Video originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)
• Published in Proceedings of the National Academy of Science (PNAS), March 2026