Nudibranch color diversity shares a common origin in guanine photonic structures

Nudibranchs are well known for their bright and diverse color patterns. This coloration is typically a form of aposematism, warning predators against toxic compounds sequestered from their prey and weaponized as a form of defense. Although many of the hues in nudibranchs have pigmentary origin, multilayer structures composed of guanine nano-platelets have been suggested as the source of color enhancement in the nudibranch Flabellina iodenea. Here, using a combination of white light and Raman microspectroscopy techniques, we report that such guanine-based multilayer structures are a widespread mechanism to create angular-independent structural color across the dorid and aeolid groups. Additionally, by using cryo-FIB tomography, we were able to access the complex 3D organization of the guanine nano-platelets responsible for the strong blue coloration of Chromodoris annae. We propose that the multilayer organization of guanine platelets with varying orientations across the tissue offers a particularly effective strategy for producing diverse optical effects. In this configuration, hue is mainly governed by interlayer spacing, while the angular dependence of color can be tuned through the degree of local order, allowing a single structural motif to generate a broad palette of optical appearances. Significance statementNudibranchs are an extraordinarily diverse group of marine animals, renowned for their dazzling range of colors and striking patterns. Whilst their pigmentary coloration is well understood, so far, structural coloration, obtained only by nanostructures, has only been reported in the nudibranch Flabellina iodenea. In this work, we present a comparative analysis of structural coloration across nudibranch species from both benthic and coral reef environments, and we show that guanine-based nano-structures are a common motif responsible for a wide range of colors, spanning the dorid and aeolid groups. We foresee that the 3D imaging conducted here may serve as inspiration for bio-photonics studies in other marine organisms, and that the structures themselves could serve as inspiration for bio-inspired materials.