Nudibranch color diversity shares a common physical basis in guanine photonic structure 'pixels'

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 were thought to be of pigmentary origin, here we show, using a combination of white light and Raman microspectroscopy, that hierarchically organized micron-scale guanine multilayer structures are responsible for many of these colors. Such architectures are widespread across the dorid and aeolid groups and are responsible for a striking array of angular-independent structural colors. By using cryogenic focused ion beam (cryo-FIB) SEM tomography, we were able to access the complex 3D organization of the guanine nanoplatelets responsible for the strong blue coloration of Chromodoris annae. We propose that the multilayer organization of guanine platelets with varying orientations across the tissue and their micron-scale size offers a particularly effective strategy for producing diverse optical effects. The macroscopic angular independent color results from individual multilayers which we describe as "pixels", these "pixels" reflect light at a wavelength governed by their interlayer spacing and guanine platelet thickness. The macroscopic hue can be spectrally tuned by altering the statistical distribution of pixels with each color, while the angular dependence of color can be changed through the relative orientation of the multilayer stacks and their size, allowing for a single structural motif to generate a broad palette of optical appearances.