Blue Sea Slugs and Nudibranchs (102 species) 102
True blue pigments are rare in the sea. The vivid blues of Chromodoris and Hypselodoris come from pixel-organised guanine nanocrystals in the skin — a matte, angle-independent structural colour sharpened as an aposematic signal. The pelagic Glaucus atlanticus instead floats upside-down at the surface, exposing its blue ventrum to the sky and its silvery dorsum to the depths in an inverted countershading.
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