Blue color of ribbontail stingray skin stems from a core-shell photonic glass ultrastructure

Abstract Blue structural colors, produced by diverse tissue nanostructures, are known from all major vertebrate clades except cartilaginous fishes (e.g. sharks, rays). We describe a bright angle-independent structural blue from ribbontail stingray skin, arising from a novel cell type with unique quasi-ordered arrays of nano-vesicles enclosing guanine nanoplatelets. This natural architecture —an intracellular photonic glass— coherently scatters blue, while broadband absorption from closely-associated melanophores obviates the low color-saturation typical for photonic glasses. This first demonstration of structural color in elasmobranchs (the oldest extant clade of jawed vertebrates) illustrates that the capacity for guanine-based colors likely arose extremely early in vertebrate evolution. The structure-function mechanisms underlying ribbontail stingray coloration point to selective pressures driving elasmobranch visual ecology and communication, but also strategies for biomimetic color production.