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.