Fluorescence is a notable adaptation in marine environments, helping to counteract the loss of longer wavelengths as light diminishes with depth. Studied to some extent in cnidarians and reef fish, its presence and functions in crustaceans are less understood. Recently, fluorescence was discovered in gall crabs (Cryptochiridae). To investigate the evolutionary significance of fluorescence in these coral-dwelling decapods, we combined a multivariate examination of 27 fluorescent morphological traits with phylogenomic analysis across 14 crab genera from the Red Sea and Indian Ocean. Fluorescence first evolved in the genus