Abstract
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 Opecarcinus and was subsequently retained showing varying levels of expression. We identified four distinct fluorescent morphologies (fluotypes) with high phenotypic variability, some of which show distinct distributions across the phylogeny. Along with differences in the crabs' microhabitats, these findings suggest that fluorescence may be shaped by selective pressures, such as visibility to potential viewers, and could thus play a role in camouflage, aiding concealment against complex coral reef backgrounds. This study provides a deeper understanding of evolutionary dynamics in cryptochirids and introduces a new workflow, providing guidance for future research on fluorescence in marine invertebrates. Further research into behavioural functions and fluorophore identification are required to explain the observed variability in Cryptochiridae.