Damsels in a Hidden Colour: Development of Ultraviolet Sensitivity and Colour Patterns in Damselfishes (Pomacentridae)

Damselfishes (Pomacentridae) are widespread and highly abundant on tropical coral reefs. They exhibit diverse body colouration within and between the ∼250 species and across ontogenetic stages. In addition to human visible colours (i.e., 400-700 nm), most adult damselfishes reflect ultraviolet (UV, 300-400 nm) colour patches. UV sensitivity and UV colour signals are essential for feeding and form the basis for a secret communication channel invisible to the many UV-blind predatory fish on the reef; however, how these traits develop across ontogenetic stages, and their distribution across the damselfish family is poorly characterised. Here, we used UV photography, phylogenetic reconstructions of opsin genes, differential gene expression analysis (DGE) of retinal samples, to investigate the development of UV vision and colour patterns in three ontogenetic stages (pre-settlement larval, juvenile, and adult) of eleven damselfish species. Using DGE, we found similar gene expression between juveniles and adults, which strongly differed from larvae. All species and all stages expressed at least one UV-sensitive _sws1_ opsin gene. However, UV body colour patterns only started to appear at the juvenile stage. Moreover, _Pomacentrus_ species displayed highly complex UV body patterns that were correlated with the expression of two _sws1_ copies. This could mean that some damselfishes can discriminate colours that change only in their UV component. We demonstrate dramatic shifts in both UV sensitivity and UV colouration across the development stages of damselfish, while highlighting the importance of considering ontogeny when studying the coevolution of visual systems and colour signals.