Ocular and extraocular expression of opsins reveals evolutionary trends underlying visual and non-visual functions in bivalves (Pteriomorphia)

Opsin proteins are central to photoreception and have diversified extensively to support both visual and non-visual functions in animals. While the molecular conservation is striking, the evolutionary pathways that shaped opsin diversity, particularly in relation to visual and extraocular roles, remain poorly understood. To address this gap, we examined opsin expression in Pteriomorphia, a diverse group of mostly sessile marine bivalves with multiple photoreceptor systems and ecologies. We generated transcriptomes for 12 species, including eyed and eyeless adults, and analysed published RNA-seq data from five additional species covering all taxonomic orders and eye types. We identified and classified opsins expressed in the photosensitive mantle margin and eyes using phylogenetically informed annotation. We tested whether opsin diversity is associated with eye presence or mobility and determined which opsins are widely used. Our results suggest that bivalve eyes generally express more opsins than extraocular tissues. Additionally, convergent expansions and reductions of opsin diversity were estimated, likely derived from an ancestral repertoire in the mantle margin. Curiously, differences in mobility among species do not influence opsin repertoire. Our findings further support that the rhodopsin-retinochrome system is pervasive across bivalves and identify visual and non-visual opsins used in both eyes and dermal photoreception.