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 nonvisual 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 repertoires 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 analyzed 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, opsin expression in
photosensitive tissues suggests an ancestral repertoire for the mantle
margin, with convergent changes in repertoire size. 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 nonvisual opsins used in both eyes and
dermal photoreception.