Molecular and structural divergence of c-opsin-based phototransduction in the radiolar eyes of Sabellid fan worms

The radiolar eyes of sabellid fan worms represent a unique evolutionary model for studying photoreception outside the cephalic region. Building upon previous findings of c-opsin-based phototransduction in Acromegalomma , this study investigates the structural and molecular features of radiolar eyes across three species with distinct eye morphologies, Acromegalomma vesiculosum (compound-inside), Bispira guinensis (compound-paired), and Pseudopotamilla occelata (compound-single). Utilising transmission electron microscopy (TEM) alongside comparative transcriptomic analysis, incorporating newly sequenced data for B. guinensis and P . occelata and existing datasets for Acromegalomma interruptum , we explore the potential correlation between photoreceptor ultrastructure and molecular signalling components. Our results confirm that while c-opsin is the primary photopigment across these lineages, the downstream G-protein recruitment is more heterogeneous than previously recognised. In contrast to the predominantly GNAO-mediated pathway identified in Acromegalomma interruptum , our transcriptomic profiling revealed distinct G-protein repertoires in the other two species: B. guinensis expresses GNAO and GNAQ, while P. occelata exhibits a highly diverse complement comprising GNAO, GNAS, GNAI, and GNAQ subunits. These findings demonstrate that the c-opsin signalling cascade in annelids possesses considerable molecular plasticity, which occurs alongside a pronounced structural variation in photoreceptor lamellar bodies. This work extends our understanding of how distributed visual systems evolve structural and molecular diversity through the flexible deployment of conserved toolkits.