Annelid eye evolution revealed by developmental, ultrastructural, and connectome analyses of cerebral eyes in Malacoceros fuliginosus

Eye evolution has long attracted interest, yet how multiple cerebral eyes within a lineage originate and diversify remains unclear. Annelids display exceptional diversity in eye number and structure, but the homology and function of distinct eye pairs are poorly understood. Here we investigate the cerebral eyes of the sedentary annelid Malacoceros fuliginosus using an integrated developmental, molecular, ultrastructural, and connectomic approach. We show that both the early-developing ventral and later-developing dorsal eyes are simple, few-celled, inverted rhabdomeric eyes that express transcription factors with conserved roles in animal eye development. Two r-opsin paralogs and distinct neurotransmitters are differentially expressed in different photoreceptor cells of the eyes. Ultrastructural reconstructions across larval development reveal differences in cellular composition and growth dynamics, while axonal tracing shows that photoreceptors from ventral and dorsal eyes project to overlapping regions of the larval brain. The overall organization and projections resemble those described in the errant annelid Platynereis dumerilii. Together, these data support the hypothesis that an ancestral cerebral eye duplicated early in annelid evolution, giving rise to multiple eye pairs with stage-specific functions.