The monoaminergic system in a bivalve larva: temporal deployment and spatial organization

Originating at the base of the bilaterian tree of life, the monoaminergic (MOA) system is a pivotal and evolutionarily conserved regulator of animal development and of responses to changing environmental conditions. Investigating the ontogeny of monoaminergic modulation in model systems such as marine bivalve molluscs is therefore particularly relevant, as their life cycle and developmental transitions are strongly influenced by environmental cues. Here, we characterized the spatio-temporal and tissue-specific expression of components of the MOA system during early larval development of the Mediterranean mussel Mytilus galloprovincialis using both time resolved transcriptomics and in situ Hybridization Chain Reaction (HCR). Our results identify serotonin and dopamine as the predominant and interconnected monoaminergic pathways deployed during early mussel development, with receptors, enzymes, and selective transporters broadly expressed across both neuronal and non-neuronal tissues. Notably, the expression of receptors preceding that of the corresponding biosynthetic enzymes indicates early, non-neuronal roles of monoaminergic signalling, supported by their localization in peripheral tissues such as ciliated epithelia. Altogether, These findings support the hypothesis that the MOA system acts as a pervasive and tightly regulated modulator of larval morphogenesis and could therefore play an evolutionary conserved role in mediating development and environmental plasticity in developing bilaterian organisms.