Dopaminergic Central Neurons and Peripheral Sensory Systems in Pteropod and Nudibranch Molluscs

In Euthyneuran molluscs, the distribution and plethora of dopamine (DA) functions are likely coupled to the feeding ecology with a broad spectrum of modifications both in the central and peripheral neural systems. However, studies of benthic grazers currently dominate the analysis of DA-mediated signaling, whereas adaptations to pelagic lifestyles and other feeding strategies are unknown. Here, we characterize the distribution of central and peripheral neurons in representatives of distinct ecological groups: the pelagic predatory pteropod Clione limacina (Pteropoda, Gymnosomata) and its prey - Limacina helicina (Pteropoda, Thecosomata), as well as the plankton eater Melibe leonina (Nudipleura, Nudibranchia). By using tyrosine hydroxylase immunoreactivity as a reporter, we mapped their dopaminergic systems. Across all studied species, despite their differences in ecology, small numbers of dopaminergic neurons in the central ganglia contrast to an incredible density of these neurons in the peripheral nervous system, primarily representing sensory-like cells, which are predominantly concentrated in the chemotactic areas and project afferent axons to the central nervous system. Combined with tubulin immunoreactivity, this study illuminates the complexity of sensory signaling and peripheral neural systems in Euthyneuran molluscs with lineage-specific adaptations across different taxonomical and ecological groups.