Neuromuscular organization of the benthic ctenophore, Vallicula multiformis

Abstract The Ctenophora is the earliest metazoan taxon with neurons and muscles. Recent studies have described genetic, physiological, and cellular characteristics of the neural and muscular systems of this phylogenically important lineage. However, despite the ecological diversity of ctenophore niches, including both pelagic and benthic forms, studies have focused predominantly on pelagic species. In this study, we describe neural and muscular architectures of the benthic ctenophore, Vallicula multiformis (Order Platyctenida), employing immunohistochemical analysis to compare it to pelagic species. In V. multiformis, which lacks characteristic comb rows seen in pelagic ctenophores, neural structures that develop beneath the comb were not detected, whereas the subepithelial and tentacle neural networks exhibited considerable similarity to those of pelagic species. Despite significant differences in morphology and lifestyle, muscle organization in V. multiformis closely resembles that of pelagic species. Detailed analysis of neuropeptide-expressing neurons unveiled a neural architecture composed of various neural subtypes. This included widely distributed subepithelial neural networks (SNNs) and neurosecretory cells located primarily in the peripheral region. The consistent distribution patterns of the VWYa-positive SNN and tentacle nerves between V. multiformis and the pelagic species, Bolinopsis mikado, suggest evolutionarily conserved functionality of these neurons in the Ctenophora. In contrast, NPWa-positive neurons, which extend neurites connecting the apical organ and comb rows in B. mikado, showed a neurite-less neurosecretory cell morphology in this flattened, sessile species. Shared characteristics and variations in neural and muscular architectures among benthic and pelagic ctenophore species offer valuable insights for unraveling the biology of this rapidly evolving yet enigmatic metazoan lineage. These findings also provide important insight into neural control modalities required in early evolutionary processes of metazoans.