Ancient nervous system architecture in a living ctenophore

The evolutionary origin of nervous systems in animals remains elusive and is largely hidden from the fossil record. Ctenophores, one of the earliest-branching animals possessing neurons, are instrumental to our understanding of nervous system origin, and a few rare ctenophore fossils preserve traces of nervous tissue as carbonaceous remains. Cambrian ctenophores appear to exhibit a more diverse neuroanatomy than that of modern species, suggesting secondary loss in extant ctenophores. However, much remains unknown about the origin and ontogeny giving rise to the structural organization of modern ctenophore nervous systems. Here, by investigating the neural anatomy of the ctenophore Mnemiopsis leidyi during development, we identified a ladder-like nerve net (LNN) beneath the comb rows that converges into condensed neurites and connects to the aboral organ. Examination of carbon-rich areas of Ctenorhabdotus capulus, an extinct ctenophore from the Burgess Shale, reveals a pattern similar to that of M. leidyi, consistent with a shared neural organization. Furthermore, M. leidyi exhibits a condensed comb nerve, resembling the longitudinal nerve preserved in the Cambrian ctenophore Fasciculus vesanus and the giant axon of extant Euplokamis dunlapae. Our study reveals conserved evolutionary constraints shaping nervous system architectures linked to locomotory organs and indicates that the different modes of nervous system organization observed in Cambrian ctenophores are variably retained in modern species.