The phylogenetic position of the phylum Ctenophora and the nature of their nervous systems are highly debated topics in modern evolutionary biology. However, very little is known about the organization of ctenophore neural and muscular systems and virtually nothing is reported about their embryogenesis. Here, we have characterized the neural and muscular development of the sea gooseberry, Pleurobrachia bachei, starting from the cleavage stages to post-hatching larvae. Scanning electron microscopy and immunochemistry were used to describe the formation of the embryonic mouth, tentacles, combs, the aboral organ, and putative sensory cells. The muscles started their specification at the end of the 1st day of Pleurobrachia development. In contrast, neurons appeared two days after the myogenesis, just before the hatching of fully formed cydippid larvae. The first tubulin immunoreactive neurons, a small group of 4-6 cells with neuronal processes, were initially recognized at the aboral pole during the 3rd day of development. Surprisingly, this observed neurogenesis occurred after the emergence of distinct behavioral patterns in the embryos. Thus, the embryonic behavior associated with comb cilia beatings and initial muscle organization do not require morphologically defined neurons and their elongated neurites. This study provides the first description of neuromuscular development in the enigmatic ctenophores and establishes the foundation for future research using emerging genomic tools and resources. This article is protected by copyright. All rights reserved.
© 2015 Wiley Periodicals, Inc.
KEYWORDS: Ctenophora; Cydippid Larvae; Mnemiopsis; Nervous System and Muscle Development; Neuronal Evolution; Origin of Neurons and Muscles; Phylogeny; Sensory cells