How to build a larval body with less than a hundred cells? Insights from the early development of a stalked jellyfish (Staurozoa, Cnidaria)

Abstract

The stalked jellyfish (Staurozoa) is an extraordinary clade within medusozoan cnidarians in which the medusa is attached to the substrate unlike the pelagic jellyfishes which compose the rest of the medusozoans. Along with this remarkable feature, staurozoans are characterized by an extremely low number of cells (< 100) in the embryos and larvae. The aim of the present study is to explore early development of the staurozoan Lucernaria quadricornis and to elucidate morphogenetic events evolved to overcome the constraints imposed by low cell number. Using bright field, confocal, and electron microscopy, we create a normal table of development of Lucernaria, describe cell number dynamics, and visualize organization of embryos and larvae. From these data, we infer a crosstalk between cell reshaping, cell rearrangement, and mechanical stress, involved in gastrulation, anterior-posterior axis differentiation, and even locomotion of the larva. Our work also demonstrates that staurozoans convergently developed morphogenetic pathways similar to other very distant animals with low cell number in the early development. We consider Lucernaria as an EvoDevo model with potential for further research to answer the question of how evolutionary forces act on developmental trajectories.