Retinoic acid influences antero-posterior positioning of peptidergic neurons in the planula larva of the hydrozoan Clava multicornis

Retinoic acid (RA) is a derivative of vitamin A known to be involved in the regulation of many developmental and physiological processes in chordates. Recently, evidence showing its presence and function in invertebrate deuterost omes and protostomes indicated the early evolutionary origin and conservation of the RA developmental machinery, with a key role also in neural differentiation. Moreover, it is known that retinoids can influence pattern specification in polyps of the hydroid Hydractinia echinata. The planula larvae of the hydrozoan Clava multicornis are characterized by a complex nervous system formed by a frontal neural plexus composed by two distinct peptidergic cell populations with antero-posterior organization: a first, anterior-most GLWamide immunoreactive (GLW-IR) population and an RFamide immunoreactive (RF-IR) cell belt just posterior to the first population. In contrast to most swimming planulae, C. multicornis larvae display a smooth gliding movement on the substrate, characterized by alternate bending of the anterior pole. We tested the effects of RA and of an RA antagonist on C. multicornis development by analyzing the nervous system organization and the photoresponsive behav- ior of larvae exposed to RA during their embryogenesis. After the exposure, the nervous system became completely disorganized, leading to the displacement of RFamide-IR cells. Moreover, RA-treated larvae did not exhibit the typ- ical phototropic behavior of control specimens. Our results suggest that RA can alter the normal development of nervous elements in this hydroid species, supporting the hypothesis that the morphogenetic activity of RA predates the origin of chordates.