Neural System Rearrangement during the Metamorphosis of the Hydroid Clava multicornis

The planula larva of the hydroid Clava multicornis has a complex nervous system. It is characterized by the presence of distinct populations of neural elements concentrated at the anterior end representing the sensory pole of this life stage. The neurons can be identified by the presence of different neuropeptides: GLWamide positive cells are arranged in a domelike organization in the anterior most region and are followed by a belt of RFamide positive ones, both types being connected to a basiectodermal anterior plexus.1 During the metamorphosis, the larva attaches to the substrate with the anterior end that develops into the basal foot region of the polyp, while the posterior end forms the oral region. In the polyp of the hydrozoan Hydractinia echinata GLWamide and RFamide positive neurons are concentrated around the mouth and in the tentacles, at the opposite side respect to the position occupied in the larva.2 To elucidate the fate of the neural cells in C. multicornis, we investigated the distribution pattern of GLWamide and RFamide positive sensory cells at different stages of metamorphosis. We observed that immunoreactivity is still present at the anterior end of the larva during early settlement, but gradually disappeared through the following stages of metamorphosis. Only in late stages positive cells appeared around the mouth of the newly formed polyp. By TUNEL assay, apoptotic nuclei were identified in the anterior end of the settled larva, in the same region occupied by sensory cells. These results suggest that at least part of the neurons of the larva degenerates during metamorphosis by apoptosis and that at least part of the adult nervous system is formed by de novo differentiation. Understanding how the nervous system is rearranged during the metamorphosis of basal metazoans can help to elucidate mechanisms of neural plasticity in higher metazoans