Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)

The organization of the cnidarian nervous system has been widely documented in polyps and medusae, but little is known about the nervous system of planula larvae, which give rise to adult forms after settling and metamorphosis. We describe histological and cytological features of the nervous system in planulae of the hydrozoan Clava multicornis. These planulae do not swim freely in the water column, but rather crawl on the substrate by means of directional, coordinated ciliary movement coupled to lateral muscular bending movements associated with positive phototaxis. Histological analysis shows pronounced antero-posterior regionalization of the planula`s nervous system, with a different neural cell types highly concentrated at the anterior pole. Transmission electron microscopy of planulae shows the nervous system to be unusualy complex, with a large orderly array of sensory cells at the anterior pole. In the anterior half of the planula the basi-ectodermal plexus of neurites forms an extensive orthogonal network, whereas more posteriorly neurites extend longitudinally along the body axis. Additional levels of nervous system complexity are uncovered by neuropeptide-specific immunocytochemistry, which reveal distinct neural subsets having specific molecular phenotypes. Together these observations imply that the nervous system of planulae of Clava multicornis manifests a remarkable level of histological, cytological and functional organization, whose features may be reminiscent of those present in early bilaterian animals. J. Comp. Neurol., 2011. (c) 2011 Wiley-Liss, Inc.