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

Abstract

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 anteroposterior regionalization of the planula's nervous system, with different neural cell types highly concentrated at the anterior pole. Transmission electron microscopy of planulae shows the nervous system to be unusually complex, with a large, orderly array of sensory cells at the anterior pole. In the anterior half of the planula, the basiectodermal 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 reveals distinct neural subsets having specific molecular phenotypes. Together these observations imply that the nervous system of the planula of Clava multicornis manifests a remarkable level of histological, cytological, and functional organization, the features of which may be reminiscent of those present in early bilaterian animals.

Nervous systems of the sea anemone Nematostella vectensis are generated by ectoderm and endoderm and shaped by distinct mechanisms

“…For example, strictly aboral orientation of early ectodermal neurites in the hydrozoan Podocoryne is restricted to neurons in the oral region (Groger and Schmid, 2001;Momose and Schmid, 2006), whereas RFamide-positive ectodermal neurites in the scyphozoan Aurelia (Nakanishi et al, 2008) and the anthozoan Acropora millepora (Hayward et al, 2001) develop oral and aboral projections. Similarly, the extensive endodermal nervous system development in planulae observed in N. vectensis has not been described in other cnidarians (de Jong et al, 2006;Groger and Schmid, 2001;Martin, 2000;Nakanishi et al, 2008;Piraino et al, 2011). Additional studies of nervous system formation at high temporal and spatial resolution in these and other cnidarians are needed for a detailed reconstruction of the evolutionary histories of cnidarian neural development.…”

Conserved MIP receptor–ligand pair regulates Platynereis larval settlement

“…In cnidarians, GLWamides are expressed in sensory cells at the anterior pole of the larva (8,56). This anterior territory shares a conserved regulatory signature with apical organs in many bilaterian ciliated larvae (24,57).…”

The Evolution of Early Neurogenesis

“…Neurons occur at all levels, but are concentrated closer to the anterior pole (Martin, 1992; Gröger and Schmid, 2001; Nakanishi et al, 2008; 2012; Seipp et al, 2010; Piraino et al, 2011). This gradient is established by a Wnt-secreting signaling center located at the oral pole (i.e., posterior end of the planula), and Wnt antagonists expressed further anteriorly in the domain where neurons are concentrated (Marlow et al, 2013; Sinigaglia et al, 2013; Fig.…”