HYDROZOAN PLANULAE MAY HAVE AN ANTERIOR MOUTH, A PERIORAL NEURAL PLEXUS, AND INGEST FOOD PARTICLES: PRODROMES OF BILATERIAN CEPHALIZATION?

The presumptive "simple" nervous system of cnidarians has been recently demonstrated in the hydrozoan diploblastic planula of Clava multicornis to achieve a surprisingly high level of histological, cytological, biochemical and functional complexity, with characteristic localization of sensory neurons concentrated in the anterior ectoderm and a prominent anterior neural plexus (PIRAINO et al., 2011). These morphological features are reminiscent of those already present in early bilaterian animals, and they have been interpreted as pre-requisites for the forward-directed crawling behavior of the benthic larva of C. multicornis. In addition, circumferential GLWamide-immunoreactive neurite bundles of the anterior neural plexus are distributed around a frontal gap in the ectoderm and mesoglea, where the endoderm bulges out in contact with the external environment. Opposite to the pole where the polyp mouth will open at metamorphosis, such histological discontinuity has been interpreted as a functional anterior larval orifice, allowing effective communication between the inner planula cavity and the outer environment. To further investigate the features and the functionality of such a mouth-like anterior discontinuity, both TEM ultrastructural analysis and functional experiments with labeled dyes and particles were carried out. Transverse sections were made at the anteriormost part of the planulae and observed under TEM, clearly confirming the occurrence of a mesogleal interruption of the ectodermal layer at the anterior larval pole, i.e. a true anterior orifice, where endodermal cells are in contact with the direct environment. At the level of this orifice, a big mucous aggregate containing several bacteria was documented, surrounded by cytoplasmic protrusions of ectodermal support cells, as in the act of being engulfed into the planula inner cavity. In vivo experiments allowed to documenting the passage of labeled fluids and particles from the outer environment towards the inner planula cavity, through loose intercellular spaces in the anteriormost ectodermal layer, lacking cell-cell septate junctions. Taken together, these findings (neural anterior concentration, anterior orifice surrounded by sensory cells and circumferential plexus) corroborate the hypothesis that a certain degree of ancestral cephalization and a bilateral bauplan organization was already achieved at the dawn of metazoans.