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The plastic nervous system of Nemertodermatida

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Abstract

Nemertodermatida are microscopic marine worms likely to be the sister group to acoels, forming with them the earliest extant branch of bilaterian animals, although their phylogenetic position is debated. The nervous system of Flagellophora cf. apelti, Sterreria spp. and Nemertoderma cf. westbladi has been investigated by immunohistochemistry and confocal microscopy using anti-tubulin, anti-5-HT and anti-FMRFamide antibodies. The nervous system of F. cf. apelti is composed of a large neuropile and a loose brain at the level of the statocysts with several nerve fibres surrounding them and innervating the broom organ. Sterreria spp. shows a commissural-like brain and several neurite bundles going frontad and caudad from this. At the level of the statocysts there is also a thicker aggregation of immunoreactive fibres. The nervous system of N. cf. westbladi consists of a nerve ring lying outside the body wall musculature at the level of the statocyst and a pair of ventro-lateral neurite bundles, from which extend thinner fibres innervating the ventral side of the animal. Numerous bottle-shaped glands were observed, innervated by fibres starting both from the brain and the neurite bundles. The nervous system of the nemertodermatids studied to date displays no common pattern; instead, there is considerable plasticity in the general morphology of the nervous system. In addition, the musculature of Sterreria spp. has been studied by phalloidin staining. It shows diagonal muscles in the anterior quarter of the body and a simple orthogonal grid in the posterior three quarters, being simpler than that of the other nemertodermatids. High-resolution differential interference contrast microscopy permitted to better visualize some morphological characters of the species studied, such as statocysts, sperm and glands and, in combination with anti-tubulin staining, describe in detail the broom organ in F. cf. apelti. Finally, we note an apparent absence of innervation of the gut in Nemertodermatida similar to the condition in Xenoturbella.

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Acknowledgments

Thanks are extended to the staff of Sven Lovén Centre for Marine Sciences (Sweden), the CCMAR in Faro (Portugal) and the Biologische Anstalt Helgoland (Germany) for their help with collecting the material. We are deeply grateful to Professor Marco Curini-Galletti, to Professor Mark Martindale and to Professor Philippe Bouchet for organizing sampling in Italy, at Hawai’i and in Papua New Guinea, respectively. Collections in Papua New Guinea took place during the Our Planet Reviewed Papua Niugini Expedition in November–December 2012, organized by the Muséum National d’Histoire Naturelle (MNHN), Pro Natura International, the Institut de Recherche pour le Développement (IRD) and the University of Papua New Guinea. The principal investigators of this expedition were Philippe Bouchet, Sarah Samadi (MNHN) and Claude Payri (IRD), and funding was provided by the Total Foundation, Prince Albert II of Monaco Foundation, Fondation EDF, Stavros Niarchos Foundation and Entrepose Contracting, with support from the Divine Word University and operated under a permit delivered by the Papua New Guinea Department of Environment and Conservation. The confocal microscopic observations were carried out at the newly equipped Research Resource Centre “Molecular and Cellular Technologies” at St.-Petersburg State University (Russia). We wish to express our gratitude to the most helpful staff of the Centre, in particular to Nikolai A. Kostin, specialist in confocal microscopy. Financial support was received from the Zoological Institute RAS project 0120135194 and the Russian Basic Research Foundation grant 13-04-02002 to Olga Raikova, the Swedish Research Council through a grant to Ulf Jondelius (grant numbers 2009-5147 and 2012-3913), the Föreningen Riksmusei Vänner (stipend 2011), Stiftelsen Lars Hiertas Minne grant FO2011-0248 and the Royal Swedish Academy of Sciences grant FOA11H-352 to Inga Meyer-Wachsmuth, and from the European Community through an ASSEMBLE grant (agreement no 227799).

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Olga I. Raikova and Inga Meyer-Wachsmuth contributed equally to this work.

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Raikova, O.I., Meyer-Wachsmuth, I. & Jondelius, U. The plastic nervous system of Nemertodermatida. Org Divers Evol 16, 85–104 (2016). https://doi.org/10.1007/s13127-015-0248-0

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