Sensory systems of Lineus ruber (Nemertea, Pilidiophora)
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ORIGINAL PAPER
Sensory systems of Lineus ruber (Nemertea, Pilidiophora) O. V. Zaitseva1 · S. A. Petrov1 · A. A. Petrov1 Received: 2 May 2020 / Revised: 26 September 2020 / Accepted: 30 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The aim of this study was to examine the organization of sensory systems in the heteronemertean Lineus ruber (Müller, 1774). Sensory systems of the head region (cerebral and frontal organs), body wall, proboscis and digestive tract of L. ruber were studied using Golgi–Colonnier silver impregnation, glyoxylic acid-induced fluorescence method for catecholamines (CAs) and immunochemical staining for serotonin (5-HT) and FMRFamide-related peptides (FaRPs). The study revealed a large number of intra- and subepithelial sensory cells in all of these structures and body regions. Only some of the sensory cells identified by silver impregnation were positive for CAs, 5-HT or FaRPs; the remaining cells appear to have different neurotransmitter modality. CA-containing (CA-C) intra- and subepithelial sensory cells were the most common cell types and were present in the epidermis and all the organs studied. All CA-C cells had a stiff cilium, which showed glyoxylic acid-induced fluorescence in the blue–green range indicative of CAs. In addition to CA-C cells, the cerebral organs had FaRP-immunoreactive sensory cells and argyrophilic cells containing an unidentified neurotransmitter and the frontal organs contained the neurites of 5-HT-immunoreactive, most likely efferent neurons. The presence of presumably mechanoreceptive CA-C cells in the frontal and cerebral organs indicates that these organs may perform not only a chemosensory, but also mechanosensory function. Keywords Nemerteans · Sensory cells · Sense organs · Nervous system · Silver impregnation · Immunochemistry
Introduction Nemerteans are a relatively small and little-known group of predatory unsegmented worms distinguished by a unique eversible proboscis used primarily for capturing prey. Molecular phylogenetic studies place nemerteans within Lophotrochozoa, but their phylogenetic affinities to other lophotrochozoan groups remain equivocal (Struck and Fisse 2008; Podsiadlowski et al. 2009; Kocot et al. 2017; Luo et al. 2018; Laumer et al. 2019; Marlétaz et al. 2019). The nervous and sensory systems show different levels of internalization and complexity in different nemertean groups and figure prominently in nemertean taxonomy (Gibson 1972; Chernyshev 2011; Beckers and von Döhren 2015). Many structural and functional aspects of these systems, however, are still poorly known, as the bulk of the literature on nemerteans are * A. A. Petrov [email protected] 1
Laboratory of Evolutionary Morphology, Zoological Institute RAS, Universitetskaya emb. 1, 199034 Saint Petersburg, Russia
histological and electron-microscopical studies that provide information only on the general architecture of the nervous system and sense organs (see reviews in Bullock and Horridge 1965; Beckers and von Döhren 201
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