Neuroanatomy of mud dragons: a comprehensive view of the nervous system in Echinoderes (Kinorhyncha) by confocal laser scanning microscopy

Abstract

Background The Scalidophora (Kinorhyncha, Loricifera and Priapulida) have an important phylogenetic position as early branching ecdysozoans, yet the architecture of their nervous organ systems is notably underinvestigated. Without such information, and in the absence of a stable phylogenetic context, we are inhibited from producing adequate hypotheses about the evolution and diversification of ecdysozoan nervous systems. Here, we utilize confocal laser scanning microscopy to characterize serotonergic, tubulinergic and FMRFamidergic immunoreactivity patterns in a comparative neuroanatomical study with three species of Echinoderes , the most speciose, abundant and diverse genus within Kinorhyncha. Results Neuroanatomy in Echinoderes as revealed by acetylated α-tubulin immunoreactivity includes a circumpharyngeal brain and ten neurite bundles in the head region that converge into five longitudinal nerves within the trunk. The ventral nerve cord is ganglionated, emerging from the brain with two connectives that converge in trunk segments 2–3, and diverge again within segment 8. The longitudinal nerves and ventral nerve cord are connected by two transverse neurites in segments 2–9. Differences among species correlate with the number, position and innervation of cuticular structures along the body. Patterns of serotoninergic and FMRFamidergic immunoreactivity correlate with the position of the brain neuropil and the ventral nerve cord. Distinct serotonergic and FMRFamidergic somata are associated with the brain neuropil and specific trunk segments along the ventral nerve cord. Conclusions Neural architecture is highly conserved across all three species, suggesting that our results reveal a pattern that is common to more than 40% of the species within Kinorhyncha. The nervous system of Echinoderes is segmented along most of the trunk; however, posterior trunk segments exhibit modifications that are likely associated with sensorial, motor or reproductive functions. Although all kinorhynchs show some evidence of an externally segmented trunk, it is unclear whether external segmentation matches internal segmentation of nervous and muscular organ systems across Kinorhyncha, as we observed in Echinoderes . The neuroanatomical data provided in this study not only expand the limited knowledge on kinorhynch nervous systems but also establish a comparative morphological framework within Scalidophora that will support broader inferences about the evolution of neural architecture among the deepest branching lineages of the Ecdysozoa. Electronic supplementary material The online version of this article (10.1186/s12862-019-1405-4) contains supplementary material, which is available to authorized users.

Insights into mud dragon morphology (Kinorhyncha, Allomalorhagida): myoanatomy and neuroanatomy of Dracoderes abei and Pycnophyes ilyocryptus

“…Positional information used for the identification of external characters, internal anatomy, functional morphology and associated molecular labeling followed the commonly accepted terminology for kinorhynchs (Herranz et al 2014(Herranz et al , 2019aSørensen and Pardos 2008). Nervous system terminology follows the neuroanatomical glossary of Richter et al (2010).…”

“…To date the sources of the neuroanatomical studies in kinorhynchs have been quite diverse (TEM, LM and CLSM) and very often focused on specific regions in TEM studies (Brown 1989;Moritz and Storch 1972;Nebelsick 1992a;Neuhaus 1994), or using specific molecular markers in CLSM studies (Altenburger 2016;Herranz et al , 2019a) (for overview, see table 1 in Herranz et al (2019a). Discrepancies in the quantity of longitudinal neurite bundles in the neck and trunk among studies, especially the ultrastructural ones, most likely relate to the challenging interpretation of ultrastructural data (Herranz et al 2019a).…”

“…To date the sources of the neuroanatomical studies in kinorhynchs have been quite diverse (TEM, LM and CLSM) and very often focused on specific regions in TEM studies (Brown 1989;Moritz and Storch 1972;Nebelsick 1992a;Neuhaus 1994), or using specific molecular markers in CLSM studies (Altenburger 2016;Herranz et al , 2019a) (for overview, see table 1 in Herranz et al (2019a). Discrepancies in the quantity of longitudinal neurite bundles in the neck and trunk among studies, especially the ultrastructural ones, most likely relate to the challenging interpretation of ultrastructural data (Herranz et al 2019a). Thus far all studied kinorhynchs, including those of the present study, exhibit a very conserved nervous system composed of: (i) a circumpharyngeal ring-like brain with a somata-neuropil-somata organization; (ii) ten longitudinal neurite bundles arising from the anterior brain region and converging in the first or second trunk segment into 5 nerves (2 subdorsal, 2 ventromedial/ventrolateral and a ventral nerve cord); (iii) a ganglionated ventral nerve cord formed anteriorly by the fusion of two longitudinal bundles that bifurcate in the posterior end; (iv) longitudinal nerves encircled in the trunk by at least one pair of transverse neurites per segment originating from the ventral nerve cord; (v) an introvert innervated from the anterior part of the neuropil; and (vi) a mouth cone innervated from the posterior part of the neuropil (Altenburger 2016;Herranz et al 2019a;Nebelsick 1993).…”

“…During the last decade, morphological studies of metazoans using modern techniques have started to cover a broader selection of animal groups, providing new insights into the diversification of organ systems (e.g., Bekkouche et al 2014;Bekkouche and Worsaae 2016;Gross and Mayer 2015;Kerbl et al 2015;Martin et al 2017). Some of these studies have focused on lesser-known taxa, such as the Scalidophora, which includes ecdysozoan groups with a radial head bearing scalids (Kinorhyncha, Loricifera and Priapulida) (Altenburger 2016;Herranz et al , 2014Herranz et al , 2019aMartín-Durán et al 2015;Neves et al 2013;Rothe and Schmidt-Rhaesa 2010). Despite their similarities in head composition, scalidophorans differ significantly in their external morphology, most dramatically in kinorhynchs, commonly known as mud dragons, which show a segmented body plan.…”

“…Recent morphological studies on kinorhynchs have relied on fluorescent labelling of different components of the nervous system and musculature combined with the use of confocal laser scanning microscopy (CLSM) and three-dimensional rendering. The myoanatomy has been studied in Antygomonas (Müller and Schmidt-Rhaesa 2003), Echinoderes (Herranz et al 2014), and Setaphyes (Rothe and Schmidt-Rhaesa 2004;Schmidt-Rhaesa and Rothe 2006); neuroanatomical studies have been carried out for the cyclorhagid genera Antygomonas, Zelinkaderes and Echinoderes (Herranz et al , 2019a and the allomalorhagid genus Setaphyes (Altenburger 2016) (see Herranz et al (2019a) for an overview). Despite these recent efforts to understand the internal morphology of kinorhynchs, there are still important gaps of knowledge and an absence of consistent comparable data sets.…”