Abstract
BackgroundBryozoans are sessile aquatic suspension feeders in mainly marine, but also freshwater habitats. Most species belong to the marine and calcified Cheilostomata. Since this taxon remains mostly unstudied regarding its neuroanatomy, the focus of this study is on the characterization and ground pattern reconstruction of the autozooidal nervous system based on six representatives.ResultsA common neuronal innervation pattern is present in the investigated species: a cerebral ganglion is located at the base of the lophophore, from where neurite bundles embrace the mouth opening to form a circumoral nerve ring. Four neurite bundles project from the cerebral ganglion to innervate peripheral areas, such as the body wall and parietal muscles via the tentacle sheath. Five neurite bundles comprise the main innervation of the visceral tract. Four neurite bundles innervate each tentacle via the circumoral nerve ring. Mediofrontal tentacle neurite bundles emerge directly from the nerve ring. Two laterofrontal- and one abfrontal tentacle neurite bundles emanate from radial neurite bundles, which originate from the cerebral ganglion and circumoral nerve ring in between two adjacent tentacles. The radial neurite bundles terminate in intertentacular pits and give rise to one abfrontal neurite bundle at the oral side and two abfrontal neurite bundles at the anal side. Similar patterns are described in ctenostome bryozoans.ConclusionsThe present results thus represent the gymnolaemate situation. Innervation of the tentacle sheath and visceral tract by fewer neurite bundles and tentacular innervation by four to six tentacle neurite bundles support cyclostomes as sister taxon to gymnolaemates. Phylactolaemates feature fewer distinct neurite bundles in visceral- and tentacle sheath innervation, which always split in nervous plexus, and their tentacles have six neurite bundles. Thus, this study supports phylactolaemates as sistergroup to myolaemates.
Highlights
Bryozoans are sessile aquatic suspension feeders in mainly marine, and freshwater habitats
Owing to the lack of more complete modern investigations on the cheilostome nervous systems, and its variation among different taxa, the aim of this study is to analyze the nervous system of cheilostome bryozoans on a broader scale including several taxonomic high-level groups and modern methods
Samples were incubated in 20% ethylenediaminetetraacetic acid (EDTA) in distilled water for 1 h or 24 h depending on their calcification state prior to staining
Summary
Bryozoans are sessile aquatic suspension feeders in mainly marine, and freshwater habitats. Most species belong to the marine and calcified Cheilostomata. Since this taxon remains mostly unstudied regarding its neuroanatomy, the focus of this study is on the characterization and ground pattern reconstruction of the autozooidal nervous system based on six representatives. Bryozoans are a group of colonial, aquatic suspensionfeeders consisting of over 6.000 recent and over 15.000 fossil described species [1]. Two distinct taxa are distinguishable among bryozoans: Phylactolaemata and Myolaemata The former is a small group of freshwater bryozoans, whereas the latter is a large bulk of mostly marine bryozoans [5]. Cyclostomes and cheilostomes have mineralized skeletons and are the largest, dominant groups of bryozoans, comprising around 800 and over 5.000 species, respectively [7]. Stenolaemates were dominant in the Paleozoic, whereas cheilostomes originated from the Mesozoic and immensely started to thrive during that period [8, 9]
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