The muscular system of Musellifer delamarei (Renaud-Mornant, 1968) and other chaetonotidans with implications for the phylogeny and systematisation of the Paucitubulatina (Gastrotricha)

We studied comparatively the muscle organization of several gastrotrich species, aiming at shedding some light onthe evolutionary relationships among the taxa of the suborder Paucitubulatina. Under confocal laser scanningmicroscope, the circular muscles were present in the splanchnic position as incomplete circular rings in Muselliferdelamarei (Chaetonotidae) and Xenotrichula intermedia (Xenotrichulidae) and as dorsoventral bands in Xenotrichulapunctata, Heteroxenotrichula squamosa and Draculiciteria tesselata (Xenotrichulidae); in the somaticposition, M. delamarei shares the presence of dorsoventral muscles with all the Xenotrichulidae, in contrast withthe remaining Chaetonotidae that lack these muscles. Maximum parsimony analysis of the muscular charactersconfirmed monophyly of Paucitubulatina and Xenotrichulidae, while the Chaetonotidae was paraphyletic, with theexclusion of Musellifer, which is the most basal genus within the Paucitubulatina. Xenotrichulidae is the sistertaxon to Chaetonotidae, which in turn has Polymerurus as the most basal taxon. In general, the results agree withrecent phylogenetic inferences based on molecular characters and support the hypothesis that, within Paucitubulatina,dorsoventral muscles are plesiomorphies retained in marine, interstitial, hermaphroditic gastrotrichs.Dorsoventral muscles were subsequently lost during changes in lifestyle and reproduction modality that took placewith the invasion of the freshwater environment. This new information prompted us to reconsider the systematizationof Chaetonotidae, proposing the establishment of Muselliferidae fam. nov. to include the genera Muselliferand Diuronotus.