Are freshwater Macrodasyida as rare as previously thought? Integrative taxonomy identifies a new taxon of Gastrotricha Macrodasyida

Macrodasyida are worm-like, hermaphroditic and marine gastrotrichs with about 350 species. Among all known Macrodasyida only Marinellina flagellata, Redudasys fornerise and Redudasys sp. (Kånneby & Wicksten, 2014), were found in fresh waters respectively from an Austrian stream, a Brazilian artificial reservoir and a USA aquifer. Herein we describe a new freshwater taxon of Macrodasyida from different streams in Minas Gerais State, Southeastern Brazil. The external morphology and internal anatomy were investigated using Differential Interference Contrast (DIC), Confocal Laser Scanning Microscopy (CLSM) Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). The systematization of the new taxon within Macrodasyida was inferred by 18S rRNA gene, and the relationships with the other freshwater Macrodasyida species were investigated by COI mtDNA. The adult has a body length from 193 to 376 m and 1+1 anterior adhesive tubes, 2 pairs of posterior adhesive tubes of unequal length, numerouscephalic cilia arranged into irregular bands, short around the mouth and longer on the anterolateral head margins, 6 pairs of lateral tactile bristles. Pharynx surrounded dorsally, laterally and ventrally by 10–12 longitudinal muscles which are inserted on the mouth rim and lie externally to circular muscles; the pair of main ventrolateral longitudinal muscles is inserted at the level of the anterior adhesive tubes. Intestine circular musculature external to the splanchnic longitudinal one and not surrounding dorsal and ventrolateral somatic longitudinal muscles. Maximum Likelihood and Bayesian Inference analyses yielded topologies congruent with each other and the new taxon nested within the family Redudasyidae, but COI mtDNA analyses showed clear distances between Redudasys fornerise, Redudasys sp. and the new taxon. Results suggest that Macrodasyida invaded freshwater environment only once and the Neotropical region seems to be a peculiar place for the evolution of the freshwater macrodasyidan clade.