Untangling the Echiniscus Gordian knot: paraphyly of the "arctomys group" (Heterotardigrada: Echiniscidae).

The genus Echiniscus C.A.S. Schultze, 1840, one of the earliest established and speciose tardigrade genera, has been hypothesized to be polyphyletic. Moreover, the presence or absence of trunk appendages, the type of cuticular sculpturing and body colour have been argued to hold taxonomic significance at the genus level in Echiniscus-line taxa. Here, by combining morphological and genetic analyses, we demonstrate that the so-called "arctomys group", i.e. Echiniscus spp. lacking trunk appendages, comprises numerous separate evolutionary lineages within the family Echiniscidae. As a result, we erect five new echiniscid genera: Barbariagen. nov., presumably of Neotropical (Gondwanan) origin, previously classified as the Echiniscus bigranulatus group; the pantropical and subtropical Kristenseniscusgen. nov. (the tessellatus group), characterized by a peculiar subdivision of dorsal plates; Claxtoniagen. nov. (the wendti group), with large and evident endocuticular pillars in the form of polygons; Nebularmisgen. nov. (the reticulatus group), with an elusive dorsal sculpturing; and Viridiscusgen. nov. (the viridis group), with body colour ranging from light green through brownish to even almost black. Additionally, we briefly address appendaged Echiniscus s.s. and divide the genus into several groups based on dorsal plate sculpturing and suggest that these could also represent separate supraspecific entities.