Set to its place: first DNA data on freshwater tardigrades of Sakhalin (Far East Russia) shed light on the phylogenetic position of Mixibius (Eutardigrada: Hypsibioidea)

In this paper I present the results of the first faunistic investigation of the Sakhalin freshwater tardigrade fauna with the use of the integrative taxonomy. For the analysis, samples of bottom sediments were collected in several rivers of South Sakhalin. I found eight species belonging to three superfamilies of Eutardigrada. For most of those species I obtained data on mitochondrial COI gene and on 18S rRNA, 28 rRNA and ITS-2 sequences in addition to the morphological analysis using light and scanning electron microscopy. The number of tardigrade species known for Sakhalin Island increased from 1 to 9. Among 8 species found during my research one is similar to Dactylobiotus dervizi previously known from Komandorskiye Islands and one to Murrayon hastatus. Other 6 species belonging to the genera Mixibius, Hypsibius, Dianea and Mesobiotus are new for science, yet their formal description is not possible because of the small number of specimens. Inclusion of obtained molecular data in the phylogenetic analysis of the superfamily Hypsibioidea revealed for the first time the presence of the well-supported clade of the enigmatic genus Mixibius and gave possibility to recognise its phylogenetic position as sister clade to the genus Acutuncus within the family Acutuncidae. Two species of the genus Mixibius (M. parvus and M. tibetanus) are transferred into the genus Isohypsibius on the basis of the morphological analysis. Amended diagnoses for the family Acutuncidae and the genus Acutuncus are given.