Phylogenetic position of the enigmatic clawless eutardigrade genus Apodibius Dastych, 1983 (Tardigrada), based on 18S and 28S rRNA sequence data from its type species A. confusus

•Phylogenetic position of the most enigmatic clawless eutardigrade genus Apodibius was resolved.•The type species of the genus Apodibius, A. confusus, reconstructed within Isohypsibioidea.•Apodibius confusus is a member of the clade grouping Isohypsibius asper, I. granulifer, and Eremobiotus alicatai.•The genus Isohypsibius is an artificial paraphyletic taxon.•Novel sequence data for the eutardigrade 18S and 28S rRNA genes are published.

The systematics of Eutardigrada, the largest lineage among the three classes of the phylum Tardigrada, is based mainly on the morphology of the leg claws and of the buccal apparatus. However, three members of the rarely recorded and poorly known limno-terrestrial eutardigrade genus Apodibius have no claws on their strongly reduced legs, a unique character among all tardigrades. This absence of all claws makes the systematic position of Apodibius one of the most enigmatic among the whole class. Until now all known associates of the genus Apodibius have been located in the incertae sedis species group or, quite recently, included into the Necopinatidae family. In the present study, phylogenetic analyses of 18S and 28S rRNA sequence data from 31 tardigrade species representing four parachelan superfamilies (Isohypsibioidea, Hypsibioidea, Macrobiotoidea, Eohypsibioidea), the apochelan Milnesium tardigradum, and the type species of the genus Apodibius, A. confusus, indicated close relationship of the Apodibius with tardigrade species recently included in the superfamily Isohypsibioidea. This result was well-supported and consistent across all markers (separate 18S rRNA, 28S rRNA, and combined 18S rRNA + 28S rRNA datasets) and methods (MP, ML) applied.

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