Revision of Testechiniscus Kristensen, 1987 (Heterotardigrada : Echiniscidae) refutes the polar-temperate distribution of the genus

The family Echiniscidae comprises limno-terrestrial heterotardigrades with a strongly sclerotised dorsum, typically covered with plates. Among other members of the Echiniscus evolutionary line, the genus Testechiniscus Kristensen, 1987 stands out with well-developed ventral armature and polygonal sculpturing of the dorsal plates. It has alleged bipolar distribution (with satellite alpine records in the Holarctic). Thanks to fresh material from terra typica (Svalbard), we integratively redescribe (i.e. using light and electron microscopy imaging, morphometry, and molecular methods) the nominal species for the genus, Testechiniscus spitsbergensis (Scourfield, 1897). A comparison of the neotype series with a number of Holarctic records revealed morphological variability suggesting that the species may encompass several taxa, which, most likely, will be possible to delineate primarily with molecular tools. Moreover, based on material from Simien Mountains (Northern Ethiopia), we describe a new sibling subspecies, Testechiniscus spitsbergensis tropicalis ssp. nov. Extensive morphometric datasets are provided for the genus members for the first time. A new generic definition is proposed, embracing the two subspecies of T. pitsbergensis and T. laterculus (Schuster et al., 1980), but excluding two circum-Antarctic species, T. macronyx (Richters, 1907) and T. meridionalis (Murray, 1906). The later species are likely to be erected in the future as separate genera, and their autapomorphies are described here. In the light of our findings, the genus Testechiniscus should be recognised as a native element of the Northern Hemisphere, with mainly circum-Arctic distribution and additional, insular alpine records from the Nearctic, Palearctic and Eastern Afrotropic