| Summary: | Tardigrades are small micrometazoans able to resist several environmental stresses in any stage of their life cycle. An integrated analysis of tardigrade specimens collected in Tsukuba (Japan) revealed a peculiar morphology and a new sensory field in the cloaca. Molecular taxonomy and phylogenetic analysis on different genes (COI, ITS2, 18S and 28S) confirmed that this population is a new species, <i>Macrobiotus kyoukenus</i> sp. nov., belonging to the widespread <i>Macrobiotus hufelandi</i> group. The stress resistance capabilities of <i>M. kyoukenus</i> sp. nov. have been tested by submitting animals to extreme desiccation, rapid freezing, and high levels of ultraviolet radiations (UVB and UVC). Animals were able to survive desiccation (survivorship 95.71 ± 7.07%) and freezing up to −80 °C (82.33 ± 17.11%). Both hydrated and desiccated animals showed a high tolerance to increasing UV radiations: hydrated animals survived to doses up to 152.22 kJ m<sup>−2</sup> (UVB) and up to 15.00 kJ m<sup>−2</sup> (UVC), while desiccated specimens persisted to radiations up to 165.12 kJ m<sup>−2</sup> (UVB) and up to 35.00 kJ m<sup>−2</sup> (UVC). Present data contribute to the discovery of a larger tardigrade biodiversity in Japan, and the tolerance capabilities of <i>M. kyoukenus</i> sp. nov. show that it could become a new emerging model for stress resistance studies.
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