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Intraspecific variability of tardigrades is a poorly investigated field. The great majority of tardigrade species were described using single populations, sometimes even single individuals. Moreover, intraspecific variability between multiple, integratively verified populations has never been described for any tardigrade species. In this study, we analyzed morphological, morphometric, and genetic variability of nine European populations of Milnesium tardigradum, one of the first described tardigrade species that until recently was thought to be cosmopolitan. We found that the nine populations of M. tardigradum exhibited low to moderate diversity in the nuclear ITS‐2 marker (0.2%–4.0%, 1.1% on average) and low to high divergence in the mitochondrial COI fragment (0.2%–11.4%, 4.4% on average). Despite the considerable variability in COI, all M. tardigradum populations exhibited mostly uniform morphology and morphometry. Moreover, we analyzed ontogenetic variability in claw and lamellae configuration under scanning electron microscopy, which resulted in the description of novel taxonomically important traits for M. tardigradum. The phylogenetic analysis unambiguously showed that all GenBank sequences predating the redescription of M. tardigradum and labeled as "M. tardigradum" or "M. cf. tardigradum" represent other species. Thus, the neotype population from Germany and the additional eight European populations analyzed in this study are the only confirmed records of M. tardigradum to date. Finally, in order to clarify ontogenetic nomenclature, we propose distinct names for the first and second immature instars in the order Apochela (hatchling and juvenile, respectively) and concise definitions of time and direction of ontogenetic changes in claw configuration (i.e., early/late and positive/negative change). Intraspecific, interpopulation variability of a tardigrade Milnesium tardigradum was investigated by means of integrative taxonomy, including morphology (light and scanning electron microscopy), morphometry, and barcoding of a nuclear (ITS‐2) and a mitochondrial marker (COI). The nine analysed European populations revealed low to moderate variability in morphology and in ITS‐2 (≤4.0%). In contrast, COI exhibited low to surprisingly high divergence (≤11.4%), questioning the possibility of setting a universal barcoding threshold for tardigrade species delineation. Additionally, novel, taxonomically important traits for M. tardigradum are described. [ABSTRACT FROM AUTHOR]

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