**Genetic variability**

Aligned sequences (obtained in our study and downloaded from GenBank) were trimmed to 609, 554, 682 and 200 bp for COI (21 sequences selected from GenBank—one sequence per species; (see Supplementary Materials—SM2)), 18S rRNA (20 sequences selected from GenBank—one sequence per species; see Supplementary Materials—SM3), 28S rRNA (16 sequences selected from GenBank—one sequence per species; see Supplementary Materials—SM4) and ITS2 (12 sequences selected from GenBank one sequence per species; see Supplementary Materials—SM5) molecular markers, respectively.

The analysis of the p-distances between our COI sequence (GenBank accession number: MW656266) and sequences of species from the genus *Macrobiotus* ranged from the most similar 17% for *Mac. canaricus* (GenBank accession number: MH063925 [67]), to least similar 25% for *Mac. kristenseni* Guidetti, Peluffo, Rocha, Cesari and Moly de Peluffo [74] (GenBank accession number: KC193575 [74]). In the conservative 18S rRNA gene fragment we observed no differences between our sequence (GenBank accession number: MW680641) and sequences of *Mac. hannae* (GenBank accession number: HQ604975 [75]). In turn, the uncorrected genetic p-distances between the least similar *Mac. polonicus* Pilato, Kaczmarek, Michalczyk and Lisi [37] (GenBank accession number: HM187580 [76]) was 4%. The analysis of the p-distances between our sequence of 28S rRNA (GenBank accession number: MW680644) and similar sequences of the genus *Macrobiotus* are as follows: the most similar was *Mac. hannae* (GenBank accession number: MH063924, Nowak and Stec [68]) with p-distance of 1% and the least similar was *Mac. polypiformis* (GenBank accession number: KX810009 [38]) and *Mac. scoticus* Stec, Morek, Gasiorek, Blagden and Michalczyk [77] (GenBank accession number: KY797266 [68]) with p-distance of 10%. In turn, the ranges of p-distances between our ITS-2 sequence (GenBank accession number: MW680418) and the most similar *Mac. hannae* (GenBank accession number: MH063923 [68]) was 11% and the least similar was *Mac. polypiformis* (GenBank accession number: KX810010 [38])—33%.

**Genus**: *Mesobiotus* Vecchi, Cesari, Bertolani, Jönsson, Rebecchi and Guidetti, 2016 [78] *Mesobiotus skorackii* Kaczmarek, Zawierucha, Buda, Stec, Gawlak, Michalczyk and Roszkowska, 2018 [15]

(Tables 6 and 7)

**Table 6.** Measurements (in μm) and *pt* values of selected morphological structures of individuals of *Mesobiotus skorackii* mounted in Hoyer's medium (N—number of specimens/structures measured; RANGE refers to the smallest and the largest structure among all measured specimens; SD—standard deviation; *pt*—ratio of the length of a given structure to the length of the buccal tube expressed as a percentage).


**Table 7.** Measurements (in μm) of selected morphological structures of eggs of *Mesobiotus skorackii* mounted in Hoyer's medium (N—number of specimens/structures measured; RANGE refers to the smallest and the largest structure among all measured eggs; SD—standard deviation).


*Locality*: 51◦24 21" N, 116◦14 27" W, 1900 m asl, Canada, Alberta, Banff National Park, near east end of the Louise Lake, moss on stone, May 2019, leg. Milena Roszkowska and Łukasz Kaczmarek.

*Material examined*: The 63 specimens; 44 animals and 19 eggs were mounted on microscope slides in Hoyer's medium, four eggs and four animals prepared for SEM and six animals prepared for molecular analyses. However, DNA sequences were obtained from one female specimen (exoskeleton) which was later mounted on microscope slide in Hoyer's medium and included into type series.

*Depositories*: All specimens (slides: CN8.\*, where the asterisk can be substituted by any of the following numbers: 5–7, 31–40, 42–43, 45, 115/S) are deposited at the Department of Animal Taxonomy and Ecology, Institute of Environmental Biology, Adam Mickiewicz University in Pozna ´n, Uniwersytetu Pozna ´nskiego 6, 61-614 Pozna ´n, Poland.

*Short diagnosis:*

Adults (measurements and statistics in Table 6). Body white in living animals and transparent after fixation in Hoyer's medium, eyes present, cuticle smooth. Bucco– pharyngeal apparatus of the *Macrobiotus* type, with ventral lamina and ten peribuccal lamellae. Mouth antero-ventral. Oral cavity armature of the *harmsworthi* type. Pharyngeal bulb spherical with triangular apophyses, three rod-shaped macroplacoids and a triangular microplacoid. Macroplacoid length sequence 2 < 3 < 1. The first macroplacoid narrower anteriorly, the second without constrictions and the third with a small, subterminal constriction. Claws of the *Mesobiotu*s type. Lunules under claws I–III smooth and slightly dentated under claws IV. Thin cuticular bars under claws I–III present. Granulation hardly visible on legs I–III, whereas on legs IV always clearly marked.

Eggs (measurements and statistics in Table 7). Eggs laid freely, white and spherical. Egg processes in the shape of short and wide sharpened cones. Egg processes reticulated and surrounded by six areolae delimited by thin brims which are often discontinuous, thus areolae are not always fully formed (semi-areolation). Surface inside the areolae with clearly visible wrinkles.

**DNA sequences**

We obtained good quality sequences for the applied molecular markers:

**COI**: single sequence; 631 bp long;

**18S rRNA**: two sequences; 667–715 bp long;

**28S rRNA**: single sequence; 735 bp long.
