Species delimitation
The bPTP analyses identified 76 entities in both the Maximum Likelihood and BI approaches, with supports ranging from 0.65 to 1.00 (0.97 on average). However, after considering the lack of morphological differences (data not shown) and low genetic variance among some of the delimited entities, and considering that bPTP and other tree-based species delimitation methods usually oversplit species into multiple entities (Dellicour & Flot, 2018), we concluded that the following populations represent single species: the eleven M. tardigradumpopulations (indicated as two entities by the bPTP; following Morek et al., 2019a), the six populations of M. eurystomum Maucci, 1991 (recovered as three entities by the bPTP; following Morek, Blagden, Kristensen, Michalczyk, 2020a), the three populations of M. quadrifidum Nederström, 1919 (indicated as two entities by the bPTP) both populations of M. variefidum Morek et al., 2016 (recovered as two entities by the bPTP), Milnesium sp. TZ.075, ZA.204 and ZA.180+ZA.218 (indicated as three entities by the bPTP);Milnesium sp. ID.432+ID.940, ID.711+ID.950, PH.014 and VN.045 (recovered as four entities by the bPTP); and Milnesium sp. AR.437+AR.470 and GF.089+GF.093+GF.193 (indicated as two entities by the bPTP). Thus, we concluded that our dataset comprises 64 species recognised as distinct taxa when both molecular and morphological data were in agreement. Slightly more than a half of the species (around 56%) were represented by a single population (singletons), whereas 44% were formed by multiple populations.
The most abundant species were: M. tardigradum (represented by eleven populations), M. eurystomum (six populations); M. inceptum Morek et al., 2019b (five populations), M. reductumTumanov, 2006 (five populations), and two undescribed species, one from the Oriental realm, Milnesium sp. #54 (six populations), and the second one from the South America, Milnesium sp. #55 (five populations). Whether the Oriental and South American populations represent single species or groups of species, should be a subject of further research. Among the 64 delimited species, as little as 15 include previously described taxa. The other 49 species are most likely new to science, which would sharply increase the number of recognised species in the genus Milnesium , currently amounting to 43 (as of 23.09.2020; Degma, Bertolani, & Guidetti, 2009–2020; Morek, Ciosek, & Michalczyk, 2020b).