Towards a better understanding of echiniscid intraspecific variability: A redescription of Nebularmis reticulatus (Murray, 1905) (Heterotardigrada: Echiniscoidea)

Abstract

The interplay between tardigrade genetic and ontogenetic diversity, resulting in morphological variation within a species, remains a poorly studied field. In this work, by combining qualitative (light and scanning electron microscopy), quantitative (morphometrics) and genetic analyses (DNA sequencing of five molecular markers) on five Palearctic populations of Nebularmis reticulatus (Murray, 1905) and a Tanzanian population of Nebularmis cirinoi (Binda and Pilato, 1993), all rare members of the Echiniscidae, we discuss intra- and interspecific variability in limno-terrestrial Heterotardigrada. We show that in these parthenogenetic (thelytokous) echiniscid species: (a) morphology strongly varies among life stages and between populations; (b) between populations of a single species there are statistically significant differences in the lengths of sclerotised structures commonly used in species delineation; (c) populations are dominated by one or two haplotypes of quickly evolving DNA fragments (ITS-1, ITS-2, COI), which suggests that such populations are characterised by low genetic diversity. Additionally, here we establish Loch Ness (Scotland) as the neotype locality for N. reticulatus. Finally, challenges of the taxonomy of the genus Nebularmis Gąsiorek & Michalczyk, 2019 are highlighted.