Acanthechiniscus under scrutiny: ontogenetic, morphological, and phylogenetic evidence of polyphyly entails the erection of a new Holarctic echiniscid genus (Heterotardigrada)

All limno-terrestrial heterotardigrades share a broadly uniform ontogenetic pattern, progressing from two-clawed larvae lacking an anus and gonopore to four-clawed, sexually mature adults through one or two moulting stages. Within the family Echiniscidae, some species undergo changes in the number or length of trunk appendages during development, whereas others exhibit more stable morphologies. Acanthechiniscus is a rarely encountered echiniscid genus characterized by lateral appendages on each trunk segment. Owing to its rarity, detailed ontogenetic data have previously been available only for Acanthechiniscus goedeni. In this study, by integrative analyses of seven Holarctic populations, we characterize the ontogeny of Acanthechiniscus victor and Acanthechiniscus islandicus, and a new species, Acanthechiniscus aestivalis sp. nov., is described based on material collected from North Greenland. Both A. victor and A. aestivalis sp. nov. exhibit stable morphologies across post-embryonic stages, without significant morphometric shifts, whereas A. islandicus and A. goedeni have larvae that differ in chaetotaxy from later stages. In addition to ontogenetic differences, A. victor and A. aestivalis sp. nov. display morphology more similar to Cornechiniscus, whereas A. islandicus and A. goedeni share several distinct phenotypic peculiarities. Phylogenetic analyses based on partial 18S rRNA, 28S rRNA, and COI sequences suggest the diphyly of Acanthechiniscus, with the clade (A. victor and A. aestivalis sp. nov.) most likely sister to Cornechiniscus, based on morphological clues. These findings led to the erection of the Holarctic Polyacanthechiniscus gen. nov., and underline the significance of the Arctic for tardigrade biodiversity research.