Systematics of tardigrada: A reanalysis of tardigrade taxonomy with specific reference to Guil et al. (2019)

The Tardigrada are a clade with a disputed and complex taxonomy. The three traditional tardigrade classes are the Heterotardigrada, Eutardigrada and the dubious, monotypic Mesotardigrada. A recent molecular study by Guil et al (Zoologica Scripta, 48, 2019, 120) suggested that the Apochela, previously considered an order of Eutardigrada, should become a new class, Apotardigrada, with the Parachela becoming THE new Eutardigrada. This new diagnosis was presented alongside compelling morphological evidence. Here, we test the assumptions of the molecular evidence for Apotardigrada by analysing tardigrade 18S and 28S data under four separate conditions, along with new BUSCO data, across both Bayesian- and maximum-likelihood phylogenetic approaches. We found variable conclusions regarding the status of Apotardigrada could be drawn by comparing the ribosomal RNA and BUSCO results, and caution against using branch length data to determine taxonomic hierarchy.