An integrative redescription of Echiniscus testudo (Doyère, 1840), the nominal taxon for the class Heterotardigrada (Ecdysozoa: Panarthropoda: Tardigrada)

The phylum Tardigrada is divided into three classes: Eutardigrada, Mesotardigrada, and Heterotardigrada. Armoured limno-terrestrial taxa within the family Echiniscidae represent over half of the described species in the last class, and exhibit more consistent morphology in comparison with their closest marine relatives. The main aim of this study was to redescribe comprehensively the nominal species for Heterotardigrada, Echiniscus testudo, applying an integrative approach. Our integrative study confirmed that both forms E. testudo trifilis and E. t. quadrifilis (with three and four lateral body cirri, respectively), although differing in the COI sequence, indeed represent a single species. Finally, we discuss the taxonomic status of E. glaber, E. filamentosus, and E. filamentosus mongoliensis. As a result, we designate the first two species as junior synonyms of E. testudo but we recognise the validity of the last taxon and we elevate it to the species level as Echiniscus mongoliensis comb. nov. This study, by providing a detailed description of the nominal taxon and the diagnosis for the genus Echiniscus, opens the way for confident descriptions of closely related species and for erecting new genera from within the largest polyphyletic tardigrade genus.