A laboratory strain identified as “Hypsibius dujardini” is one of the best studied tardigrade strains: it is widely used as a model organism in a variety of research projects, ranging from developmental and evolutionary biology through physiology and anatomy to astrobiology. Hypsibius dujardini, originally described from the Île-de-France by Doyère in the first half of the 19th century, is now the nominal species for the superfamily Hypsibioidea. The species was traditionally considered cosmopolitan despite the fact that insufficient, old and sometimes contradictory descriptions and records prevented adequate delineations of similar Hypsibius species. As a consequence, H. dujardini appeared to occur globally, from Norway to Samoa. In this paper, we provide the first integrated taxonomic redescription of H. dujardini. In addition to classic imaging by light microscopy and a comprehensive morphometric dataset, we present scanning electron photomicrographs, and DNA sequences for three nuclear markers (18S rRNA, 28S rRNA, ITS-2) and one mitochondrial marker (COI) that are characterised by various mutation rates. The results of our study reveal that a commercially available strain that is maintained in many laboratories throughout the world, and assumed to represent H. dujardini sensu stricto, represents, in fact, a new species: H. exemplaris sp. nov. Redescribing the nominal taxon for Hypsibiidae, we also redefine the family and amend the definitions of the subfamily Hypsibiinae and the genus Hypsibius. Moreover, we transfer H. arcticus (Murray, 1907) and Hypsibius conifer Mihelčič, 1938 to the genus Ramazzottius since the species exhibit claws and eggs of the Ramazzottius type. Finally, we designate H. fuhrmanni as subjectively invalid because the extremely poor description precludes identifying neotype material.

Outdated and inaccurate original descriptions of species nominal for genera or species complexes give birth to notorious difficulties in species identification and often translate into erroneous classifications of multiple species as one. This, in turn, results in severe overestimations of specific geographic ranges and ultimately in underestimations of the diversity of organisms living on our planet. Tardigrades, ubiquitous microscopic metazoans exhibiting relatively few taxonomically useful traits, bear many such problematic species. In this paper, we investigated, morphologically and molecularly, 10 European populations of Ramazzottius cf. oberhaeuseri (Doyère, 1840), a putatively cosmopolitan nominal species for the family Ramazzottiidae. The analysis of COI sequences revealed eight potential species, including one from Paris, the type locality of R. oberhaeuseri, which we used to integratively redescribe the nominal taxon. The identification of seven further species in a handful of samples collected from a single continent explicitly shows the considerable undescribed species diversity within this group and puts in question the cosmopolitan distribution of R. oberhaeuseri. The integrative redescription of R. oberhaeuseri opens the opportunity for descriptions of other species in the complex. http://zoobank.org/urn:lsid:zoobank.org:pub:50B99B53-C317-4DBD-8CF1-16297B85C4F3

Given the constantly growing number of described tardigrade species and the scarcity of taxonomically meaningful morphological characters that these m

Isohypsibioidea are most likely the most basally branching evolutionary lineage of eutardigrades. Despite being second largest eutardigrade order, phylogenetic relationships and systematics within this group remain largely unresolved. Broad taxon sampling, especially within one of the most speciose tardigrade genera, Isohypsibius Thulin, 1928, and application of both comparative morphological methods (light contrast and scanning electron microscopy imaging of external morphology and buccal apparatuses) and phylogenetic framework (18S + 28S rRNA sequences) resulted in the most comprehensive study devoted to this order so far. Two new families are erected from the currently recognised family Isohypsibiidae: Doryphoribiidae fam. nov., comprising all aquatic isohypsibioids and some terrestrial isohypsibioid taxa equipped with the ventral lamina; and Halobiotidae fam. nov., secondarily marine eutardigrades with unique adaptations to sea environment. We also split Isohypsibius into four genera to accommodate phylogenetic, morphological and ecological variation within the genus: terrestrial Isohypsibius s.s. (Isohypsibiidae), with smooth or sculptured cuticle but without gibbosities; terrestrial Dianea gen. nov. (Isohypsibiidae), with small and pointy gibbosities; terrestrial Ursulinius gen. nov. (Isohypsibiidae), with large and rounded gibbosities; and aquatic Grevenius gen. nov. (Doryphoribiidae fam. nov.), typically with rough cuticle and claws with branches of very similar heigths. Claw morphology is reviewed and, for the first time, shown to encompass a number of morphotypes that correlate with clades recovered in the molecular analysis. The anatomy of pharynx and cuticle are also shown to be of high value in distinguishing supraspecific taxa in Isohypsibioidea. Taxonomy of all isohypsibioid families and genera is discussed, with special emphasis on the newly erected entities. Finally, a dychotomous diagnostic key to all currently recognised isohypsibioid families and genera is provided.

In this paper we describe a new apochelan species, Milnesium variefidumsp. nov. from Scotland and provide novel morphological and molecular data for Milnesium berladnicorumCiobanu et al., 2014. The new species differs from the most similar M. berladnicorumby the presence of developmental dimorphism in claw configuration, absent or weakly developed cuticular bars under claws I-III, a different arrangement of cuticular pseudoplates, and by differences in the sequences of three nuclear DNA fragments: 18S rRNA (p-distance: 0.6%), 28S rRNA (2.0%), ITS-2 (9.3%), and on mitochondrial gene COI (12.4%). Although ontogenetic claw configuration change was suspected to occur in some Milnesiumspecies, we are the first to document it through the combined use of traditional, molecular and experimental methodologies. We discuss the implications of the observed phenomenon for the taxonomy of the genus and propose a new diagnostic key to all Milnesiumspecies described up to the end of 2015. We also review other traits used for species differentiation in the genus and offer recommendations to improve the quality of future descriptions as well as suggest a need for integrative redescriptions of the known species. Finally, we propose to suppress M. dujiangensisand M. tardigradum trispinosumand suggest that M. alpigenumand M. quadrifidumare valid species that require thorough redescriptions.

Tardigrade taxonomy is based largely on classical methodology, with morphological and morphometric traits being most often the sole basis for species