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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. [ABSTRACT FROM AUTHOR]

In this paper we describe a new apochelan species, Milnesium variefidum sp. nov. from Scotland and provide novel morphological and molecular data for Milnesium berladnicorum Ciobanu et al., 2014. The new species differs from the most similar M. berladnicorum by 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 Milnesium species, 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 Milnesium species 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. dujiangensis and M. tardigradum trispinosum and suggest that M. alpigenum and M. quadrifidum are valid species that require thorough redescriptions. [ABSTRACT FROM AUTHOR]