Untangling the Echiniscus Gordian knot: Stellariscus gen. nov. (Heterotardigrada: Echiniscidae) from Far East Asia

Species constituting the family Echiniscidaeare highly derived, armouredand inhabit terrestrial habitats, in contrast to other heterotardigrades that are predominantly marine. The genusEchiniscus C.A.S. Schultze, 1840, nominal for the family Echiniscidae, is currently the most speciose tardigrade genus. However, the great morphological variability, in comparison with other heterotardigrade genera, suggests the polyphyletic character of the genus. Here, we analyse new specimens of Echiniscus pseudelegans Semeria, 1994 collected in Japan and conclude that the species as well as two other related taxa, E. elegans Richters, 1907 and E. latifasciatus Dudichev and Biserov, 2000, represent a new genus, Stellariscus, gen. nov. The new genusis characterised by a mixture of peculiar morphological apomorphies: black eyes, star-like dorsal plate sculpturing, no trunk appendages (only cephaliccirri present), two types of ventral plates, and striking sexual dimorphism in both qualitative and quantitative traits. Morphological phylogeny of the family Echiniscidaesuggests a close affinity between Stellariscus, Hypechiniscus Thulin, 1928 and PseudechiniscusThulin, 1911. The polyphyletic status of both Echiniscus and Testechiniscus Kristensen, 1987 is also inferred. The taxonomic significance of ventral armature in echiniscid phylogeny and taxonomy is discussed.