Phenotypically exceptional Echiniscus species (Heterotardigrada: Echiniscidae) from the Neotropics (Argentina)

Abstract One of the most extensive species radiations in the tardigrade evolution took place in the Echiniscus phylogenetic line, which contains the genus Echiniscus with over 120 species described to date. The internal classification of Echiniscus is historically based on two major sources of variability: chaetotaxy, that is the arrangement and the shape of trunk appendages, and the sculpturing of dorsal plates. These two criteria allowed for the delineation of species complexes, such as the blumi-canadensis or the spinulosus group. Recent phylogenetic analyses confirmed that they represent monophyletic lineages. However, the affinities of some species that have not been sequenced are not straightforward given their uncommon morphologies. In this paper, we present five Echiniscus species found in Argentina, two of which constitute such exemplary cases. Echiniscus aonikenk sp. nov. exhibits extraordinarily elongated cirri A, exceeding 50% of the body length, and dorsoventrally flattened trunk spines, including particularly wide spines E. The species is a probable sister group of the spinulosus complex. Furthermore, Echiniscus evelinae de Barros, 1942, reported here only third time ever, is characterised by an extreme elongation of cirri E, often exceeding the body length, and by autapomorphic dorsal sculpturing comprising epicuticular granules, pseudopores, distinct endocuticular reticulum and pillars. Analogously to E. aonikenk sp. nov., E. evelinae apparently constitutes a separate evolutionary lineage. Finally, Echiniscus pellucidus sp. nov. (the merokensis complex), Echiniscus peruvianus Binda & Pilato, 1994 syn. nov. (the testudo complex) and Echiniscus manuelae da Cunha & do Nascimento Ribeiro, 1962 (the spinulosus complex) are analysed. Morphological peculiarities revealed in this study are discussed in the light of the Echiniscus taxonomy.