Phylogenetic position, validity and diversity of the genus Milnesioides (Eutardigrada: Apochela)

Abstract

Abstract The order Apochela, the only exclusively carnivorous tardigrade lineage, comprises four genera, of which Milnesium contains multiple species and is common and cosmopolitan, whereas the remaining genera (Bergtrollus, Limmenius and Milnesioides) are monotypic, rare and have restricted geographical distributions. More importantly, although there are ample genetic data for numerous Milnesium species, no other apochelan genus has been sequenced so far, which limits our understanding of milnesiid evolution. In this contribution, we analysed two populations of Milnesioides from the south coast of Western Australia. The obtained molecular phylogenetic tree clearly indicated Milnesioides as a sister lineage to all remaining Milnesium species, supporting the validity of the genus. Moreover, the sister relationship of Milnesioides and Milnesium lends support to the Gondwanan origin of the order Apochela. Phenotypic differences between the type population of Milnesioides exsertum from New South Wales and the populations from Western Australia analysed in this study suggest that the latter represent a new species. The new data and morphological differences between populations reported in the original description of Milnesioides strongly suggest that the genus is not monotypic.