Molecular phylogeny of Arthrotardigrada (Tardigrada)

Tardigrades are microscopic ecdysozoans with a worldwide distribution covering marine, limnic and terrestrial habitats. They are regarded as a neglected phylum with regard to studies of their phylogeny. During the last decade molecular data have been included in the investigation of tardigrades. However, the marine arthrotardigrades are still poorly sampled due to their relative rarity, difficult identification and minute size even for tardigrades. In the present study, we have sampled various arthrotardigrades and sequenced the 18S and partial 28S ribosomal subunits. The phylogenetic analyses based on Bayesian inference and maximum parsimony inferred Heterotardigrada (Arthrotardigrada + Echiniscoidea) and Eutardigrada to be monophyletic. Arthrotardigrada was inferred to be paraphyletic as the monophyletic Echiniscoidea is included within the arthrotardigrades. The phylogenetic positions of Stygarctidae and Batillipedidae are poorly resolved with low branch support. The Halechiniscidae is inferred to be polyphyletic as the currently recognized Styraconyxinae is not part of the family. Archechiniscus is the sister-group to the Halechiniscidae and Orzeliscus is placed as one of the basal halechiniscids. The phylogeny of the included eutardigrade taxa resembles the current molecular phylogenies. The genetic diversity within Arthrotardigrada is much larger (18S 15.1–26.5%, 28S 7.2–20.7%) than within Eutardigrada (18S 1.0–12.6%, 28S 1.3–8.2%). This can be explained by higher substitution rates in the arthrotardigrades or by a much younger evolutionary age of the sampled eutardigrades.