Higher-level molecular phylogeny of the water mites (Acariformes: Prostigmata: Parasitengonina: Hydrachnidiae)

- Monophyletic Hydrachnidiae, including Hydrovolzioidea, is strongly supported.
- Terrestrial Parasitengonina do not form a monophyletic sister group to water mites.
- Stygothrombidioidea is close to water mites but is not nested within this clade.
- All superfamilies, except paraphyletic Hydryphantoidea, are strongly supported.
- Protohydrachnidia, Euhydrachnidia, and Neohydrachnidia are strongly supported.

With nearly 6000 named species, water mites (Hydrachnidiae) represent the largest group of arachnids to have invaded and extensively diversified in freshwater habitats. Water mites together with three other lineages (the terrestrial Erythraiae and Trombidiae, and aquatic Stygothrombiae), make up the hyporder Parasitengonina, which is characterized by having parasitic larvae and predatory nymphs and adults. Relationships between the Hydrachnidiae and other members of the Parasitengonina are unclear, as are relationships among the major lineages of water mites. Monophyly of water mites has been asserted, with the possible exception of the morphologically distinctive Hydrovolzioidea.Here we infer the phylogeny of water mites using multiple molecular markers and including representatives of all superfamilies of Hydrachnidiae and of almost all other Parasitengonina. Our results support a monophyletic Parasitengonina including Trombidiae, Stygothrombiae, and Hydrachnidiae. A monophyletic Hydrachnidiae, including Hydrovolzioidea, is strongly supported. Terrestrial Parasitengonina do not form a monophyletic sister group to water mites. Stygothrombiae is close to water mites but is not nested within this clade. Water mites appear to be derived from ancestors close to Stygothrombiae or the erythraoid group Calyptostomatoidea; however, this relationship is not clear because of extremely short branches in this part of the parasitengonine tree. We recovered with strong support all commonly accepted water mite superfamilies except for Hydryphantoidea, which is clearly paraphyletic. Our data support the previously proposed clades Protohydrachnidia (Hydrovolzioidea and Eylaoidea), Euhydrachnidia (all remaining superfamilies), and the euhydrachnid subclade Neohydrachnidia (Lebertioidea, Hydrachnoidea, Hygrobatoidea, and Arrenuroidea). We found that larval leg structure and locomotory behavior are strongly congruent with the molecular phylogeny. Other morphological and behavioral characters, including host choice, are not as strongly correlated with phylogeny. Molecular dating suggests that the Hydrachnidiae arose about 235 MYA, and that Neohydrachnidia began to diversify about 155 MYA. Our results provide a strong framework for classification and for further elaboration at finer taxonomic scales, which will allow testing of ecological and behavioral hypotheses associated with the transition from terrestrial to aquatic life.

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