Updating the phylogeny and taxonomy of pleurostomatid ciliates (Protista: Ciliophora) with establishment of a new family, a new genus and two new species

The order Pleurostomatida is one of the most confusing groups in the ciliate class Litostomatea. In the present study, two new species, Paralitonotus foissneri gen. & sp. nov. and Loxophyllum apochlorelligerum sp. nov., were investigated using an integrative approach. Paralitonotus foissneri differs from other pleurostomatids by its distinctive ciliary pattern (i.e. the anterior ends of the right somatic kineties are progressively shortened from the middle towards dorsal and perioral ones). Therefore, we place it in the new genus Paralitonotus. Loxophyllum apochlorelligerum is characterized by seven to nine left and 12–14 right somatic kineties, three to five anterior dorsal warts and a subterminal contractile vacuole. Phylogenetic analyses show that Pa. foissneri clusters with Apolitonotus lynni and Pseudolitonotus gracilis to form a deep-branching subclade that is basal to other core pleurostomatids. All three genera have distinctive anteriorly shortened right kineties, suggesting that this might be a familial character state. We therefore establish a new family, Paralitonotidae fam. nov., for pleurostomatids with this character and propose a new taxonomic scheme and a hypothetical evolutionary trajectory of the order Pleurostomatida. In addition, the generic diagnostic characters for pleurostomatids are updated, and a key to the identification of pleurostomatid genera is provided.