Taxonomy and molecular phylogeny of pleurostomatid ciliates from China with a description of two new species.

Ciliates in the order Pleurostomatida are found free-living in many habitats including within biofilms, but some (e.g. Pseudoamphileptus spp.) are ectocommensal on various hosts. Due to issues involving overall undersampling, the exact diversity and molecular phylogeny of this group remain largely underexplored. To combat this deficiency, detailed investigations were undertaken in northern China. As a result of these studies, we provide the morphological descriptions of two new species. Pseudoamphileptus apomacrostoma sp. nov., a new ectocommensal species, is characterized by the broadly oval cell shape, numerous scattered contractile vacuoles, and unique densely bounded extrusomes; Amphileptus qingdaoensis sp. nov., a marine form, is characterized by possessing oblong extrusomes with a conical anterior end, a single contractile vacuole and 5-7 left and 18-23 right kineties. In addition, a new population of Amphileptus orientalis Zhang et al., 2022, a freshwater representative, was documented and an improved diagnosis is provided. The phylogenetic analyses based on the SSU rDNA sequences imply that the genus Pseudoamphileptus is monophyletic whereas the genus Amphileptus is paraphyletic. The new molecular sequences presented here further support the establishment of two new species.