Phylogeny of the anaerobic ciliate genus Sonderia (Protista: Ciliophora: Plagiopylea), including the description of three novel species and a brief revision of the genus

Anaerobic protists in general, and ciliates in particular, are important components of anoxic or hypoxic environments, however, their diversity remains underestimated. Sonderia is a poorly studied genus that is distributed worldwide and is commonly found in anaerobic environments. In the present study, the taxonomy and phylogeny of three new species, namely Sonderia aposinuata sp. nov., Sonderia paramacrochilus sp. nov. and Sonderia steini sp. nov., collected from China, were investigated based on microscopic observations and SSU rRNA gene sequencing methods. Sonderia aposinuata sp. nov. is diagnosed mainly by having a relatively large body size, a crescent-shaped oral opening, numerous slender extrusomes, one suture on the ventral side and two on the dorsal side, and a buccal cavity that occupies the anterior third of the cell. Sonderia paramacrochilus sp. nov. closely resembles S. macrochilus but differs mainly by its oral opening being located closer to the anterior cell margin and its spindle-shaped extrusomes. Sonderia steini sp. nov. is a freshwater species that can be recognized by its shallow buccal cavity, sparsely distributed rod-shaped extrusomes, and having 68–79 monokinetidal somatic kineties that form sutures on both sides of the body. Phylogenetic analyses based on small subunit ribosomal RNA (SSU rRNA) gene sequence data support the monophyly of the family Sonderiidae, however, Sonderia is paraphyletic. The genus Sonderia is briefly revised and a key to the identification of species belonging to this genus is supplied.