The systematics of the anaerobic family Sonderiidae (Alveolata: Ciliophora), with the establishment of one new genus and five new combinations, and the description of two new species

Abstract The class Plagiopylea represents a distinct evolutionary lineage of obligate anaerobes, yet its diversity, particularly within the family Sonderiidae Small and Lynn, 1985, remains poorly understood due to limited morphological and molecular data. We here describe three sonderiid ciliates from marine coastal habitats in China, including two new species, Heterosonderia faurei gen. et sp. nov. and Sonderia paravorax sp. nov., and the little-known Heterosonderia macrochilus (Kahl, 1931) gen. et comb. nov. A novel, fundamentally distinct oral morphology is revealed within Sonderiidae, which justifies the establishment of a new genus, Heterosonderia gen. nov., to accommodate species with a belt-shaped, oral opening and a buccal cavity extending posteriorly on both sides. In addition, four other new combinations are established, namely H. schizostoma (Kahl, 1931) gen. et comb. nov., H. labiata (Fauré-Fremiet and Tuffrau, 1955) gen. et comb. nov., H. paramacrochilus (Li et al., 2022) gen. et comb. nov., and H. steini (Li et al., 2022) gen. et comb. nov. Phylogenetic analyses based on SSU rRNA gene sequences support the monophyly of Heterosonderia and its separation from Sonderia.