The end of a winding path: The anaerobic ciliate Spirorhynchus is a member of the class Muranotrichea

Ciliated protists (Ciliophora) are important members of freshwater, brackish, marine, and hypersaline benthic microbial communities. As part of our broader studies of anaerobic protists, we encountered a ciliate in hypoxic sediment samples from three geographically distant saline habitats and identified it as Spirorhynchus verrucosus Cunha, 1915. This highly unusual ciliate has a complicated nomenclatural and taxonomic history and has been assigned to the armophorean family Metopidae. The distinctive cell shape and unique arrangement of ectosymbionts allows identification of this species by in vivo observation. Two populations of another ciliate, found in deep-sea sediments, were recognized as a morphologically and genetically distinct but not yet formally described Spirorhynchus species. Muranotrichea Rotterová et al., 2020 is a recently established class of obligately anaerobic marine ciliates that bear prokaryotic ectosymbionts, and includes one family with two genera and three species. Phylogenetic analyses, based on the first 18S rRNA gene sequences from the genus, place Spirorhynchus in Class Muranotrichea. In single-gene trees, Spirorhynchus is monophyletic, branching as the closest relative of a marine environmental sequence and the muranotrichean genus Thigmothrix, with which Spirorhynchus also shares some morphologic similarities. Here, we also provide the first protargol impregnations and scanning electron microscopy images from Spirorhynchus.