Morphology and molecular phylogeny of two freshwater ciliates, with notes on a new species and redescription of a known species (Ciliophora, Oligohymenophorea)

The class Oligohymenophorea comprises a diverse array of ciliates, however, the diversity and phylogeny of freshwater taxa, particularly from inland waters, remain inadequately explored. Therefore, we investigated two freshwater ciliates from Harbin, China, one of which was identified as a new species, Cyclidium orientale n. sp. The new species is characterized by a small body size (20–25 × 10–12 μm in vivo), a buccal field occupying approximately 80% of the body length, 10 somatic kineties, and a scutica composed of eight kinetosomes arranged in two groups. A redescription of the known species Lembadion bullinum is also provided, along with notes on its novel epibiotic lifestyle on the fish Pelteobagrus fulvidraco. Phylogenetic analyses based on SSU rDNA sequences revealed that Cyclidium orientale clusters with Cyclidium marinum, while L. bullinum forms a fully supported clade with its congeners, thereby corroborating their morphological identification.