Abstract

Two freshwater hypotrichous ciliates from China, a new species, Hypotrichidium flosaquae n. sp., and, as a first Chinese record, H . bichromum Omar et al., 2025, were studied based on live observations, protargol-stained preparations, and molecular analyses. The new species is characterized by: (1) a pyriform body with an inconspicuous tail-like tip; (2) a question mark-shaped adoral zone occupying about 64% of the body length; (3) undulating membranes in an Australocirrus -like pattern; (4) biconcave, light yellowish-green, sparsely distributed cortical granules; and (5) a cortex densely covered with rod-shaped ectosymbiotic bacteria. Morphogenesis during binary fission generally follows the generic pattern. The Chinese population of H . bichromum aligns with the type population from Korea. However, it displays only pale red cortical granules, probably due to a brief cultivation period, and also shows rod-shaped ectosymbionts. Phylogenetic analyses based on 18S rDNA sequences confirm the monophyly of Hypotrichidium and support the diagnostic value of undulating membrane patterns for species delimitation, recovering two clades: one (including H . paraconicum and H . flosaquae n. sp.) with an Australocirrus -like pattern, and the other (including H . tisiae and H . bichromum ) with a Cyrtohymena -like pattern.