Two freshwater Hypotrichidium species (Ciliophora, Hypotrichia) from China, including H. flosaquae n. sp., with notes on undulating membrane patterns

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.