Morphological and molecular phylogenetic study of two oxytrichid ciliates from the middle and lower reaches of the Yangtze River

In the present study, we investigated two hypotrichous ciliates, Architricha indica Gupta et al., 2006, collected from Donghu Lake in Wuhan, and Crassohymena primicirrata (Berger & Foissner, 1987) Omar et al., 2025, obtained from an aquaculture pond in Honghu, Jingzhou. Morphological analyses based on live observation and protargol staining were combined with small-subunit ribosomal DNA (SSU rDNA) sequencing and phylogenetic reconstruction. Our results show that C. primicirrata is recorded for the first time in China, whereas A. indica represents a new record for central China. The Wuhan population of A. indica was consistent with the Indian and Shanghai populations in living morphology, cortical granules, frontal-ventral-transverse cirral pattern, number of marginal cirri (three right and two left rows), and the dorsal ciliature (four dorsal and two dorsomarginal rows). Nevertheless, the Wuhan population exhibits a broader morphometric range in the number of postoral ventral cirri (1—5 vs. 3) and transverse cirri (4—6 vs. 5), with larger body size (150—180) μm× (50—80) μm vs. 140 μm in the Indian population and (100—140) μm× (30—40) μm in the Shanghai population. Despite these differences, the overlapping diagnostic characteristics support their interpretation as intraspecific variation. The Honghu population of C. primicirrata closely resembled the Austrian population in features such as the length of the fourth dorsal kinety and the number of micronuclei, but differs in certain respects from the Korean populations, thereby enriching the knowledge of the morphological diversity across different geographic regions. Molecular phylogenetic analyses further support the species identifications. The SSU rDNA sequences of the Wuhan and Shanghai populations of A. indica show 99.76% similarity, differing by only four nucleotides, and consistently cluster together in the phylogenetic trees. The Honghu and Korean populations share identical sequences and consistently form a well-supported clade in the phylogenetic tree. By integrating morphological redescriptions with molecular data, this study contributes to the taxonomy of the family Oxytrichidae, and provides new insights into the biodiversity and biogeography of ciliates in the middle and lower reaches of the Yangtze River.