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

Nevertheless, the Wuhan popula tion 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 characteris tics 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 morphologi cal 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.