The morphology, morphogenesis, and molecular phylogeny of a new hypotrichous ciliate, Sterkiella zhangi n. sp., which originated from terrestrial moss collected in east China, were studied using live observation, protargol staining, and the macronuclear SSU rRNA and mitochondrial cytochrome c oxidase subunit 1 gene sequences. The new species can be distinguished from its congeners by the following characteristics: three ordinary frontal cirri (an extra small frontal cirrus present in half of the individuals examined), an average of 15 left and 16 right marginal cirri, five transverse cirri, two macronuclear nodules, and cyst surface with irregular wrinkles but without spines. The 1:3:3:3:4:4 frontal-ventral-transverse anlagen fragmentation pattern of S. zhangi n. sp. indicates that the extra frontal cirrus is very likely a retained parental structure. Phylogenetic analyses revealed that the new species branches early in the subfamily Stylonychinae. The comparison of mitochondrial cytochrome c oxidase subunit 1 gene sequences of Sterkiella species shows that the intra-specific variation is much smaller than the inter-specific divergence. The well-known model organism Oxytricha trifallax has accumulated significant evolutionary differences and should be treated as a distinct species rather than a synonym of S. histriomuscorum.
