Morphologic and phylogenetic studies of two hypotrichous ciliates, with notes on morphogenesis in Gastrostyla steinii Engelmann, 1862 (Ciliophora, Hypotrichia)

Two oxytrichid ciliates, Stylonychia (Metastylonychia) nodulinucleata Shi and Li, 1993 and Gastrostyla steinii Engelmann, 1862; collected from Huguangyan Maar Lake and Shenzhen Mangrove Nature Protection Area, southern China, respectively, were investigated using standard methods. The uncommon species Stylonychia (Metastylonychia) nodulinucleata can be recognized by its large body size, the conspicuous caudal cirri in vivo, and a moniliform macronucleus. We provide the first record of the small subunit ribosomal RNA (SSU rRNA) gene sequence for the species. The division of Stylonychia into two subgenera based on morphological and morphogenetic data is supported by the phylogenetic analyses, in which Stylonychia (Metastylonychia) nodulinucleata is placed as a sister branch to the clade including all the sequences of Stylonychia (Stylonychia) mytilus complex. The first brackish water population of Gastrostyla steinii is described in detail, with emphasis on its morphogenesis, which corresponds well with previous populations. In the phylogenetic trees, all the available Gastrostyla sequences, except for Gastrostyla sp. Y2 (KT780432) (probably a misidentification), nest together in the big group of the subfamily Stylonychinae with moderate to high support (ML/BI, 91%/0.95), very likely revealing the monophyly of the genus Gastrostyla.