Morphological redescription and molecular phylogeny of a little-known hypotrichous ciliate, Cyrtohymena torrenticola (Šrámek-Hušek, 1957) Foissner, 1989 (Ciliophora, Oxytrichidae) from Baiyangdian Lake, China

The present study provides a detailed redescription of the very little-known oxytrichid ciliate, Cyrtohymena torrenticola, collected from Baiyangdian Lake in northern China. The morphology of the Chinese population agrees very well with the original description, including a slender, Urosoma-like body outline, a cyrtohymenid oral apparatus, and terminally arranged transverse cirri. In addition, our study shows that C. torrenticola has (i) six dorsal kineties and (ii) yellowish, about 1 µm-sized cortical granules. An improved diagnosis is provided. From four dividers we can conclude (i) that the oral primordium originates de novo, (ii) that all postoral ventral cirri are involved in primordia formation, (iii) that the anlagen V and VI of both proter and opisthe, very likely originate via primary primordia, and (iv) that the dorsal ciliature is formed in the Oxytricha pattern, that is, kinety 3 fragments to form kinety 4 and two dorsomarginal kineties originate from/near the right marginal row anlagen. A phylogeny using SSU rDNA sequence data basically supports the morphological classification in Cyrtohymena. Both the type locality and our record strongly indicate that C. torrenticola prefers limnetic habitats. Our population cannot be used for neotypification because it was collected far away from the type locality.