A new contribution to the taxonomy and molecular phylogeny of three, well-known freshwater species of the ciliate genus Spirostomum (Protozoa: Ciliophora: Heterotrichea)

Members of the heterotrich genus Spirostomum are commonly found in freshwater or low salinity biotopes. In the present study, three species (S. minus, S. subtilis and S. teres) collected from freshwater habitats in Qingdao, China, are investigated using morphological and molecular methods. Detailed morphometric data are documented and improved diagnosis are supplied based on a combination of previous and present studies. In addition, small subunit ribosomal DNA (SSU rDNA) sequences are obtained from the clonal cultures. Phylogenetic analyses show that all three species are placed in the Spirostomum clade. However, isolates of the morphospecies S. minus are divided into two paraphyletic clades, while 'populations' of the nominal species, S. teres, are placed in at least four separate groups in the tree. After comparing morphological and molecular differences in closely related forms available, we hypothesized that S. minus and S. teres might represent species complexes. A key to the identification of the ten valid species of Spirostomum is also supplied.