Molecular data suggests the ciliate Mesodinium(Protista: Ciliophora) might represent an undescribed taxon at class level

The well-known ciliate, Mesodinium Stein, 1863, is of great importance to marine microbial food webs and is related to the ＂red tides＂. However, it is possibly one of the most confusing ciliate taxa in terms of its systematic position： either the morphological or the molecular data excluded it from all the other known assemblages or groups. In the current work, the sequences of small subunit ribosomal RNA （SSU rRNA） genes for all isolates available are analysed and an examination of the secondary structure patterns of related groups is carried out. The results indicate that （1） Mesodiniurn invariably represents a completely separated and isolated clade positioned between two subphyla of ciliates with very deep branching, which indicates that they should be a primitive or ancestral group for the subphylum Intramacronucleata; （2） the secondary structure of the SSU rRNA of Mesodinium species is unusual in that, while the secondary structure of V4 in Mesodinium sp. has the deletions common to all litostome ciliates, it has more extensive deletions in helix E23_8 and a longer helix E23_1; （3） combining the phylogenetic and morphological information, we suggest establishing Mesodiniea el. nov., including the order Mesodiniida Grain, 1994, belonging to the subphylum Intramacronucleata.