Balantidium duodeni; Phylogeny; 18S rRNA; Transmission electron microscopy

In this work, we present reisolation and redescription of Balantidium duodeni Stein, 1867 from the European common brown frog Rana temporaria Linnaeus, 1758 using light and electron microscopy. This species has a unique morphological feature-its cells are flattened along the dorsoventral axis. Because of its unique morphology and localization (duodenum) in the gastrointestinal tract of the host, it has been proposed to recognize B. duodeni as a member of separate genus, Balantidiopsis Penard, 1922. Molecular phylogenetic analysis demonstrates it to be close to the type species Balantidium entozoon (Ehrenberg, 1838). We argue that its placement into separate genus is not substantiated. We also propose to reinstate the genus Balantioides Alexeieff, 1931 with the type species Paramecium coli (Malmstein, 1857). The recently proposed generic name for this taxon, Neobalantidium Pomajbikova et al., 2013, is a junior synonym of the previously recognized name.