Taxonomy and molecular phylogeny of two poorly known ciliate genera, Balantidion and Acropisthium (Protista: Ciliophora: Litostomatea), including a new species of Balantidion

The class Litostomatea Small & Lynn, 1981 is a morphologically diverse ciliate group including hundreds of free-living and endocommensal species. The genera Acropisthium Perty, 1852 and Balantidion Eberhard, 1862 previously consisted of one free-living freshwater species each. Here, we not only highlight additional morphological features of the two type species, but also investigate a new species, Balantidion foissneri sp. nov., isolated from a river flowing through Lake Weishan, China, based on complementary methods, i.e., living morphology, stained preparations, and 18S rRNA gene sequence data. Balantidion foissneri sp. nov. can be distinguished from the type species, B. pellucidum Eberhard, 1862, by the body size (115–170 × 50–80 μm vs. 70–100 × 25–45 μm in B. pellucidum), oral bulge (distinct vs. indistinct), extrusome shape (filiform vs. rod-shaped), and the number of somatic kineties (46–60 vs. 25–40). In Balantidion species, pre-encystment trophonts show similarly-shaped polymorphic cytoplasmic lepidosomes destined to adorn the outer surface of the resting cyst. Based on the current knowledge, assignment of Balantidion to the family Acropisthiidae Foissner & Foissner, 1988 is proposed. In addition, phylogenetic analyses based on molecular data show that the two Balantidion species form a fully-supported clade to which Acropisthium mutabile has a sister relationship.