The enigmatic genus Malacophrys Kahl, 1926 (Ciliophora: Intramacronucleata) belongs to the class Oligohymenophorea

The ciliate genus Malacophrys has been incertae sedis for more than 50 years in what is now subphylum Intramacronucleata, provisionally assigned to three different classes by various authors. Of the three species included in the genus, M. sphagni and the type species, Malacophrys rotans, have not been studied by modern methods and M. viridis, although morphologically well-described, lacks molecular characterization. We identified two freshwater ciliates as morphologically consistent with two members of Malacophrys, namely M. rotans and M. viridis. We studied one population of M. rotans, using in vivo observation and silver carbonate and silver nitrate impregnation, and two populations of M. viridis, using in vivo observation, silver carbonate impregnation, and 18S rRNA gene sequencing. Phylogenetic analyses strongly support the position of M. viridis in the class Oligohymenophorea. Morphologic features suggest Malacophrys rotans belongs to order Tetrahymenida, but a confident phylogenetic assignment awaits its molecular sequencing. Differences between their oral structures and silverline patterns indicate that M. rotans and M. viridis likely belong to different genera. A formal separation at the genus level awaits molecular characterization of the type species, M. rotans.