Expanding the Family Tropidoatractidae: Characterization of Two New Tropidoatractus Species (Armophorea: Metopida) From South Korea Based on Morphology and Molecular Phylogeny

While oxygen is essential for metabolic processes, many ciliate species thrive in low-oxygen or even oxygen-free environments. Anaerobic ciliates have adapted to a broad range of such habitats, yet have long remained understudied compared to their aerobic counterparts. In recent years, however, interest in these lineages has grown, including in the family Tropidoatractidae Rotterová et al. 2018 (Order Metopida), which currently comprises of two genera, Tropidoatractus Levander, 1894 and Palmarella Jankowski, 1975, and five described species. This report presents two novel freshwater species of Tropidoatractidae, identified through a combination of morphological analysis and 18S rRNA gene phylogeny. Tropidoatractus songi n. sp. is identified by its arcuate body and 11–13 oblique rows of cortical granules between ciliary rows. Tropidoatractus rostrum n. sp. possesses an elongated, ellipsoidal body; a prominent, dark, beak-like anterior end to the preoral dome brim; a macronucleus that is closely associated with rod-shaped endosymbiotic prokaryotes; and sparse, irregular cortical granules. Both species exhibit a diplostichomonad paroral membrane. Phylogenetic analyses of the 18S rRNA gene confirm their affiliation with Tropidoatractus, revealing previously unrecognized species diversity among anaerobic freshwater ciliates.