Contribution to the genus Planometopus (Armophorea, Metopidae), with the description of a new species and the type species.

The family Metopidae Kahl, 1927 comprises diverse anaerobic ciliates, prompting several taxonomic re-evaluations. Among its constituent genera, Planometopus Rotterová et al., 2018 was established as a monotypic genus based on the type species P. contractus (Penard, 1922) Rotterová et al., 2018, and its diagnosis was derived solely from that species. This study presents an integrative revision of this genus, including a new species, Planometopus striatulus n. sp. This new species is characterized by a body size of 100-130 × 50-70 μm in vivo, conspicuous cortical ribs, obliquely arranged somatic kineties comprising 15-22 rows of densely packed dikinetids, an adoral zone with 29-37 membranelles, and an almost entire portion of the perizonal stripe segmented into 71-86 false kineties. The Korean population of P. contractus exhibits a diplostichomonad-type paroral membrane, a feature that has not been described previously. Based on the new species and a comparative reassessment of P. contractus, our results suggest an improved diagnosis of the genus. Body size and the arrangement of somatic dikinetids demonstrate interspecific variability. Furthermore, molecular phylogeny based on 18S rRNA gene sequences robustly confirms the monophyly of Planometopus and supports the distinctiveness of P. striatulus n. sp.