Redescription and molecular phylogeny of the type species for two main metopid genera, Metopus es (Müller, 1776) Lauterborn, 1916 and Brachonella contorta (Levander, 1894) Jankowski, 1964 (Metopida, Ciliophora), based on broad geographic sampling.

Metopid ciliates occupy terrestrial, freshwater, and marine habitats worldwide, playing important roles as predominant consumers of bacteria, flagellates, algae, and diatoms in hypoxic environments. Metopus and Brachonella are the most species-rich metopid genera, however most of their species have not been studied by modern methods Here, we report the morphologic, morphometric and molecular characterization, and phylogeny of Metopus es and Brachonella contorta, both types of their respective genera, collected in a broad global sampling effort. Five strains of M. es and three strains of B. contorta were studied in detail, providing the first correlation of morphology, morphometrics, and 18S rRNA gene sequencing for both. We submitted 29 new 18S rRNA gene sequences to GenBank. Phylogenetic analyses yielded trees of similar topology. A strongly supported Metopus es clade is sister to the Brachonella contorta clade. Our analysis shows genus Metopus is not monophyletic. The monophyly of Brachonella cannot yet be determined due to lack of sequences for other species of this genus in molecular databases. Both species appear to have a global distribution. Metopus es was not found in Africa, probably reflecting low sampling effort. Strains of both species showed low 18S rRNA gene sequence divergence despite wide geographic separation.