Morphological and phylogenetic investigations shed light on evolutionary relationships of the enigmatic genus Copemetopus (Ciliophora, Alveolata), with the proposal of Copemetopus verae sp. nov.

Copemetopus Villeneuve-Brachon, 1940 is a rare, poorly known sapropelic ciliate genus composed of only two valid nominal species. Over time, Copemetopus was taxonomically assigned to Heterotrichea and Armophorea classes, but its phylogenetic affinities remained unknown. Until the present study, there were no molecular data available for Copemetopus representatives. Here, we present the 18S and 28S-rDNA sequences and the phylogenetic position of Copemetopus verae sp. nov., as well as its detailed morphological description based on live observations, protargol impregnation, and scanning electron microscopy. Transmission electron micrographs of the type species C. subsalsus Villeneuve-Brachon, 1940 reveal new morphological traits and a unique somatic ciliature pattern of Copemetopus, composed by short segments of dikinetids with one or two supplementary kinetosomes. The phylogenetic trees recovered Copemetopus as the sister group of the genus Protocruzia, both constituting early-divergent lineages that split first from a common ancestor of Intramacronucleta. Morphological and molecular evidence suggest that Copemetopus is neither a heterotrichean nor an armophorean ciliate, but a distinct clade related to Protocruzia.