Armophorean ciliates constitute an important component of microeukaryotic community in anaerobic or hypoxic environments. Yet, their diversity remains poorly known due to under-sampling or the scarcity of knowledge. In this study, three metopid ciliates, i.e., Metopus paraes sp. n., Metopus spiculatus sp. n., and Metopus parapellitus sp. n., collected from coastal sediments in Qingdao and Rizhao, China, were investigated using live observation, protargol staining, and molecular phylogenetic methods. M. paraes sp. n. can be distinguished by its long caudal cilia. M. spiculatus sp. n. resembles M. vestitus in many ways, but differs mainly in having a beak-like preoral dome end and a conspicuous tail. The most remarkable features of M. parapellitus sp. n. include an ovate body shape, caudal cilia located at the rear end and right posterior body, and an adoral zone that never extends onto the dorsal surface. Sequence divergences supported the species identification of these three species. Phylogenetic analyses confirmed that the Metopus is not monophyletic, and first revealed that all marine species of Metopus form a well-supported clade. The clustering of these marine forms with M. es (type species) is not rejected by the AU test, which infers that the marine clade represents the genus Metopus together with M. es.
Anaerobic ciliates are key organisms for studying adaptations to an anaerobic lifestyle, the evolution of eukaryotic cell organelles, mechanisms of energy metabolism, and symbiosis with prokaryotes; however, the biodiversity of their marine representatives remains poorly known. In this study, three species of ciliates belonging to the class Plagiopylea, i.e. Trimyema foissneri sp. nov., Plagiopyla rariseta sp. nov., and Plagiopyla frontata, collected from various marine anoxic habitats from two geographically distant continents, Asia and North America, were investigated using morphological and molecular methods. Trimyema foissneri can be recognized by three complete somatic ciliary girdles, special arrangement of kinetosomes near the anterior end of the oral kineties, and the presence of a ventrolateral fragment. Plagiopyla rariseta is distinguished by sparse cilia on the frontal region and particular orientation of the buccal cavity. Plagiopyla frontata is newly recorded and is redescribed in detail and redefined based on new populations. Phylogenetic analyses based on the small subunit ribosomal ribonucleic acid (SSU rRNA) gene supported the generic assignment of the three species and corroborated the monophyly of the families Trimyemidae and Plagiopylidae. An improved key to the identification of the valid species of genus Trimyema is also supplied.
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