New insights and investigation: Morphomolecular notes on the infraciliature, taxonomy, and systematics of pleurostomatid ciliates (Protozoa, Ciliophora), with establishment of a new suborder, two new genera, and three new species.

Pleurostomatid ciliates, as a highly diverse and widely distributed unicellular eukaryote group, play a crucial role in the cycling of nutrients and energy in microbial food webs. However, research on pleurostomatids remains insufficient, resulting in a paucity of molecular information and substantial gaps in knowledge of their phylogenetic relationships. In recent years, we investigated pleurostomatid diversity in various Chinese habitats, including their systematic relationships and taxonomic circumscriptions, which were comprehensively analyzed using an integrative morphomolecular approach. Results revealed that: (1) pleurostomatids can be categorized into two suborders, Protolitonotina and Amphileptina Jankowski, 1967; (2) Protolitonotina represents the ancestral pleurostomatid group and includes two genera, and , characterized by right kineties progressively shortened along rightmost full kineties and absence of a left dorsolateral kinety; (3) represents an orphan lineage and is defined by an oral slit extending to its dorsal margin; (4) " " does not group with congeners but instead represents an orphan lineage, thus is established to which is transferred as ; (5) three new species, , , and , are described; and (6) helices 21es6a to 21es6d within the V4 region of small subunit ribosomal RNA (SSU rRNA) may serve as a useful tool for discriminating pleurostomatids. The evolutionary relationships among all main lineages of pleurostomatids are discussed and a key to the identification of pleurostomatid genera is provided.