Notes on the life cycle of a new marine ciliate, Metafolliculina songi n. sp. (Protozoa, Ciliophora, Heterotrichea), with description of its infraciliature and phylogeny

With a total of 33 genera, the family Folliculinidae Dons, 1914 represents the most genus-rich lineage within the class Heterotrichea. Its members are characterized by a transparent lorica, conspicuous peristomial lobes, and a dimorphic life cycle, which distinguishes them from other heterotrichs. Although approximately 80 nominal species have been recognized within the family, most aspects of their morphology, life cycle, and molecular information are still unknown. In the present study, a new species, Metafolliculina songi n. sp., collected from a wetland in China, is investigated using morphological and molecular methods. The complete life cycle and morphology of this species are documented, along with detailed ecological information and cultivation protocols. Notably, this folliculinid exhibits unique vulnerabilities: its short, wide neck and lack of a closure mechanism make it susceptible to predation by the ciliate Loxophyllum salinum and infestation by rotifers. Phylogenetic analysis based on small subunit ribosomal DNA (SSU rDNA) sequences confirms the monophyletic nature of the family Folliculinidae. However, M. songi n. sp. unexpectedly formed a sister relationship with Metafolliculina + Eufolliculina, suggesting evolutionary relationships within this family require further investigation through additional data.