Taxonomy and phylogeny of two planktonic hypotrichs, with establishment of a new family Pelagotrichidiidae (Protista: Ciliophora: Hypotrichia)

Hypotrich ciliates are a highly diverse, predominantly benthic group, with a few planktonic representatives identified. This study examines two planktonic hypotrich species from China: Pelagotrichidium faurei (Tuffrau, 1972) Jankowski, 1978, and Psilotrichides hawaiiensis Heber et al., 2018, using live observation, protargol impregnation, and SSU ribosomal DNA sequencing. The Chinese population of Ps. hawaiiensis closely resembles previous reports but has more ventral cirri. This is the first detailed account of the morphology, ontogenesis, and SSU ribosomal DNA sequence of Pe. faurei. Key ontogenetic features of Pe. faurei include: (i) the proter retains the parental adoral zone, while the opisthe develops a new oral primordium; (ii) five frontoventral anlagen form five cirral rows per daughter cell; (iii) marginal cirral rows and dorsal kineties develop intrakinetally, while dorsomarginal kineties develop de novo; and (iv) macronuclear nodules fuse into a single mass before division. Morphological, ontogenetic, and phylogenetic differences distinguish Pe. faurei from the spirofilid type genus Hypotrichidium Ilowaisky, 1921. Phylogenetic analyses place Pe. faurei within the strongylidiid clade, separate from the spirofilids. Based on these findings, we propose a new family, Pelagotrichidiidae fam. nov., to accommodate the genus Pelagotrichidium Jankowski, 1978.