Abstract
Oligotrich ciliates are common members of marine microplankton. However, their biodiversity is not well documented. In this study, the morphology and phylogenetic positions of three new oligotrich species, Spirostrombidium apourceolare spec. nov., Spirostrombidium subtropicum spec. nov. and Parallelostrombidium conicum spec. nov., collected from coastal habitats of southern China, were investigated. Spirostrombidium apourceolare is characterized by the girdle kinety which encircles the cell twice as two dextrally oriented whorls with some undulations and by the presence of several macronuclear nodules. Spirostrombidium subtropicum is recognized by the girdle kinety encircling the cell as two dextrally oriented whorls and extrusomes arranged in a stripe along the girdle kinety. Parallelostrombidium conicum differs from its congeners by the obconic body shape and the posterior portion of the girdle kinety extending downwards on the left ventral side to reach the posterior pole. In small-subunit rRNA gene trees, S. subtropicum clusters with Omegastrombidium elegans and Varistrombidium kielum, and Parallelostrombidium conicum is sister to the clade containing Novistrombidium sinicum, Novistrombidium orientale and Parallelostrombidium sp.