Morphology and taxonomy of Pseudokeronopsis rubra (Ehrenberg, 1836) and Pseudokeronopsis parasongi sp. nov. (Ciliophora, Hypotrichia, Urostylida) from the Yellow Sea.

Pseudokeronopsis rubra (Ehrenberg, 1836), type species of Pseudokeronopsis, is a widely reported but taxonomically confused species with most available sequences either misidentified or dubious. Based on the Yellow Sea population, of which the molecular data have been analyzed in a previous work, we redescribe P. rubra and try to clarify the identifications of the populations designated as P. rubra or those allocated to this species. Nonetheless, there is still a need of integrated investigation of the species combing both morphological and molecular data based on specimens from its type locality. Recent molecular analyses indicate a Yellow Sea population of Pseudokeronopsis (marked as P. cf. songi) likely represents a new species. Here we describe it as P. parasongi sp. nov. based on the specimens from the same population. The new species possesses a yellow-brownish body with both pigmented cortical granules and colorless blood cell-shaped structures underneath the body surface, invariably one contractile vacuole at 66% of body length and occasionally a second one at 25%, and the midventral complex terminating 2-23 μm ahead of 2-4 transverse cirri. It differs from closely related species by its yellow-brownish body color and separates from them in the ITS1-5.8S-ITS2 and COI genetic distances.