Morphological, physiological, and molecular evidences suggest that Euplotes parawoodruffi is a junior synonym of Euplotes woodruffi (Ciliophora, Euplotida).

Euplotes woodruffi Gaw 1939 is a striking ciliate species commonly used in many fields of protistology. Euplotes parawoodruffi Song and Bradbury 1997 is separated from E. woodruffi by several morphological differences (e.g. the absence of pre-oral pouch) and in particular the brackish habitat (vs. freshwater). However, recent molecular data dispute the separation of E. woodruffi into two species. Based on a brackish isolate from the southeast of China, we investigated the morphology, molecular phylogeny, and adaptation of E. woodruffi using living observation, protargol impregnation, gene sequencing, and cultivation experiments. The morphological variability present in our isolate encompasses the previous descriptions of E. woodruffi and E. parawoodruffi. The pre-oral pouch, which occurs in living cells of freshwater isolates, is also recognizable in the brackish specimens. Laboratory experiments suggested that the brackish isolate could acclimatize and reproduce in both brackish and fresh water. The molecular analyses using 13 available small subunit (SSU) rRNA gene sequences showed that the minimum pairwise sequence similarity is as high as 99.2% and nine sequences from both brackish and freshwater isolates are even 100% matched. Therefore, we conclude that E. parawoodruffi is a junior synonym of E. woodruffi.