First record of Chilodonella piscicola (Ciliophora: Chilodonellidae) from two endangered fishes, Schizothorax o'connori and Oxygymnocypris stewartii in Tibet.

Schizothorax o'connori and Oxygymnocypris stewartii are two endangered endemic Tibetan fishes that thrive in the Lhasa River at an average altitude over 4000 m. During artificial reproduction of endemic Tibetan fishes, the juvenile fish of S. o'connori and O. stewartii experienced mass mortality event. The causative agent is diagnosed to be a ciliate parasite, Chilodonella piscicola (syn. C. cyprini), which is common in various fishes. Here, we supplemented its description based on the morphological and molecular data. The body of C. piscicola is oval, 30-60 × 25-40 μm in vivo. Cyrtos is hook-like, composed of 9-10 toothed nematodesmal rods. Somatic kineties usually contain seven right kineties and nine left kineties. Two parallel circumoral kineties revolve round the cyrtos, and one preoral kinety extends to the anterior end of the fourth left-most kinety. Terminal fragment kinety is linear and on the top left of dorsal side. Sequence alignments revealed that the present SSU rDNA and ITS1-5.8S-ITS2 sequences are both most similar to the sequences of C. uncinata with the similarities of 98.2 and 99.5%. The phylogenetic analyses showed that C. piscicola is sister to other Chilodonella species, whereas C. cyprini (FJ873805) cluster with Tetrahymena species. Molecular analysis shows that the ITS1-5.8S-ITS2 sequence of C. cyprini in GenBank is unreliable. Our study extended the host range of C. piscicola and supplemented and revised the molecular data. Besides, as far as we know, this is the first record of C. piscicola in Tibetan plateau.