Two new colonial peritrich ciliates (Ciliophora, Peritrichia, Sessilida) from China: With a note on taxonomic distinction between Epistylididae and Operculariidae.

Two new colonial sessilid species, Opercularia miaoxinensis spec. nov. and Epistylis conica spec. nov., were isolated from the pereopods of freshwater crayfish, Procambarus clarkii in Hubei Province, China from 2016 to 2017. Both species were investigated by living observation, protargol impregnation, and molecular methods. Opercularia miaoxinensis spec. nov. is morphologically characterized by the following characteristics: spindle-shaped zooid, zooid size of 48-74×20-35µm in vivo, contractile vacuole ventrally located above macronucleus, and dichotomously branched stalk. Epistylis conica spec. nov. is characterized by conical zooid shape, zooid size of 56-70×22-39µm in vivo, C-shaped macronucleus with transverse orientation, dorsally located contractile vacuole, and dichotomously branched stalk. To further identify both species, phylogenetic trees were constructed based on small subunit ribosomal DNA (SSU rDNA) and ITS1-5.8S-ITS2 sequences. Both results showed that E. conica spec. nov. was part of a clade consisting of the majority of Epistylis species. Surprisingly, O. miaoxinensis spec. nov. also clustered within this large clade of Epistylis species and had a distant relationship with the other Opercularia species. These findings challenged the distinguishing morphological characteristics between Epistylididae and Operculariidae.