Systematic reassessment of Ambiphrya and Apiosoma (Ciliophora, Peritrichia), with morpho-molecular characterization of two epibiotic ciliates from rainbow trout in Korea

Abstract

Many peritrich ciliates are parasites of, or epibionts on, aquatic animals. Among them, the genera Ambiphrya and Apiosoma are common fish ectocommensals in aquacultures. Here, we report infections by two poorly known forms, Ambiphrya cf. ameiuri and Apiosoma pseudopiscicola, on rainbow trout, a commonly farmed and economically important food fish in South Korea, and provide detailed descriptions and diagnostic illustrations of both species. Their SSU rDNA sequences were obtained, filling the molecular data gap for Apiosoma, a genus for which no sequences had previously been available. Our phylogenetic analysis supports the exclusion of Ambiphrya from the family Scyphidiidae, although its familial placement remains unresolved. The valid species of Ambiphrya are clarified, and populations identified as Ambiphrya ameiuri appear to represent a species complex. The family rank of Apiosomatidae, proposed by Jankowski (2007) but not widely adopted, is validated. Our findings confirm that Apiosoma belongs to Apiosomatidae rather than to the family Epistylididae in the widely used classifications of Corliss (1979) and Lynn (2008). Parapiosoma, previously of uncertain familial placement, should likewise be assigned to Apiosomatidae. Notably, the proposed synonymization of Scyphidiidae with Apiosomatidae, as well as the inclusion of other genera in Apiosomatidae by Jankowski (2007), is unsupported.