Observations on dinoflagellate parasites of aloricate ciliates in Korean coastal waters

Parasites are an understudied but ecologically significant component of marine planktonic food webs. Syndinean dinoflagellates that infect tintinnid ciliates and free-living dinoflagellates cause host mortality that can lead to the decline of blooms and promote species succession. Far less is known about the role of parasitism in aloricate ciliates and other protistan groups. Here, we provide data on parasitism of aloricate ciliates for seasonal samples collected from the southern coast of Korea over a 3 yr period. Aloricate ciliates were parasitized by species from 2 syndinean dinoflagellate genera and an unidentified genus of core dinoflagellates (Dinokaryota). Morphological and developmental differences among parasites of different host taxa suggest high parasite diversity. Infections generally peaked in fall, but notable interannual variation was evident within seasons. Parasites were more often encountered in commonly occurring and abundant hosts, but were also detected in occasional and rare host species. Most host taxa were rarely or sporadically parasitized, but Strombidium pollostomum and Strombidium bilobum were infected in 31 and 13% of the samples, respectively, where the species were present. Parasite prevalence in those species reached 22 and 17%, respectively. Overall, 29% of the samples analyzed contained infected aloricate ciliates. Results indicate that parasites of aloricate ciliates are usually a minor source of host mortality in coastal waters of Korea, but may periodically produce high infection levels contributing to top-down control of particular host species.