In the deep Skagerrak I found several specimens of Nephtys hystricis at different stations with ciliates attached to its gills. Obviously only this species was suitable to host the ciliates. Other Nephtys-species and also other polychaetes were not "infected". I konw the paper by Alvarez-Campos et al. (2014) describing ciliate species on Syllidae. Maybe somebody knows about similar observation on Nephtyidae.