Direct and indirect impact of two common rotifer species (Keratella spp.) on two abundant ciliate species (Urotricha furcata, Balanion planctonicum)

1. We investigated the potential competition and feeding impact of the common rotifer species, Keratella cochlearis and K. quadrata, on the abundant prostomatid ciliates, Urotricha furcata and Balanion planctonicum, in laboratory batch culture experiments. All four species have similar feeding preferences, co-occur in many freshwater environments, and are thus potential competitors for the same algal food. 2. Two small Cryptomonas species served as food for the ciliates and the rotifers in the experiments. Growth rates of each ciliate species were measured when they grew alone and when they were paired with one of the rotifer species. 3. Both rotifer species reduced the growth rate of U. furcata, probably primarily by direct feeding on the ciliates. Growth rate of B. planctonicum was unaffected by K. cochlearis, but was drastically reduced by grazing and/or mechanical interference of K. quadrata. 4. These results suggest niche partitioning of the sympatric ciliates with respect to their rotifer competitors/predators