Species-specific interactions between small planktonic ciliates (Urotricha spp.) and rotifers (Keratella spp.)

We investigated the potential impact of four isolates of smallciliates of the genus Urotricha on the common rotifer speciesKeratella cochlearis and Keratella quadrata in laboratory batchculture experiments. Two small Cryptomonas species served asfood for the ciliates and the rotifers in the experiments. Populationgrowth rates of the rotifers were measured when they grew aloneand when they were paired with one of their potential ciliatecompetitors. Growth rates of K. cochlearis were enhanced andtheir mortality rates reduced in the presence of Urotricha furcata,most likely because the rotifers preyed upon the ciliates. Thelarger rotifer species, K. quadrata, also feeds upon small Urotricha,yet their population growth rates were negatively affected byUrotricha. This was because the mortality rate of K. quadrataincreased in the presence of all three isolates of U. furcataand the one isolate of Urotricha farcta. It needs to be investigatedwhether this effect is chemically mediated and ecologicallyrelevant.