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

We investigated the potential impact of four isolates of small ciliates of the genus Urotricha on the common rotifer species Keratella cochlearis and Keratella quadrata in laboratory batch culture experiments. Two small Cryptomonas species served as food for the ciliates and the rotifers in the experiments. Population growth rates of the rotifers were measured when they grew alone and when they were paired with one of their potential ciliate competitors. Growth rates of K. cochlearis were enhanced and their mortality rates reduced in the presence of Urotricha furcata, most likely because the rotifers preyed upon the ciliates. The larger rotifer species, K. quadrata, also feeds upon small Urotricha, yet their population growth rates were negatively affected by Urotricha. This was because the mortality rate of K. quadrata increased in the presence of all three isolates of U. furcata and the one isolate of Urotricha farcta. It needs to be investigated whether this effect is chemically mediated and ecologically relevan