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Abstract

We characterized the ecological niches of several planktonic prostome ciliates with respect to their food demand and temperature. We found intergeneric differences between Balanion planctonicum and the 2 Urotricha spp., U. furcata and U. farcta. There were also significant interspecific differences within the genus Urotricha and intraspecific differences between 2 Balanion spp. and 3 U, furcata isolates from distant lakes. Relative to Urotricha spp., Balanion appeared to be the superior competitor at low to medium food concentrations and reached high growth rates at moderate temperatures. The threshold prey concentration for positive population growth of B. planctonicum was lower than that obtained for the 2 Urotricha spp., but higher than that reported earlier for the marine species, B. comatum. A third Urotricha species, U. castalia, was investigated for its temperature response only, The temperature response revealed species-specific temperature adaptation between B. planctonicum and the sympatric U. furcata, and further differences within the genus Urotricha: U, farcta grew fastest at high temperatures; U. castalia was adapted to low temperatures; and U, furcata peaked at moderately warm temperatures