Functional responses of the rotifers Brachionus calyciflorus and Brachionus rubens feeding on armored and unarmored ciliates

Density dependent grazing experiments were performed to investigate the feeding behaviour of the rotifers B. calyciflorus and Brachionus rubens on the ciliates Coleps sp. and Tetrahymena pyriformis . The ciliates are similar sized but differ in their body surface texture. The surface of Coleps sp. consists of calcareous plates while T. pyriformis is a soft bodied ciliate. The two rotifers, which differ in their body size, were allowed to feed for 4 h on the ciliates, and the clearance and ingestion rates were calculated to fit functional response models. For B. calyciflorus fed with Coleps sp., the curvilinear functional response Type 2 gave the best fit to the data (maximal clearance and ingestion rate of 30 μl rotifer −1 h −1 and 5.7 ciliates rotifer −1 h −1 ), whereas the functional response for the ciliate T. pyriformis changed to the rectilinear Type 1 model (maximal clearance and ingestion rate of 8.5 μl rotifer −1 h −1 and 4.2 ciliates rotifer −1 h −1 ). In contrast, B. rubens could not eat Coleps sp., but when fed with T. pyriformis a functional response Type 2 was observed (maximal clearance and ingestion rate of 8 ml rotifer −1 h −1 and 3.3 ciliates rotifer −1 h −1 ). There is evidence that the surface texture of prey organisms influences the type of functional response. The change from a Type 2 ( Coleps sp.) to a Type 1 model ( T. pyriformis ) for B. calyciflorus suggests that the handling time for the armored Coleps sp. is longer than for the soft bodied T. pyriformis . The smaller rotifer B. rubens , which generally prefers smaller food items than B. calyciflorus , was able to ingest the soft bodied T. pyriformis but needed a longer handling time for this ciliate than did B. calyciflorus . The hard surface texture of Coleps sp. probably prevented its ingestion by B. rubens .