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 fee ding behaviour of the rotifers B. calyciflorus and Brachionus rubens on the ciliates Coleps sp. and Tetrahymena pyribormis. The ciliates are similar s ized but differ in their body surface texture. The surface of Coleps sp. co nsists of calcareous plates while T. pyriformis is a soft bodied ciliate. T he 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 mu l rotifer(-1) h(-1) and 5.7 c iliates rotifer(-1) h(-1)), whereas the functional response for the ciliate T. pyriformis changed to the rectilinear Type 1 model (maximal clearance a nd ingestion rate of 8.5 mu l 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 8.5 rotifer(-1) h(-1) and 3.3 ciliates rotifer(-1) h( -1)). Then is evidence that the surface texture of prey organisms influence s 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 handl ing time for the armored Coleps sp, is longer than for the soft bodied T. p yriformis. The smaller rotifer B, rubens, which generally prefers smaller f ood items than B. calyciflorus, was able to ingest the soft bodied T. pyrif ormis but needed a longer handling time for this ciliate than did B. calyci florus. The hard surface texture of Coleps sp, probably prevented its inges tion by B. rubens.