Summary

The effects of cytochalasin B and dimethyl sulphoxide (DMSO) on ciliate ingestion and growth were investigated in laboratory cultures of the freshwater ciliate Tetrahymena pyriformis and the marine ciliate Uronema marinum, in order to assess whether cytochalasin B could be used to inhibit grazing selectively in natural ciliate communities. The ciliates were exposed to a final concentration of 0–10 μg ml⁻¹ cytochalasin B dissolved in 0.4% DMSO, and to 0–0.4% DMSO only, for between 1 and 48 hours, then fed carmine particles as tracers of particle ingestion. DMSO had no effect on particle ingestion. Cytochalasin B reduced particle ingestion in U. marinum and T. pyriformis at respective concentrations of ≥0.1μg.ml⁻¹ and ≥1.0 μg ml⁻¹, achieving a maximum effect after 4–8 hours. The effect of cytochalasin B was greater on U. marinum, with a minimum of only 4% of the population feeding after 4 hours exposure to 10 μg ml⁻¹, as compared to a minimum of 48% recorded for T. pyriformis after 8 hours exposure to the same concentration. Cytochalasin B and DMSO at these concentrations had no effect on ciliate growth rates over 48 hours suggesting that the drug has potential as a means of reducing ciliate grazing in field samples.