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The copepod Acartia tonsa Dana exhibits raptorial feeding behaviour when encountering planktonic ciliates. It detects individual ciliates at a distance of 0.1 to 0.7 mm from the first antennae, and requires no physical contact. Detection distance is correlated with prey size, and the probability of successful captures declines with distance from the first antennae. When A. tonsa encounters the ciliate Mesodinium rubrum, the ciliate responds with rapid swimming motion, which reduces the probability of capture. Feeding experiments, in which A. tonsa was offered different concentrations of planktonic ciliates, showed that ciliates were cleared at concentrations as low as 0.3 μg Cl⁻¹. The observed differences in clearance for planktonic ciliates (15 to 86 ml d⁻¹) may be explained in terms of detection distance and ciliate swimming behaviour. Experiments with mixtures of ciliates and the microflagellate Cryptomonas baltica …