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The planktonic rotifer Ascomorpha ovalis feeds on large dinoflagellates (e.g. Ceratium sp., Peridinium sp.) and is able to extract their cell contents by means of its virgate mastax. This paper presents the results of experiments on the feeding behaviour of laboratory-cultured Ascomorpha with Ceratium furcoides as food algae. Ascomorpha are three times larger than their prey Ceratium (by volume), but with regard to total length, their prey was even 20% larger. Ascomorpha showed a hyperbolic functional response curve with a plateau of the feeding rate at 8 Ceratium cells animal⁻¹ day⁻¹ when concentrations of Ceratium were >100 cells ml⁻¹. The mean handling time (time for capturing and extracting one Ceratium cell) was 3 min. The shape of the functional response was better described by a curvilinear model than by a rectilinear model. However, handling times cannot be responsible for this, since they were too short to set limits on ingestion rates. At low food concentrations, encounter rates with prey seemed to limit the feeding rates of Ascomorpha, whereas at medium to high food concentrations, satiation effects (lower attack rates) seemed to set limits on the feeding rates. Ascomorpha showed a significant decrease in the exploitation of single Ceratium cells at high prey concentrations. This decrease could be explained by a saturation effect in which the partly filled guts of Ascomorpha did not permit the total extraction of the contents of a Ceratium cell.
