Dinoflagellate grazing on the raphidophyte Fibrocapsa japonica

Laboratory experiments were performed to determine the growth and grazing capabilities of 2 heterotrophic dinoflagellates with different feeding modes (pallium feeder: Oblea rotunda; engulfment feeder: Oxyrrhis marina) when fed the raphidophyte Fibrocapsa japonica. Both dinoflagellates readily ingested prey and exhibited positive growth when feeding on monocultures of F. japonica. Maximum growth rates at food saturation were 0.54 and 0.72 d -1 for O. rotunda and O. marina, respectively. Both dinoflagellates are thus able to grow faster than their prey, for which a maximum growth rate of 0.45 d -1 has been previously reported. In the case of O. rotunda, it was found that a rather high food concentration of 300 F. japonica cells ml -1 (corresponding to 142 g C l -1 ) was needed to sustain half-saturated growth. This is consistent with the quantification of behavioural aspects of the feeding process. In about 55% of cases, a failure to attach the tow filament after prey encounter was recorded, and in about 83% of cases, F. japonica was able to escape from the attached tow filament, indicating that motility of F. japonica is a quite effective defence mechanism against pallium-feeding dinoflagellates. In addition, qualitative observations suggest that trichocysts of F. japonica may act mechanically as a grazer deterrent.