Prey size selection, grazing and growth response of the small heterotrophic dinoflagellate Gymnodinium sp. and the ciliate Balanion comatum--a comparative study

Prey selectivity, growth and feeding responses were studied in the cillate Balanion comaturn (17 pm) and the heterotroph~c dinoflagellate Gjmnodinium sp.(7 pm).Almost identical prey size spectra were found for the 2 organisms.Optimum prey size was 8 pm, while the lower and upper hmits of prey capture were -4 and 10 pm, respectively.Maximum growth and ingestion rates of B. comatum were slightly higher than those of Gj~mnodinium sp.Threshold prey concentration for growth of B. comatum and Gymnodinium sp. was 11 and 17 pg C I-', respectively.At 15"C, both organisms needed to ingest approx. 1 to 2 % h-' of their cell volume in order to sustain basic metabolic actlvlty.M a x ~m u m specific clearance was 2 to 3 tlmes h ~g h e r for the ciliate compared to the dinoflagellate.Gymnodlnium sp.survived for a longer time than B. comatum when deprived of prey organisms.Gymnodinium sp. cells were not ingested by B. comatum, although they were of a size which is optimal for B. comatum.