GROWTH AND GRAZING RESPONSE OF A CILIATE FEEDING ON THE RED TIDE DINOFLAGELLATE GYRODINIUM-AUREOLUM IN MONOCULTURE AND IN MIXTURE WITH A NONTOXIC ALGA

The effect of the red tide dinoflagellate Gyrodinium aureolum on the growth of the tintinnid ciliate Favella ehrenbergii was studied. The ciliate is unable to sustain growth with this alga as the only food source, irrespective of concentration. Ciliate survival decreases at very high concentrations of G. aureolum probably due to toxic substances exuded from G. aureolum to the medium. This assumption is supported by the fact that growth of the ciliate is not affected by even very high concentrations of a non-toxic dinoflagellate, Heterocapsa triquetra. However, direct attempts to demonstrate toxic effects of exudates, using filtrates of dense cultures of G. aureolum, failed. Growth and grazing experiments were also carried out in which the ciliate was fed mixtures of G. aureolum and H. triquetra at relatively low algal concentrations. The growth of the ciliate was unaffected until G. aureolum accounted for about 70% of the total biomass. In cases where G. aureolum accounted for about 90% of the algal biomass, the growth rate of the ciliate was reduced by less than 25%. Grazing experiments demonstrated that F. ehrenbergii cannot selectively avoid ingestion of G. aureolum.