Feeding by the mixotrophic thecate dinoflagellate Fragilidium cf. mexicanum on red-tide and toxic dinoflagellates

We investigated prey species, prey selectivity, growth rates, grazing rates, and the effect of light and nutrient environment on feedng for the thecate mixotrophic dinoflagellate Fragilidiurn cf. mexicanum. Among the red-tide and toxic dinoflagellate prey offered, Lingulodinium polyedrum, Gymnodinium sanguineum, Prorocentrum micans, P. minimum, and Scrippsjella trochoidea were ingested by F. cf. mexicanurn, but Arnphidinium carterae and Cochlodinium polykr~koides were not. The feeding frequency (FF), based on the percent of F. cf. mexicanum containing one or more target prey cells, was significantly affected by prey species. The maximum FFs of F. cf. mexicanurn on L. polyedrum and S. trochoidea after 72 h incubation in a unialgal diet (50 and 89'>6, respectively) were much higher than those for P. micans, P. minimum, and G. sanguineum (6, 10, and 2%, respectively). FFs on L. polyedrum and S. trochoidea were significantly affected by prey concentration, but those on P. micans and G. sanguineum were not. F. cf. mexicanum strongly selected L. polyedrum over S. trochoidea in prey mixtures. With increasing mean prey concentration, growth and ingestion rates of F.