ABSTRACT: We investigated prey species, prey selectivity, growth rates, grazing rates, and the effect of light and nutrient environment on feeding for the thecate mixotrophic dinoflagellate Fragilidium cf. mexicanum. Among the red-tide and toxic dinoflagellate prey offered, Lingulodinium polyedrum, Gymnodinium sanguineum, Prorocentrum micans, P. minimum, and Scrippsiella trochoidea were ingested by F. cf. mexicanum, but Amphidinium carterae and Cochlodinium polykrikoides 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. mexicanum on L. polyedrum and S. trochoidea after 72 h incubation in a unialgal diet (50 and 89%, 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. cf. mexicanum feeding on L. polyedrum increased, with saturation at a mean prey concentration of approximately 500 cells ml^-1. The maximum specific growth rate (mixotrophic growth) of F. cf. mexicanum on L. polyedrum was 0.36 d^-1, under a 12 h light:12 h dark cycle of 20 µE m^-2 s^-1, while its growth rate (phototrophic growth) under the same light conditions without added prey was -0.05 d^-1. The maximum ingestion rate of F. cf. mexicanum on L. polyedrum, 3.9 prey eaten predator^-1 d^-1, was comparable to those of the co-occurring heterotrophic dinoflagellates Protoperidinium cf. divergens and P. crassipes for the same prey. However, maximum clearance rate of F. cf. mexicanum, 6 µl predator^-1 h^-1, was much higher than those of P. cf. divergens and P. crassipes. The ingestion rate of F. cf. mexicanum on L. polyedrum was not significantly affected by light intensity or nutrient concentration when prey was plentiful.

KEY WORDS: Dinoflagellate · Growth · Grazing · Mixotrophy · Protist · Red tide

Published in MEPS Vol. 176 (1999) on January 18
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999