Differential grazing by Acartia tonsa on a dinoflagellate and a tintinnid

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Abstract
The calanoid copepod, Acartia tonsa Dana, ingests both the dinoflagellate, Heterocapsa triquetra (Ehrenberg) Stein, and the tintinnid ciliate, Favella sp. In laboratory experiments its ingestion rate increases with increasing dinoflagellate density to a maximum at ∼650 cells ml −1 , then declines. With Favella as the sole food item, ingestion rate increases up to and possibly above prey densities of 3.4 Favella ml −1 . In mixtures of the two prey, the clearance rate of Acartia for Favella decreases with increasing concentration of Heterocapsa . At a Favella concentration of ∼1 ml −1 and a Heterocapsa concentration of 280 cells ml −1 , Acartia ingests the same biomass of each prey type. The copepods preferentially feed on Favella even when the dinoflagellate is more abundant in terms of carbon and nitrogen than the tintinnid. If the effect of food density on the growth of Favella is considered as well as copepod predation, it is evident that both of these factors, and their interaction, can be important in regulating populations of this ciliate.

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