Negative effects of prey species on consumers could be due to deterrence, nutritional insufficiency, or toxicity of the prey. These effects can be discerned in experiments in which the suspect prey is offered to the consumers in a food mixture containing another prey item that is not toxic and in which the ingestion rates on the prey and the grazers ’ responses (e.g., egg production) are measured. We used this framework to determine whether several algae that have been reported to have harmful effects on grazers (Prorocentrum minimum, low- and high-toxin Alexandrium sp., Heterosigma carterae, Thalassiosira rotula, and Phaeodactylum tricornutum) are toxic to females of the calanoid copepod Acartia tonsa. Ingestion, egg production, and egg-hatching rates were measured for A. tonsa offered sole diets of the suspect alga and mixed diets containing the suspect alga and a control alga (the green flagellate Tetraselmis sp.) at an ecologically relevant concentration (250 �g CL �1) and duration (3 d). With the exception of the Alexandrium strain with the high-toxin content (16.3 pgSaxitoxin [pgSTX] equivalents cell �1), none of the diets studied can be considered toxic. The high-toxin Alexandrium reduced A. tonsa’s total ingestion rate, and thus egg production, as the proportion of Alexandrium increased in the diet. A. tonsa exhibited significantly reduced ingestion and egg production rates when feeding on sole foo
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