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The role of mesozooplankton grazers in the development of monospecific algal blooms has often been examined in a scenario in which grazers, depending on their abilities of recognition, select against toxic species and increase grazing pressure on non-toxic species. Here, we present a different ecological scenario in which grazers may select between different strains (toxic and non- toxic) of the same species, which may coexist in similar densities in natural environments prior to bloom initiation. The calanoid copepod Acartia clausi was fed with single and mixed diets of 2 strains of the dinoflagellate Alexandrium minutum, a producer of paralytic shellfish poisoning (PSP) toxins. One strain produced high, and the other low, quantities of PSP toxins. We examined feeding strate- gies and estimated copepod responses based on their food selection abilities, toxic effects on mainte- nance physiology and fitness, and benefits produced by a toxin-dilution strategy in a mixed diet. Copepods were found to feed selectively on A. minutum strains. Diet composition had a strong effect on parameters such as food ingestion, mortality, egg hatching, and egg production. The effect on copepod mortality and egg production was greatly reduced when a mixed diet (toxic + non-toxic) was provided to the copepods. However, the negative effects on egg hatching were dose-dependent, and this parameter was not recovered by toxin-dilution mechanisms. We conclude that copepods did not effectively reject the toxic strain and that the effect of A. minutum on mortality and egg production, but not on egg hatching, is reduced by dilution mechanisms. Therefore, we suggest that feeding pressure by grazers does not appear to be an important mechanism that favors toxic over non-toxic strains prior to bloom initiation.

Feeding strategies of the copepod Acartia clausi on single and mixed diets of toxic and non-toxic strains of the dinoflagellate Alexandrium minutum