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Omnivorous copepods are capable of discriminatory feeding using mechano- and chemosensory mechanisms. The presence of phycotoxins in phytoplankton often results in reduced consumption of such potential prey by copepods, though it has not been clear if this is the result of discriminatory feeding by either tactile (mechanosensory) or chemosensory recognition of toxic prey, or perhaps a physiological response to ingested neurotoxic compounds. In this study, experiments were performed to determine whether 3 species of marine copepods (Acartia tonsa, Centropages hamatus, and Eurytemora herdmani) that commonly co-occur with toxic Alexandrium spp. dinoflagellates were capable of discriminating between cultured Alexandrium spp. strains on the basis of paralytic shellfish poisoning (PSP) toxin content, i.e. by chemosensory means, using live fluorescently labeled cells. Additional experiments investigated whether toxic cells in mixtures with non-toxic alternate species of dinoflagellates affected either prey selection or total carbon consumption rates of copepods, and whether daily carbon rations could be maintained on both toxic and non-toxic Alexandrium spp. monoculture diets. Results indicated that all 3 copepod species could discriminate between toxic and non-toxic Alexandrium spp. cells by chemosensory means, suggesting that selective behavior, rather than physiological effects, governs the grazing response of copepods exposed to toxic prey. Prey selection in mixtures of several dinoflagellate species depended on whether the Alexandrium spp. cells present were toxic or non-toxic. C. hamatus and E. herdmani (but not A. tonsa) maintained daily carbon rations despite the presence of toxic Alexandrium spp., chiefly through increased consumption of alternate prey. For A. tonsa and C. hamatus, carbon rations were not equivalent between toxic and non-toxic Alexandrium spp. monoculture diets, indicating strong aversions to PSP toxins, and the potential for physiological effects when no other food is available. In all experiments feeding behavior varied among copepod species, suggesting that grazing pressure on toxic Alexandrium spp. is not uniform throughout the zooplankton community. The grazer-deterrent effects observed have implications for the function of PSP toxins