Grazer‐induced bioluminescence and toxicity in marine dinoflagellates

Abstract Marine copepods are the most abundant multicellular zooplankton in the global oceans. They imprint their surrounding waters with a unique bouquet of chemical compounds, including polar lipids such as copepodamides. Prey organisms can detect copepodamides and respond by inducing defensive traits including bioluminescence, toxin production, changes in colony size, and structural modifications. This mechanism has been suggested to contribute to harmful algal bloom formation, but to date only a limited number of species and strains have been experimentally exposed to copepodamides. Here, we quantify bioluminescence and toxin content in response to increasing concentrations of copepodamides in three harmful algal bloom‐forming species of marine dinoflagellates: Alexandrium catenella , Protoceratium reticulatum , and Gymnodinium catenatum . All three species up‐regulated their defensive traits in response to copepodamide exposure, including the first example of copepodamide‐induced GC‐toxin production. Neither bioluminescence nor toxin production was associated with measurable costs in terms of reduced growth rates. The results support the role of copepodamides as general alarm cues in marine phytoplankton. Moreover, the expression of simultaneous defensive traits may confound studies addressing the costs and benefits of these co‐varying traits.