Abstract

The toxic dinoflagellate Alexandrium catenella, a Paralytic Shellfish Poison (PSP) producer occurs in estuarine fjords and recently in offshore waters of southern Chile (39 ° to 55 ° S). This dinoflagellate co-exist with zooplankton assemblages (mainly copepods), being their direct grazers regulating the distribution and abundance of microalgae. Grazing, growth and mortality rates are scarce for A. catenella and almost unavailable for Chilean fjords. Consequently, the objective of this study is to generate information about these rates through bioassays with strains of A. catenella. This information is relevant for biological modelling of the distribution and abundance of A. catenella in estuarine waters. To evaluate the "predator-prey" relationship two treatments were performed subjecting two species of calanoid copepods (Acartia tonsa y Calanus chilensis) to A. catenella unique diets (Treatment 1) and natural phytoplankton assemblage diets inoculated with A. catenella (Treatment 2). When A. catenella is available as single diet (Treatment 1) both copepods predate on it, differing in a greater clearance and ingestion rates of C. chilensis over A. tonsa. When a greater diversity food diet is available (Treatment 2), grazing pressure decreases on A. catenella in both grazers. Different hypotheses show that copepods “learn” to discriminate toxic cells post-ingestion through chemosensory methods. They can also choose for an alternative diet to reject the toxic prey. On the other hand, an important ecological advantage, it has been pointed out, in organisms that synthesize toxins, because they diminish the consume pressure on them and derive it to other species.