Abstract.We investigated the impact of neurotoxins produced by the dinoflagellate Alexandriurn excavatum on survival of Atlantic mackerel (Scomber scombrus) and American lobster (Homarus americanus) larvae, respectively reared from eggs and from female lobster, collected in 1988 from the southern Gulf of St. Lawrence, Canada. Sensitivity to the toxins was first verified by exposing larvae of both species to various concentrations of toxic A. excavatum (treatment) and non-toxic A. tamarense (control). Daily mortality rates ranged from 65 to 96% among mackerel larvae directly fed upon toxic cells and reached 36% in postlarvae exposed to toxic microzooplankton. Lobster larvae were apparently immune to the toxins, which they concentrated up to five times relative to vector toxicities. Bioassays conducted on mackerel larvae by exposure to natural plankton samples collected in situ during a bloom of toxic A. excavatum confirmed that exposure to the toxins could also have lethal effects in natural ecosystems. We conclude that the current proliferation of toxic dinoflagellates threatens early survival of finfish larvae and their recruitment to adult populations.

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