Grazing of toxic dinoflagellates, Alexandrium spp., by adult copepods of coastal Maine: Implications for the fate of paralytic shellfish toxins in marine food webs

Abstract

Dinoflagellates of the genus Alexandrium synthesize potent neurotoxins known as paralytic shellfish poisoning (PSP) toxins. The grazing responses of two abundant copepod species from the Gulf of Maine, Acartia tonsa and Eurytemora herdmani, were compared using cultured isolates of Alexandrium spp., which differed in toxicity per cell and toxin composition and a non-toxic dinoflagellate Lingulodinium polyedrum. Additional experiments were performed to determine if copepods were incapacitated over time by ingested toxins, or if they were capable of avoiding toxic cells by selecting alternative food. Toxin analyses were performed by high performance liquid chromatography with fluorescence detection (HPLC-FD) on both dinoflagellate prey and copepod consumers, to determine the rate and levels of PSP toxins accumulated in copepods. Toxin content (ng saxitoxin equivalents [STXeq] per individual) and relative composition (% molar) were used to establish whether or not toxin biotransformation and detoxification kinetics reflect the ability of these Zooplankton to act as vectors of PSP toxins in marine food webs. Significant differences in grazing rate, behavior, and toxin accumulation were found between the two copepod species tested. Eurytemora herdmani demonstrated marked selective capabilities, unrelated to the toxicity of prey items offered. Neither species showed strong evidence of incapacitation or adverse effects from ingested toxins. These results suggested that copepods have highly sensitive and specific chemoreceptive and selective abilities, and that the effect of Zooplankton grazing on toxic dinoflagellate blooms will depend on the species composition of the grazing community. Although the toxin retention efficiencies of copepods tested were low overall, high levels of PSP toxin were accumulated in copepod grazers, supporting the hypothesis that zooplankton may serve as PSP toxin vectors to higher trophic levels.