Neurotoxins of the harmful alga Alexandrium sp. improve the performance of pelagic copepods

Background/Question/Methods  Blooms of harmful algae are common and may be proliferating in the sea. Some of these algae are toxic. Our understanding of these blooms is limited, especially with respect to the role of grazing by mesozooplankton, some of which have been found to be resistant to toxins. We used laboratory experiments to examine the effect that neurotoxins produced by the toxic dinoflagellate Alexandrium sp. had on a copepod grazer.

Results/Conclusions  We discovered novel responses of individual grazers to the toxic alga. Some copepods of the species Acartia hudsonica and Acartia tonsa responded to the toxic dinoflagellate Alexandrium sp. with increased ingestion rate, survivorship and egg production. It appeared that the responses were not due to food quality as generally conceived, nor were they due to food quantity. Further, the observed positive effects were prevalent in a laboratory population, and they were also common in wild populations. We hypothesize that the neurotoxins of Alexandrium sp. improve copepod ingestion and/or assimilation efficiency, and therefore egg production and survivorship, by affecting the acquisition and processing of food in the gut. We hypothesize further that a mutation in the voltage-gated sodium channel of the copepod may be responsible for these effects. Such positive effects, which are currently unappreciated, will affect grazing feedbacks and, therefore, the fate of toxic blooms, toxin transfer to higher trophic levels, and the co-evolution of toxic prey and their consumers.