Determination and characterization of resistance by populations of the copepod Acartia hudsonica to the toxic dinoflagellate Alexandrium sp.
The evolutionary consequences of harmful algal blooms (HABs) to zooplankton grazers remain highly unexplored. To determine if marine grazer populations are able to adapt to the presence of HAB algae, I examined whether copepods have evolved a resistance to the presence of toxic dinoflagellates. I hypothesize that the ability of copepods to feed and reproduce on the toxic dinoflagellate Alexandrium spp. should be higher in copepods from regions that are frequently exposed to toxic Alexandrium spp. blooms than those less exposed to the blooms. ^ I tested this hypothesis through common environment experiments in which adult Acartia hudsonica from five populations collected from regions with varying exposure to Alexandrium blooms were fed toxic and non-toxic Alexandrium, and the non-toxic flagellate Tetraselmis sp. When fed toxic Alexandrium, I observed significantly higher ingestion and egg production rates in the copepods from regions exposed to toxic Alexandrium blooms relative to copepods from regions naïve to Alexandrium blooms. Such differences were not observed when copepods were fed non-toxic diets. Furthermore, a life-history analysis showed that toxic Alexandrium negatively affected the survival, fecundity and growth of naïve copepods, while not affecting exposed copepods. Fitness estimates, λ, revealed that the exposed A. hudsonica population was more resistant to toxic Alexandrium than the naïve population. These results support the hypothesis of local adaptation of the historically exposed copepod populations to the toxic dinoflagellates. ^ To examine the mechanism of toxicity in the naïve copepod populations, I measured the ingestion rates of A. hudsonica at different time intervals (t = 3, 6, 12, 24, 48 hours) while feeding on different diet mixtures of toxic Alexandrium. The copepods initially ingested toxic Alexandrium at high rates followed by sharp declines in ingestion over time; this did not result from changes in prey selection. The decreases in ingestion were accompanied by reduced respiration rates. These observations are consistent with the hypothesis that toxic Alexandrium physiologically incapacitated the naïve copepods. ^ This study represents an unequivocal test of adaptation and is the first demonstration of resistance in a marine zooplankton populations. These results have important implications for our understanding of the effects of HABs on marine ecosystems.