Abstract
Phytoplankton have evolved myriad defenses against predators; yet, studies that simultaneously test for defense fitness costs and benefits are rare. We tested for relative fitness costs and benefits of defense in the marine dinoflagellate Alexandrium catenella using a framework that relates growth rates of prey genotypes (strains) that differed in constitutive toxin production (low, moderate, and high) to predator (copepod) concentration. Our approach is based on a novel molecular technique that allows one to disentangle the effect of predation mortality from the cell growth reduction due to toxin production. Results show that the strain with the highest constitutive toxin production was the only one that expressed inducible toxin production-a strategy that paid off as its fitness benefit outweighed its cost. Surprisingly, the moderate toxin strain that derived the highest relative fitness benefit increased cell division rate (akin to compensatory growth) and decreased cell size, while keeping its volume-specific toxin production constant in response to predation. These results suggest an effective antipredator defense portfolio.