Fitness benefits and costs of induced defenses in Daphnia carinata (Cladocera: Daphnidae) exposed to cyanobacteria

Induced defenses can shape the trophic interactions between prey and predators, but the presence, benefits, and costs of induced defenses against harmful algae in zooplankton are little known. Here, the hypothesis that a short-time exposure to harmful algae can induce defenses in zooplankton was tested by evaluating the fitness changes in a single clone of Daphnia carinata (Cladocera: Daphnidae) after the exposure to a bloom-forming cyanobacterium Microcystis aeruginosa. Life-table experiments revealed that the 1-day exposure to M. aeruginosa significantly enhanced the average life span, the net reproductive rate, and the intrinsic rate of population increase of D. carinata in the presence of cyanobacteria, compared to nonexposed individuals. Then, we tested the second hypothesis that the induced defense in zooplankton would carry a cost on fitness in the absence of cyanobacteria. The 1-day exposure to M. aeruginosa significantly depressed the average life span, the net reproductive rate, and the intrinsic rate of population increase of D. carinata when exclusively feeding on a green alga, Chlorella pyrenoidosa. The induced defense against M. aeruginosa in D. carinata after a short-time exposure explains some aspects of the interactions between harmful algae and zooplankton, and may provide new general insights into the predator-prey "arms race".