VARIATION IN HERBIVORE RESPONSE TO CHEMICAL DEFENSES - ZOOPLANKTON FORAGING ON TOXIC CYANOBACTERIA

Some planktonic cyanobacteria possess chemical defenses (toxins) or morphological defenses (filamentous morphology) against herbivory by zooplankton; Anabaena affinis is both toxic and filamentous. The presence of A. affinis has a differential effect on various zooplankton taxa, causing reductions in the population growth rates of large cladocerans but not of small cladocerans or rotifers. Here, we investigate the importance of differences in zooplankton foraging patterns in determining the differential inhibitory effect of A. affinis. Radiotracer feeding experiments were conducted on four cladoceran species (5 clones, and 15 body sizes), two rotifer species, and one calanoid copepod species. With one exception (the cladoceran Ceriodaphnia dubia), the ingestion rates of zooplankton on unicellular phytoplankton (Cryptomonas) were not reduced by the presence of A. affinis. Behavioral observations showed that cladocerans sometimes rejected A. affinis filaments, but did not simultaneously reject Cryptomonas cells. Thus, the differential inhibitory effect of A. affinis is not caused by mechanical interference with feeding. When offered Cryptomonas and A. affinis simultaneously, small cladocerans, rotifers, and the copepod fed more selectively than large cladocerans and avoided ingesting toxic filaments. As a result, large cladocerans received higher mass-specific doses of the toxin. Different tendencies to ingest toxic filaments, perhaps combined with different physiological sensitivities to the toxin, cause the differential effect of A. affinis on various zooplankton taxa. This variation in herbivore response to cyanobacterial defenses may change the species composition and size structure of zooplankton communities.