FORAGING ON CYANOBACTERIA BY COPEPODS - RESPONSES TO CHEMICAL DEFENSES AND RESOURCE ABUNDANCE

We used laboratory feeding trials to examine the responses of copepods to a variety of cyanobacteria of differing size, morphology, and toxicity. Diaptomus birgei exhibited behaviors ranging from strong preferences for some taxa of cyanobacteria to almost complete rejection of other taxa when allowed to feed in low-concentration mixtures of a high-quality green alga, Chlamydomonas reinhardi, and each of 16 taxa of cyanobacteria. Our results demonstrate the overriding importance of chemical factors. Copepods exhibited very high clearance rates for some filaments and colonies but rejected morphologically similar strains shown to contain potentially toxic compounds. Responses to resource abundance exhibited three patterns, each consistent with the assumptions and predictions of an optimal diet model. First, selectivity for high-ranking cyanobacteria declined in high-concentration mixtures with Chlamydomonas relative to selectivity in low-concentration mixtures. Second, a test of the effects of time without food showed that hunger, rather than food concentration per se, was responsible for these shifts in selectivity. Finally, each toxic strain was consistently ingested at very low rates, even when offered as the sole food resource. Experiments with complex mixtures of natural seston revealed similar patterns of consistently strong discrimination against a presumably toxic cyanobacterium and concentration-dependent selectivity for presumably nontoxic taxa. Our results show how the behavioral flexibility of copepods can be adaptive in lakes with significant populations of cyanobacteria. The ability of copepods to feed size-selectively on filaments and colonies and to discriminate against toxic forms suggests that these grazers may be important in the evolution and continued success of toxic strains.