Testing mechanisms driving the increased susceptibility of <i>Daphnia</i> to disease when exposed to <i>Chaoborus</i> cues

Abstract Non-consumptive effects of predators upon their prey can alter disease dynamics by shifting prey behavior and physiology. In the Chaoborus–Daphnia–Australozyma (formerly Metschnikowia) predator–prey–host–parasite system, Chaoborus cues increase the prevalence of Australozyma infection in Daphnia, but the mechanism of this effect is unknown. This study investigated two potential mechanisms for the non-consumptive effect on infection. It has been hypothesized, but not directly tested, that Daphnia encounter more Australozyma spores on exposure to Chaoborus cues as a result of increased feeding rate. We also hypothesized that the production of morphological anti-predator defenses imposes a resource allocation trade-off with immune function. We tested both hypotheses by rearing Daphnia dentifera factorially in the presence/absence of Chaoborus kairomone and the presence/absence of Australozyma spores, measuring infection, morphology, reproduction and feeding rate. Our results did not support either hypothesis. Feeding rate did not increase in response to kairomone exposure, nor was it predictive of infection. Exposure to Australozyma did not significantly decrease anti-predator morphology, nor was morphology predictive of infection. While the mechanism of the effect remains unknown, we have tested and provided evidence against two likely hypotheses as to its functioning.