Predator cues alter habitat use by the amphipod Hyalella azteca (Saussure)

The abundance and distribution of aquatic mesograzers may be regulated by both top-down (i.e., predator-mediated) and bottom-up (i.e., producer-mediated) effects. Under predation by fish, these herbivores may experience differential survivorship among different types of resource patches. Prey may attempt to maximize fitness by integrating information on predation risk and patch quality into foraging decisions. The freshwater amphipod Hyalella azteca occupies mats of the toxic cyanobacterium Lyngbya wollei and the green alga Rhizoclonium hieroglyphicum in lotic water bodies throughout the southeastern USA. We tested the hypotheses that Lyngbya is an effective refuge from Bluegill Sunfish (Lepomis macrochirus) predation and that predator cues modify habitat selection by amphipods. In no-choice assays, amphipods exposed to fish predation showed higher survivorship on Lyngbya than on Rhizoclonium. In choice assays, we observed greater proportions of amphipods on Lyngbya in tanks containing either predators or waterborne predator cues compared to control tanks containing only freshwater. These results suggest that Lyngbya is an effective refuge from predation for amphipods. Furthermore, predatory fish may indirectly influence the relative abundance of algae and cyanobacteria by reducing amphipod abundance on highly palatable species and restricting these mesograzers to less palatable species.