Predator diet, origin or both? Factors determining responses of omnivorous amphipods to predation cues

Abstract

Detection of chemical cues of predators and food resources is a key for the behaviour of many species in aquatic ecosystems. We checked whether predator origin and diet, containing potential food and/or alarm signals, affect the behaviour of omnivorous prey partly sharing their diet with a top predator. We conducted y-maze experiments to study the responses of invasive omnivorous and cannibalistic amphipods (Dikerogammarus villosus and Pontogammarus robustoides) to chemical cues of fish predators, injured amphipods and chironomid larvae (common amphipod food). As the predators, we used the goby Babka gymnotrachelus (sympatric with the amphipods) and piranha Pygocentrus nattereri (allopatric to the amphipods). The fish were either starved or fed amphipods or chironomids. D. villosus preferred predators fed chironomids and conspecifics as well as crushed conspecifics and chironomids, but avoided both hungry predators. Thus, D. villosus may perceive the presence of a top predator as an indication of both food resource and predation risk. P. robustoides avoided predators fed both amphipods (particularly conspecifics) and their alarm substances. The responses of both species depended more on the predator diet than on its species, which is likely to facilitate the recognition of allopatric predators and survival in newly invaded areas.