The alarm cue obstruction hypothesis: isopods respond to alarm cues, but do not respond to dietary chemical cues from predatory bluegill

Predator avoidance behaviours occur when prey detect a predator but the predator has not yet detected and identified prey. These defences are critical because they prevent predation at the earliest possible stages when prey have the best chance of escape. We tested for predator avoidance behaviours in an aquatic macroinvertebrate (Caecidotea intermedius; order Isopoda) in a series of three experiments. The first experiment attempted to determine if isopods possess alarm cues by exposing them to stimuli from macerated conspecifics. We then exposed isopods to kairomones from non-predatory tadpoles (Rana catesbiana) and predatory fish (Lepomis macrochirus) that had been fed a benign diet. Finally, we exposed isopods to kairomones of predatory fish that had been fed a diet exclusively of isopods. We found that isopods did not respond to any kairomone cues or dietary cues from any potential predator, but did reduce activity in response to alarm cues. These results suggest that isopods exhibit predator avoidance responses toward chemical cues in a limited setting (they do not respond unless the information suggests an attack has occurred in the immediate past) or that bluegill have the ability to modify or mask the alarm cues from their prey.