Avoidance of conspecific injury-released chemical cues by free-ranging Gammarus lacustris (Crustacea:Amphipoda)

Behavioral responses to chemical cues have been demonstrated for a range of aquatic animals. Injury-released chemical alarm cues from conspecifics are released when a prey's predator is actively foraging. Detection of these cues elicits antipredator behaviors that reduce the probability of predation. Amphipod crustaceans in the genus Gammarus are widespread denizens of ponds and streams. Antipredator responses by Gammarus to conspecific alarm cues, and subsequent reduction of predation risk, are known from experiments in the laboratory. Here, we verify laboratory findings by demonstrating an avoidance response to alarm cues using a field population of G. lacustris. We used small traps baited with sponge blocks containing either water (control) or injury-released cues from Gammarus. We repeated the experiment twice. In both experiments, significantly fewer Gammarus were captured in traps with alarm cue sponges than in traps with water sponges. Predatory leeches Dina parva were attracted to Gammarus traps in the first experiment but not the second experiment. In the second experiment, we measured the individual weight of captured amphipods. Two size classes were present; small (1-5 mg) and large (35-108 mg). Both sizes contributed to the avoidance response. Within the large size class, small individuals were significantly less responsive to the alarm cue than large individuals, implying that small adult Gammarus may have different cost/benefit decision criteria for risk assessment than large Gammarus.