Gammarus pulex show a grouping response to conspecific injury cues but not to predator kairomones

Many species gain protection from predators by forming groups, but there is also evidence that some predators are better able to detect or more likely to attack grouped prey. Given this, it might pay prey to be flexible in their group behavior, forming groups on detecting certain predators, but dispersing when detecting others. In the first of 2 experiments, we found that flounders (Platichthys flesus) were more likely to attack larger groups of gammarids (Gammarus pulex) than smaller ones, whereas sticklebacks (Gasterosteus aculeatus) showed no such bias. This gave us the opportunity to test the idea that prey might show predator-specific grouping responses. Accordingly, our second experiment compared the grouping behavior of gammarids exposed to kairomones from either of the 2 predators, to conspecific injury cues (a nonspecific predation cue), to combinations of predator kairomone plus conspecific injury cues and finally to 2 control treatments. We predicted, based on our first experiment, that the gammarids would disperse in response to flounder kairomones, and group more cohesively in response to stickleback kairomones and conspecific injury cues. In fact, only the treatments including conspecific injury cues elicited a grouping response in the gammarids, whereas predator kairomones alone had no effect whatsoever on group cohesion or dispersal. We discuss possible explanations for these findings and briefly consider other systems that might be better suited to exploring predator-specific antipredatory grouping behavior.