Abstract
Group living has evolved as an adaptation to predation in many animal species. In a multitude of vertebrates, the tendency to aggregate varies with the risk of predation, but experimental evidence for this is less well known in invertebrates. Here, we examine the tendency to aggregate in the freshwater amphipod Gammarus pulex in the absence and presence of predator fish odour. Without fish odour, the gammarids showed no significant tendency to aggregate. In contrast to this, in fish-conditioned water, they significantly preferred to stay close to conspecifics. Predation risk can, thus, influence gammarids social behaviour.