Predation pressure may affect many aspects of prey behavior, including forming groups and changes in social interactions. We studied the aggregation behavior of competing gammarids Dikerogammarus villosus and Pontogammarus robustoides (Amphipoda, Crustacea) to check whether they modify their preferences for conspecifics or heterospecifics in response to predator (the racer goby Babka gymnotrachelus) kairomones in the presence or absence of stone shelters (alternative protection source). Both species exhibited preferences toward shelters occupied by conspecifics over empty shelters and conspecifics apart from shelters, suggesting that their aggregation depends not only on habitat heterogeneity, but also on their social interactions. Moreover, gammarids in the presence of shelters (safer conditions) preferred conspecifics over heterospecifics, but predator kairomones made them form aggregations irrespective of species. In the predator presence, P. robustoides increased its aggregation level only in the sheltered conditions, whereas D. villosus exhibited this response only in the absence of shelters, suggesting that this behavior can protect it against predators. Therefore, we tested the antipredator effectiveness of D. villosus aggregations by exposing them to fish predation. Gobies foraged most effectively on immobile single gammarids compared to moving and aggregated individuals. Fish also avoided aggregated prey, confirming the protective character of aggregations. We have demonstrated that the predator presence increases aggregation level of prey gammarids and affects their social behavior by reducing antagonistic interactions and avoidance between competing species. This is likely to affect their distribution and functioning in the wild, where predator pressure is a standard situation.

Responsiveness to biotic factors is crucial for the survival of sessile aquatic animals. They cannot escape from danger, but developed a number of defences against predation, usually delayed in time. We checked the initial defence of the freshwater byssate zebra mussel, Dreissena polymorpha, associated with valve gaping. We tested the effect of chemical signals: fish predator scent (the roach Rutilus rutilus), conspecific alarm cue and a mixture of both, as well as a mechanical stimulus: the presence of an amphipod (Dikerogammarus villosus) mechanically irritating mussels. The alarm cues and amphipod presence made mussels spend more time with closed/narrowly open valves, which can be related to decreasing detection probability by reduced infochemical excretion and/or protecting soft tissues in the presence of an imminent threat. In contrast, reactions to the predator scent alone were much weaker. Moreover, the fish scent mixed with alarm substance induced weaker responses than the alarm substance alone. Thus, the fish infochemical might mask the presence of the alarm cue components, potentially benefiting the predator. A variety of defences exhibited by mussels demonstrates the importance of the predation cue type (direct/indirect, chemical/mechanical, originating from conspecifics/ predators/mixed) for the behaviour of sessile animals.

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