What scares a mussel? Changes in valve movement pattern as an immediate response of a byssate bivalve to biotic factors

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.