Environmental factors affecting behavioural responses of an invasive bivalve to conspecific alarm cues

• We tested the responses of the zebra mussel to conspecific alarm cues.
• Alarm cues increased horizontal mussel movement in light and reduced it in darkness.
• Alarm cues inhibited upward relocations and stimulated downward movement of mussels.
• Mussels were more aggregated in response to alarm cues, but only on soft substratum.
• Mussels can fine-adjust their antipredator defences depending on multiple factors.

Antipredator defences of aquatic animals depend on various environmental parameters. We studied behavioural responses of a Ponto-Caspian invasive bivalve, the zebra mussel, Dreissena polymorpha, to conspecific alarm cues. We hypothesized that mussels would change their locomotion and aggregation in response to alarm signals. We also hypothesized that body size, light, substratum quality (suitable or unsuitable for attachment) and inclination would affect mussel defences. Changes in horizontal movement of mussels exposed to the alarm substance depended on light. In the presence of crushed conspecifics illuminated mussels (all sizes) moved longer distances than control individuals, whereas in darkness their reaction was the opposite. The response of small mussels was the strongest. Furthermore, the alarm substance reduced upward relocations of all size groups on an inclined surface but at the same time stimulated their downward movement. Large and medium mussels (but not small individuals) exposed to alarm signals formed aggregations more often than control individuals. This effect was only exhibited on sand, unsuitable for mussel attachment. Mussels were generally more clumped on sand than on hard substratum, suggesting that they did not prefer conspecific shells as attachment sites when alternative substrata were available. All responses of mussels to alarm cues tended to be stronger in light, which is an indirect indication of danger. Our study shows that the responses of the zebra mussel to conspecific alarm cues are not limited to activity reduction, as previously thought. They are adjusted to particular environmental conditions and may also involve increased locomotion when relocation to a safer site gives a better chance of survival.