Changes in attachment strength and aggregation of zebra mussel, Dreissena polymorpha in the presence of potential fish predators of various species and size

We studied the attachment strength and aggregation behaviour of Dreissena polymorpha in the presence of large roach Rutilus rutilus (> 180 mm total length) (efficient molluscivore), small roach (< 110 mm) (unable to feed on zebra mussels) and perch Perca fluviatilis (not feeding on mussels). The intention was to check whether small (< 10 mm) and large (> 10 mm) mussels would respond specifically to fish capable of consuming them (i.e. large roach). After 1 day of exposure, we found no significant differences in mussel attachment strength. After 6 days in the presence of large roach, mussels were attached more strongly than in the other treatments. After a 1-day exposure to all kinds of fish, mussels were more aggregated than in the control treatment. After 6 days, the largest percentage of aggregated mussels was found in the presence of large roach, while the aggregation levels in the other treatments were lower and did not differ from one another. Perhaps, an initial response was a non-specific reaction to the presence of any fish, while a specific response to large roach appeared later. Thus, zebra mussels were able to recognize their potential predators. The observed behaviour of mussels may enhance their resistance to molluscivores in the field by limiting the access of predators to their potential prey (due to the increased aggregation of prey) and by increasing predator handling costs (due to the stronger attachment of prey).