Attachment strength, aggregation and movement of the zebra mussel (Dreissena polymorpha, Bivalvia) in the presence of potential predators

We carried out laboratory experiments to study the behaviour of zebra mussels in the presence of predators with different feeding habits: the roach Rutilus rutilus (an efficient molluscivores), the racer goby Neogobius gymnotrachelus, the crayfish Orconectes limosus (facultative molluscivores), and the perch Perca fluviatilis (prefers other kinds of food). We studied three size (length) classes of mussels: small (< 10 mm), medium (10-17 mm) and large (> 17 mm). We measured attachment strength and numbers of individuals forming aggregations after 6-d exposure to predators, as well as horizontal and vertical distances moved by mussels after 24 h of exposure. Attachment strength and tendency to form aggregations of small and medium mussels increased in the presence of roach. Medium mussels exposed to perch were also more strongly attached than those in the control treatment (no predators). Large mussels were unaffected by any predators. Predators did not affect horizontal distances moved by all sizes of mussels. Small individuals exposed to roach reduced their upward movement. In the most cases, the lack of responses to perch (a non-molluscivore) ruled out any potential influences of fish movements or faeces upon mussels, suggesting an effect of predator kairomones. The exception was the attachment of medium individuals which also responded to the presence of perch. In this case, responses to other stimuli, e.g. fish movements, could be involved.