Predator-induced morphological defences in two invasive dreissenid mussels: implications for species replacement

Induced morphological defences as a response to chemical cues from predators are common in aquatic systems. We evaluated predator-induced defences and the costs of these responses measured as growth rate reductions in two invasive mussel species. One mussel species (quagga mussel, Dreissena rostriformis bugensis) often dominates the other (zebra mussel, Dreissena polymorpha) in lakes where they co-occur. We exposed both species to cues from three important North American predators: round goby (Neogobius melanostomus), which is native in the ancestral range of the mussels, and pumpkinseed sunfish (Lepomis gibbosus) and rusty crayfish (Orconectes rusticus) that are native in North America. Both mussel species responded to risk cues by increasing shell thickness and decreasing growth rates. The response to round goby was smaller than the response to the two new predators. The strength of the induced response was negatively correlated with growth in both mussel species and both the strength of the induced defence and the decrease in growth were larger for zebra mussels than for quagga mussels. Survival was higher in zebra mussels than in quagga mussels, but there was no significant difference in survival among different risk cue treatments. The lower response and correlated faster growth of quagga mussels than zebra mussels in response to risk cues may be an additional reason for the replacement of zebra mussels by quagga mussels in North America, at least as long as native predators are not a major source of mortality. If true, we predict that both mussel species will co-occur after the exotic round goby becomes a dominant mussel predator in North America.