Does differential predation explain the replacement of zebra by quagga mussels?

We examined the hypothesis that direct predation is part of the explanation for the displacement of zebra (Dreissena polymorpha) by quagga mussels (Dreissena rostriformis bugensis) in North America. We investigated selectivity and feeding rates by 3 predators (Round Goby, Pumpkinseed Sunfish, rusty crayfish) using 2 or 3 size classes of predators feeding on different size classes of both mussel species. Round Goby, the predator with a long evolutionary history with dreissenids, consumed a larger range of dreissenid sizes than the other 2 predators. Pumpkinseed Sunfish did not show a preference for either mussel species, but both rusty crayfish and gobies preferred quagga mussels over zebra mussels. The size of mussels vulnerable to these predators increased with predator size, but all predators preferred the smallest mussels (4-8 mm). Medium (105-125 mm) and large (135-155 mm) Round Gobies consumed the largest size class (16-20 mm) of quagga mussels, but only large Round Gobies were able to consume 16-20-mm zebra mussels. Crayfish consumed a higher number of quagga than zebra mussels in both the 8-12-mm and 12-16-mm size classes. Large Pumpkinseed Sunfish (170 185 mm) were able to consume the largest size class of quagga but not zebra mussels. Our results do not support direct predation as the cause for quagga mussels replacing zebra mussels. However, predation may affect mussel size structure because small mussels were more vulnerable to predation than larger mussels.