During the early 1990s, 2 Eurasian macrofouling mollusks, the zebra mussel Dreissena polymorpha and the quagga mussel D. bugensis, colonized the freshwater section of the St. Lawrence River and decimated native mussel populations through competitive interference. For several years, zebra mussels dominated molluscan biomass in the river; however, quagga mussels have increased in abundance and are apparently displacing zebra mussels from the Soulanges Canal, west of the Island of Montreal. The ratio of quagga mussel biomass to zebra mussel biomass on the canal wall is correlated with depth, and quagga mussels constitute >99% of dreissenid biomass on bottom sediments. This dominance shift did not substantially affect the total dreissenid biomass, which has remained at 3 to 5 kg fresh mass /m2 on the canal walls for nearly a decade. The mechanism for this shift is unknown, but may be related to a greater bioenergetic efficiency for quaggas, which attained larger shell sizes than zebra mussels at all depths. Similar events have occurred in the lower Great Lakes where zebra mussels once dominated littoral macroinvertebrate biomass, demonstrating that a well-established and prolific invader can be replaced by another introduced species without prior extinction.
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