A genetic and morphological comparison of shallow- and deep-water populations of the introduced dreissenid bivalve Dreissena bugensis

The discovery of a morphologically distinct dreissenid mussel in the profundal zone of Lake Erie suggests the presence of either a third dreissenid mussel species in the Great Lakes or a previously unknown morphological phenotype of an existing dreissenid species. We examined the morphometrics and molecular systematics of the zebra mussel (Dreissena polymorpha) and the profundal and epilimnetic forms of the quagga mussel (Dreissena bugensis) from Lakes Erie and Ontario. In an attempt to resolve the taxonomic status of the profundal form of the quagga mussel, we sequenced a 710 base pair fragment of the cytochrome oxidase subunit I mitochondrial gene of the two forms of the quagga mussel. No nucleotide differences were found, supporting the hypothesis that the profundal form of the quagga mussel is a phenotype of D. bugensis, not a separate species. In contrast, the second and third principal component scores from an analysis of the morphological variables shell length, shell width, shell height, and shell mass separated the epilimnetic and profundal forms of the quagga mussel into two groups, but grouped zebra mussels from all depths together. The most parsimonious explanation for our results is that D. bugensis shows plasticity in shell morphology with respect to depth, whereas D. polymorpha does not.