A readily apparent characteristic of zebra mussels is their variability. They possess a wide range of color patterns as well as shell shapes. This variability is reflected in their genetic structure, as determined using protein electrophoresis. Zebra mussels have an average heterozygosity (a measure of within-species genetic variability) of 0.27-0.43 (Herbert et al. 1989, Marsden et al. 1995).
As zebra mussels spread throughout the Great Lakes and into inland waterways, it is possible that they established genetically differentiated subpopulations. This would have occurred if a new population comprised a sufficiently small number of individuals that did not contain all of the variations present in the source, or founder, population. If such differentiated populations exist, they may respond differently to environmental conditions or even to control measures. However, since zebra mussels likely invaded new areas in the Great Lakes watershed by ballast water transfer or drifting of veligers, most new populations probably consisted of a large number of individuals. In fact, an extensive survey of zebra mussel populations in the Great Lakes and adjacent river systems conducted in 1991-1992 indicated that these populations showed little if any differentiation (Marsden et al. 1995). Comparison of North American zebra mussel populations with zebra mussel populations from Russia, Germany, Poland, and Hungary indicated that little variation was lost when the zebra mussels were transported to North America. More than one European population likely contributed to the North American invasion.
Genetic studies also revealed the presence of a second dreissenid species in the Great Lakes. The first individual of this species was detected by accident among a collection of mussels from the Erie Canal; genetic data indicated that they were extremely different from zebra mussels (May and Marsden 1992). Examination of additional individuals revealed that the genetic distance (a measure of relatedness among populations) between these "new" mussels and zebra mussels was 1.215; the genetic distance between species is generally observed to be between 0.16 and 1.39. The new species was named the quagga mussel to distinguish it from the zebra mussel. Taxonomic work combined with genetic comparison of mussels from Russia resolved the identity of this mussel as Dreissena bugensis, native to the Bug-Dneiper estuary of the Black Sea (Spidle et al. 1994, Rosenberg and Ludyanskiy 1994). The quagga can be visually distinguished from the zebra mussel by the absence of a carina, or angle, in the shell. In cross section, a zebra mussel is approximately triangular, whereas the quagga mussel has a smoothly ovoid shape.
This photograph illustrates the noticeable differences between zebra and quagga mussels.
Quaggas also tend to have paler shells with finer lines than zebra mussels, but there is considerable variability in the color patterns of both species.
Quagga mussels are now widespread throughout Lake Erie and much of Lake Ontario, the Erie Canal, and the upper St. Lawrence River. Quagga mussels dominate the deeper waters to at least l00 m in large lakes, have lower thermal tolerances than zebra mussels, and are less tolerant of salinity (Spidle et al. 1995).
Zebra Mussel Identification