Are the two species easily distinguishable?
Sometimes the species are hard to tell apart due to considerable phenotypic plasticity of all morphological characteristics in dreissenid species that may be a result of environmental factors, meaning the same genotype may express different phenotypes to suit environmental conditions (Claxton et al., 1998). Dreissena rostriformis bugensis exhibits many different morphs; yet, there are several diagnostic features that aid in identification. They usually have dark concentric rings on the shell and are paler in color near the hinge. If quaggas are viewed from the front or from the ventral side, the valves are clearly asymmetrical; however, the zebra mussel shells are bilaterally symmetrical and join together in a midventral line (Domm et al., 1993).
