Shell morphological versus genetic identification of quagga mussel (Dreissena bugensis) and zebra mussel (Dreissena polymorpha)

Herein we provide first genetic evidence that Dreissena bugensis Andrusov, 1897 has reached the German Danube system. The occurrence of this species at several sampling sites along a 152 km river stretch of the upper Danube River thereby proved to be wrongly estimated by using solely shell morphology features for species identification. 17.5% of the dreissenid specimens (Dreissena bugensis, N=26, Dreissena polymorpha, N=34), identified according to exterior shell characteristics, were assigned to the wrong species. In contrast to RFLP identification, comparisons of shell dimension measurements by discriminant function analysis were not able to fully identify the two species, with correct classification rates between 61.1% and 94.4%, confirming their high morphological plasticity. The results of this study suggest that species records of dreissenid mussels should generally be genetically verified.