The sharp-angled (presence of an acute ridge) or rounded (absence of an acute ridge) transition of the ventral and dorsal shell surfaces is the “key feature” for the distinction of Dreissena polymorpha and Dreissena rostriformis. However, up to now it has not been possible to put this into a quantifiable framework. Therefore, the aim of this study was to develop a method, which (i) facilitates the quantification of this feature as a morphometric parameter and (ii) to test whether the presence or absence of an acute ridge can be used to distinguish the two species both in mathematical terms and under field conditions