Is a visuo-haptic differentiation of zebra mussel and quagga mussel based on a single external morphometric shell character possible?

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. We called the new parameter ‘angularity’ (An) and developed a method for its quantification. An ≤ 3.3 and An ≥ 4.7 were the discriminant thresholds for quagga mussels and zebra mussels, respectively. 3.3 < An < 4.7 did not allow for a clear distinction between species. Nevertheless, An is highly sensitive in morphometrically discriminating both species: more than 96% of the mussels were correctly classified whereas less than 1% were falsely classified. We conducted a visuo-haptic experiment in which we asked test persons to rate mussels according to their angularity (acute ridge present vs. absent). Remarkably, our results revealed that all participants were well able to distinguish the species at an error rate of 8.4 %. Nevertheless, even naïve persons who have no previous knowledge of dreissenid mussels can reliably select specimens of zebra and quagga mussels under field conditions based on the angularity alone.