Discovery of Dreissena rostriformis bugensis (Andrusov 1897) in Western Europe

Live individuals of Dreissena rostriformis bugensis were found in a trawlnet sample taken in April 2006 near Willemstad in the Hollands Diep, part of the main distributary in the Rhine delta in The Netherlands. This represented the first record of this species in Western Europe and a major westward range extension for this Ponto-Caspian species. Density was very low, with the ratio of Dreissena polymorpha to D. rostriformis bugensis in the order of 100:1. Mean length (n = 5) was 22.5 mm (range 20.4–24.6 mm), suggesting that they were at least two years old and that the original introduction into the Hollands Diep had occurred in 2004 or earlier. Species identity was confirmed by morphological and molecular methods. We speculate that the invasion occurred through the Main-Danube Canal – a pathway that is becoming increasingly recognized as a southern corridor for the potential movement of non-indigenous aquatic species between Eastern and Western Europe.