The invasive quagga mussel Dreissena rostriformis bugensis (Andrusov, 1879) (Bivalvia: Dreissenidae) in the Dutch Haringvliet, an enclosed freshwater Rhine-Meuse estuary, the westernmost record for Europe

Here the westernmost record in Europe of the invasive quagga mussel Dreissena rostriformis bugensis is discussed, i.e. in the Haringvliet, an enclosed freshwater Rhine-Meuse estuary in The Netherlands. In 2006 and 2007 small numbers of quagga mussels were found in between large numbers of zebra mussels, Dreissena polymorpha. They were recorded during the periodical analyses of settlement plates of the international biofouling monitoring project SETL. Identifications were done on the basis of morphology and confirmed by DNA-barcoding techniques. The quagga mussels show a patchy distribution in the Dutch Haringvliet and Hollands diep. No settlement of quagga and zebra mussels was recorded between the second half of March to the first half of June both in 2006 and 2007. The main settlement period of quagga and zebra mussels in the research area is concluded to be from the second half of June until the end of December. The calculated maximum shell growth rates of both invasive mussel species are similar to somewhat higher than those recorded in literature, i.e. 0.077 mm/day to 0.141 mm/day