Quagga mussels Dreissena rostriformis bugensis (Andrusov, 1897) in the Main River (Germany)

The first record of the quagga mussel Dreissena rostriformis bugensis (Andrusov, 1897) in Germany is described. This species has expanded its distribution area in Europe at a slower rate than the zebra mussel Dreissena polymorpha (Pallas, 1771). Recent records from the Danube in Romania and from an enclosed Rhine-Meuse estuary in The Netherlands, suggest that the Main-Danube canal and River Rhine have functioned as the dispersal pathway of the quagga mussel to The Netherlands. The record of this species in the River Main supports this hypothesis.

The quagga mussel Dreissena rostriformis bugensis (Andrusov, 1897) was until early twenty-first century a slower invader than the zebra mussel Dreissena polymorpha (Pallas, 1771). Both mussel species originate from the Ponto-Caspian area. The original native range of the quagga mussel is the Lower Dniepr River and Southern Bug River (Son 2007). Over the last few decades, however, the quagga mussel has considerably extended its range in Russia (Orlova et al. 2003(Orlova et al. , 2004. It reached the River Volga via the Don River and Volga-Don canal (Zhulidov et al. 2004(Zhulidov et al. , 2005. Furthermore, it invaded the Great Lakes area of North America probably simultaneous with the zebra mussel (1987), most likely as larvae via ballast water discharge. The quagga mussel was recognized for the first time in the Great Lakes in 1991 (Spidle et al. 1994). D. rostriformis bugensis seems to have an ecology sim ilar to D . polymorpha, but often colonizes deeper water than D. polymorpha (Orlova et al. 2005). However, it is known that at places where D. polymorpha occurs and D. rostriformis bugensis becomes established the latter outcompetes slowly the first species (Orlova et al. 2004). Zhulidov et al. (2004) mentions some characteristics of D. rostriformis bugensis which seems to be important in this process, viz. a better ability than D. polymorpha to cope with low food conditions and high temperatures.
Recently, D. rostriformis bugensis was recorded by Micu and Telembici (2004) and Popa and Popa (2006) from the Danube, Romania. The neighbourhood of Drobeta Turnu-Severin was considered by these authors the most western recorded occurrence in Europe of this species. However, Molloy et al. (2007) found the species for the first time on the 19th of April 2006 in the Hollands Diep near Willemstad in The Netherlands, where it occurred in relatively low numbers between the zebra mussels (1% were quagga mussels). Molloy et al. (2007) suggested that the quagga mussel entered via the M ain-Danube canal, as did so many other Ponto-Caspian invaders (Bij de Vaate et al. 2002). These records in The Netherlands and Romania however, show an enormous geographical distance gap without any records in between. The invasion of quagga mussels in The Nether-lands could also be caused by the discharge of ballast water from the Black Sea area in the Rotterdam Port. However, it should be noted that, in the Port of Rotterdam itself we have only sampled zebra mussels thus far. During a trip to Hungary in 2007 we sampled along the River M ain (19th of May), the Main-Danube canal (25th of May), the Danube at various sampling sites (19th and 25th of May) in order to obtain dreissenid mussels for genetic analysis. To our surprise, in the River M ain east of W ürzburg near Hörblach, we found quagga mussels (D. rostriformis bugensis) (13 speci mens sampled) on stones up to a depth of 1 m besides zebra mussels (Dreissena polymorpha) (26 specimens sampled within half an hour) (Figure 1). During this sampling the River Main showed hardly any flow. Yellow water lilies (Nuphar lutea (L.) Smith), showing underwater leaves only, were present. Besides zebra mussels and quagga mussels we found the native snails A ncylus fluviatilis (Müller, 1774), Radix auricularia (L., 1758) and Bithynia tentaculata (L., 1758) on riparian stones at a water depth of 20 to 100 cm. Freshwater sponges were commonly present on dreissenid colonised stones and were even found overgrowing these mussels, in particular D. polymorpha. W ithin the sediment, Unio tumidus (Philipsson, 1788) was present together with numerous Asian clams Corbicula flum inea (Müller, 1774).
The finding of quagga mussels in the M ain makes it very likely that this mussel species took the southern corridor from the mouth of the Danube, upstream to the Main-Danube canal and subsequently colonized downstream in the Main, Rhine and Rhine delta (Figure 2). Because the largest quagga mussel we sampled had a length of 29.8 mm, the Main must certainly have been invaded several years before 2007. We also tried to sample this species from the Main-Danube 4 ____________ 9 10 mm
The shells o f the Quagga m ussels 5 and 6 show an atypical stripe pattern. The largest Quagga m ussel on the figure has a length of 25 mm.
Quagga mussels in the Main River  Orlova (2003) and Zhulidov et al. (2004Zhulidov et al. ( , 2005 (Popa and Popa 2006) and M ain, G erm any (this paper), see Annex for coordinates canal and the Danube. The following localities have been sampled: the Danube west o f Regens burg near Kiefenholz (Pfatter), Vilshofen an der Donau west of Passau (stones sampled at 1 m depth); M ain-Danube canal at Kelheim, Essing, Hilpoltstein (steep walls scraped by a scrape net to a depth of 3 m). However, in spite of our efforts to sample specimens of quagga mussels from the M ain-Danube canal as well as the Danube itself, we collected only zebra mussels from these localities.