Dikerogammarus villosus (Sowinsky, 1894) (Crustacea, Amphipoda) enters Vistula – the biggest river in the Baltic basin

Dikerogammarus villosus (Sowinsky, 1894) has been recorded for the first time in the Vistula River, the biggest river in Poland and in the whole Baltic drainage system. Until now the species has been found in the Bug River that is a tributary of the Vistula. It has migrated there from the Ponto-Caspian basin through the central invasion corridor (namely Pripet-Bug canal). Another population of that species has been found in the Oder River. It came there via the Mittelland Canal from the Rhine River that had been colonised from the Black Sea basin through the southern invasion corridor. In the Vistula River, D. villosus co-occurs only with other PontoCaspian gammarids.

The Vistula River (Figure 1) is one of the major European rivers, with a length of 1047 km and a drainage area of 194,400 km 2 of which 168,699 km 2 lies within Poland (over half the area of the country).The mean flow of the river at the mouth is 1080 m 3 /s.The river empties into the Baltic Sea in the Gulf of Gdansk.Navigable for barges in its lowers course, two big artificial reservoirs were constructed on the river, Goczałkowicki (in its upper course) and Włocławski (in the middle course).The Vistula River is part of the European central invasion corridor as it is naturally connected to the Bug (and further by artificial canal to Pripet and Dnieper rivers) (Bij de Vaate et al. 2002;Galil et al. 2007).Another artificial canal (Bydgoski Canal) joins the Vistula to the Noteć and Oder rivers (and consequently to the North Sea basin).
There are few detailed studies on the native freshwater gammarids of the Vistula River.The earliest note of any gammarid species inhabiting this river is that of Seligo (1920) who observed Gammarus pulex (Linnaeus, 1758) in the lower Vistula River.Gammarus fossarum Koch in Panzer, 1836 was observed upstream of Cracov (Konopacka 2004) and in numerous small affluents (Jażdżewski and Konopacka 1995).The Vistula River is inhabited by several other invasive species.One of the earliest invaders is the Balkan species Gammarus roeselii Gervais, 1835 that has been present only in the lower part of the river, in the closest vicinity of some affluents.North American Gammarus tigrinus Sexton, 1939 and Ponto-Caspian Obesogammarus crassus (G.O.Sars, 1894) inhabit only the brackishwater parts of the Vistula deltaic system.Chaetogammarus ischnus (Stebbing, 1899), the oldest Ponto-Caspian coloniser, is present in the middle and lower sections of the river, accompanied by dominating Dikerogammarus haemobaphes (Eichwald, 1841), and in the more lentic parts by Pontogammarus robustoides (G.O.Sars, 1894).From there it has colonised the entire flow of this river in Poland (Grabowski et al. 2007).The penetration of D. villosus into the Oder basin is especially interesting since it has used first the so-called southern corridor, i.e.Danube River, for westward range expansion (Bij de Vaate et al. 2002).In 2003 this species was discovered in the Bug River in Poland (Konopacka 2004), where it migrated along the central migration corridor, through the Pripet-Bug connection.From the date of D. villosus discovery in that river, its most westward locality in the Vistula basin was the Zegrzyński Reservoir, constructed on the junction of the Bug and Narew rivers near their mouth to the Vistula.
Our recurrent monitoring (samplings twice a year since 2003) in the Vistula River discovered the species in the autumn of 2007, where it was found in that river near Wyszogród (Figure 1).This locality (Figures 3, 4) is 60 kilometres downstream from the previous findings of the species.The species, second in abundance, was accompanied only by three other Ponto-Caspian gammarids: the most numerous D. haemobaphes, and scarce Chateogammarus ischnus and Pontogammarus robustoides (Annex 1).Looking at the examples of other big European rivers, as Rhine (Dick and Platvoet 2000;van Riel et al. 2006), Meuse (Devin et al. 2001(Devin et al. , 2003;;Bollache et al. 2004), Danube (Pöckl 2006) and Oder (own unpublished data), we may expect fast colonisation of almost entire Vistula's flow by the species and drastic deterioration of benthic communities due to the very high predatory abilities of D. villosus.
Annex 1.Records of gammarid species in the Vistula River in Wyszogród.The relative abundance in the sample is presented, based on effort unit (2 people/hour) (Jażdżewski et al. 2004;Grabowski et al. 2006)

Figure 1 .
Figure 1.Study site and main waterways of the central invasion corridor in Poland.