Danube
The Danube was once a long-standing frontier of the Roman Empire, and today flows through 10 countries, more than any other river in the world. Originating in Germany, the Danube flows southeast for , passing through or bordering Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine before draining into the Black Sea. Its drainage basin extends into nine more countries. The Danube river basin is home to fish species such as pike, zander, huchen, Wels catfish, burbot and tench. It is also home to a large diversity of carp and sturgeon, as well as salmon and trout. A few species of euryhaline fish, such as European seabass, mullet, and eel, inhabit the Danube delta and the lower portion of the river.
Dikerogammarus villosus
"D. villosus" was originally found in the lower courses of large rivers in the Black Sea and Caspian Sea drainage basins. It has become an invasive species across central and western Europe, using the Danube river and its tributaries in its expansion. It is thought to have first escaped from the Danube in 1992 when the Rhine–Main–Danube Canal was opened and has since spread to nearly all the major rivers in western Europe, including the Rhône, Loire, Seine, Moselle, Meuse, Rhine and Main, as well as the Baltic Sea. It is unknown how the species is dispersed but it is probably related to shipping activity. It spread quickly through western Europe; found in the Rhine at the German–Dutch border (1995), the canals and rivers of northern Germany (1998), the Baltic Szczecin Lagoon (2001), the Moselle (2001), the Netherlands (2002), Lake Constance (2003), Lake Leman, the Rhine in France (2003), the (2003) and Lake Garda (2003). Its spread is thought to be related to the previous introduction of the zebra mussel, "Dreissena polymorpha" which it has evolved alongside. In September 2010, it was found in Grafham Water in Cambridgeshire, the first report of the species in the United Kingdom and it was found in Wales in November 2010.