Fauna of Europe of the Baltic Sea has led to the evolution of many slightly divergent species, such as the Baltic Sea herring, which is a smaller variant of the Atlantic herring. The most frequent benthic species are "Saduria entomon" and "Monoporeia affinis", which is originally a freshwater species. A great part of its bottom is anoxic and without animal life. The Baltic Sea and North Sea are also home to a variety of marine mammals (common seals, grey seals). Europe contains several important freshwater ecoregions, including the heavily developed rivers of Europe, the rivers of Russia, which flow into the Arctic, Baltic,
History of the North Sea mixing with the native Celtic populations. The Viking Age began in 793 and for the next two centuries the Vikings ruled the North Sea. From the Middle Ages until the end of the 15th century, before the development of good roads, maritime trade on the North Sea connected the economies of northern Europe, Britain, and Scandinavia with each other as well as with the Baltic and the Mediterranean. The Netherland low countries and German speaking lands were the central hub for English Channel and North Sea coastal trade and organizations such as the Hanseatic League, aimed at protecting interests of