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,
Geography of the European Union the dichotomy between highland and mountainous Southern Europe and a vast, partially underwater, northern plain ranging from United Kingdom in the west to Poland in the east. These two halves are separated by the mountain chains of Pyrenees and Alps/Carpathians. The northern plains are delimited in the west by the Scandinavian mountains and the mountainous parts of the British Isles. Major shallow water bodies submerging parts of the northern plains are the Celtic Sea, the North Sea, the Baltic Sea complex, and the Barents Sea. The northern plain contains the old geological continent of Baltica, and so may be regarded