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,
Geology of the Baltic Sea on the Baltic water body became the Yoldia Sea. About 10,700 years ago the continued land rise separed the Baltic water body from the ocean again and Yoldia Sea turned into the Ancylus Lake. This lake lasted until 10,000 years before present when a new connection to the ocean was established, this time in the Danish straits forming the Littorina Sea. Land rise did not close off Littorina sea but it made its connection the ocean over time more shallow allowing less salt water to enter it, thus gradually it evolved into the current Baltic Sea with somewhat brackish water.