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,
Ecosystem of the North Pacific Subtropical Gyre deeper layers of the NPSG, species higher up on the food chain will migrate vertically or horizontally within or in and out of the gyre. Based on analyses of the zooplankton community, the Central North Pacific has a high species diversity (or high number of species) and high equitability (meaning relatively equal numbers of each exist). There is also a low degree of seasonal variability of densities of zooplankton. Studies of mesopelagic fishes of central subtropical waters are scarce. The few studies that do exist found that mesopelagic fish species are not uniformly distributed throughout the subtropical Pacific Ocean. Their