The Baltic Sea is an evolutionary young sea that have developed a low salinity in its water from the fresh water that flows from the north and saltwater that flows from the south of the sea. The low salinity is too low for many marine organisms and too high for many freshwater organisms. Species like the blue mussel, which have adapted to the low salinity, may have developed different protein expression as a result. To study which protein that have been expressed in the organism proteome analysis is often used. 2-dimensional electrophoresis may be the only method that can do this kind of analysis.