Crustaceans in the History of the Baltic Sea
The Baltic Sea has a relatively short history. Yet nearly 20 thousand years ago, its present position was occupied by a huge ice sheet. After the glacial period it went through a series of phases, in which freshwater and salt water mingled together. These changes were the reason why the present Baltic Sea ultimately became a brackish water body. As a consequence of all those transformations and processes, species of both marine and freshwater origin now live in the Baltic. About 10 thousand years ago, when the Baltic Lake merged with the North Sea, many crustacean species were able to swim into the then sea, the so-called Yoldia Sea. Some of these relict species from that arctic period still inhabit the Baltic (Fig. 1.1).