323 Abstract A detailed account is given as to when and how alien crustaceans have entered the Baltic Sea, with special emphasis on the regions to south and west. The biogeographic origin of the species, their vectors and arrival pathways, as well as ecological and economic impacts are discussed. Altogether 19 alien crustacean species were hitherto recorded in the south-western Baltic Sea; of which 8 are of Ponto-Caspian and 4 of North American origins. The impact of these newcomers upon the Baltic ecosystem has not been perceived as significant. However, in some lagoons and estuaries, the rapid increase of population abundance in some species did cause disturbances. 1 Introduction The Baltic Sea, a semi-enclosed brackish basin, at most 12,000 years old, it is a relatively new body of water. Its history encompassed freshwater and polyhaline brackish periods leading to the present day phase of mesohaline/oligohaline basin. This latter phase has started no earlier than 3,000 years ago (Segerstråle 1957; Olenin and Leppäkoski 1999). With a surface area including Kattegat of about 412,000 km 2 , the Baltic Sea is among the world’s largest brackish water bodies. Its catchment area is over 1,700,000 km 2 and more than 200 rivers discharge their waters into the sea. The Vistula and Oder in the south and the Nemunas and Dvina (Daugava) in the southeast are the largest rivers in the Baltic drainage system. The combination of a tide-less shallow basin with an average depth of ca. 60 m and a limited exchange of seawater with the NE Atlantic, results in a surface salinity decreasing from ca. 20 psu at the south-western end to some 2 psu in eastern and northern regions of the Gulf of Finland and the Gulf of Bothnia (Fig. 1). Below locally, K. Jażdżewski (*) and M. Grabowski Laboratory of Biogeography and Invertebrate Ecology, Department of Invertebrate Zoology and Hydrobiology, University of Lodz, Banacha 12/16, 90-237 Lodz, Poland e-mail: kryjaz@biol.uni.lodz.pl; michalg@biol.uni.lodz.pl Alien Crustaceans Along the Southern and Western Baltic Sea Krzysztof Jażdżewski and Michal Grabowski B.S. Galil et al. (eds.), In the Wrong Place - Alien Marine Crustaceans: Distribution, Biology and Impacts, Invading Nature - Springer Series in Invasion Ecology 6, DOI 10.1007/978-94-007-0591-3_11, © Springer Science+Business Media B.V. 2011