Living in a sea of exotics — the Baltic case

The brackish Baltic Sea is known to function as a bridgehead for the spread of non-indigenous species (NIS) between the Eastern and Western Hemispheres and, in an ecological sense, between freshwater and the sea. The recent basin-wide invasion of the predatory fishhook water flea Cercopagis pengoi and the North American bristle worm Marenzelleria viridis in the Baltic are the latest examples of 〉100 introductions recorded over the last 200 years. A great majority of them are found in the most diluted coastal lagoons and estuaries only, these hot spot sites being known as centers of xenodiversity. The most important source areas for NIS have been the western European waters, the Atlantic coast of North America and the Ponto-Caspian realm.