Ecology of host parasite relationships in the Baltic Sea.

The Baltic Sea is one of the greatest brackish waters, and for 7000 years it has been affected by the influx of haline waters from the North Sea. Many marine and some fresh water organisms have adapted to its lower salinity. Some organisms, referred to as genuine brackish water species, predominate over their near relatives in this environment. All these elements are also present among parasites, which differ in the degree of specialization to their hosts. Analyses of parasite communities, especially those of small fish hosts, reveal characteristics which differ from communities in the marine milieu: (a) suspension of specificity, (b) new hosts, (c) reduction in the number of hosts in life cycles, and (d) adaptation to genuine brackish water hosts. The view of the Baltic Sea as a species-poor but balanced ecosystem is also valid for parasites. The Baltic Sea is endangered by the high level of eutrophication, as the organisms are not adapted to its consequences. The study of parasite communities reveals such threatening conditions for the respective environments.