Northern circumpolar distribution of brackish-water plathelminths

The composition of brackish-water faunas on both sides of North America shows a high degree of similarity with the corresponding faunas in European brackish waters in terms of species of the Plathelminthes. Twenty species are common to both the Atlantic coast of Canada and Northern Europe; a further nine brackish-water plathelminths are common to the Pacific coast of Alaska and Europe; and two species occur in all three areas. These patterns of distribution must be regarded as the result of past or present connections between the American and European populations of brackish-water species. The most parsimonious explanation is the hypothesis that a species-rich community of brackish-water plathelminths has a northern circumpolar distribution.