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After the initial opening of the Bering Strait  5.5–-5.0 My ago, there occurred several periods with exchange of marine species between the Pacific and Atlantic Oceans. The snow crab (Chionoecetes opilio) was first reported in the Barents Sea (BAR) in 1996, presumably following introduction via Ballast water. Since then, the population has increased in size and distribution and has become self-reproducing and of economic importance. We implemented genetic analysis to identify the origin and pathway of introduction of BAR snow crab. Samples from BAR, Alaska, eastern Canada, and west Greenland were genotyped with 12 microsatellite loci. Pairwise F ST between 15 samples showed three geographic clusters, each significantly different from one another. Unexpectedly, the largest genetic distance was between the BAR and Greenland clusters. This clustering may be the result of dispersals through Bering Strait from the North Pacific Ocean and passage through the Arctic Ocean. The BAR samples did not show signs of a bottleneck in population size, as would be expected after introduction of small numbers of individuals. Presence of snow crab in the BAR may thus be the result of recent natural range expansion from around Alaska. [ABSTRACT FROM AUTHOR]