Genetic affinities of the bivalve Macoma balthica from the Pacific coast of North America: evidence for recent introduction and historical distribution

The tellinid bivalve Macoma balthica (L.) has an extensive geographic range that reaches from temperate to arctic coastal waters in the North Atlantic and North Pacific oceans. Recent studies have indicated that eastern and western North Atlantic populations are morphologically and genetically different from one another, and that they may have diverged as sibling species. To determine the genetic relationship between M. balthica from the Pacific and Atlantic coasts of North America, populations from each coast were examined at 11 enzyme loci using standard starch gel electrophoresis. Allele frequency data indicate that M. balthica populations from San Francisco Bay, California appear more closely related to western North Atlantic populations than to populations from Oregon. We suggest that San Francisco Bay populations were introduced relatively recently from western North Atlantic populations. The Oregon populations are probably a natural extension of northern populations that occur along Northern Asia and in the eastern North Atlantic.