POPULATION-GENETICS OF THE HARD CLAM, MERCENARIA-MERCENARIA, AT THE NORTHERN LIMIT OF ITS RANGE

Canadian populations of Mercenaria deserve recognition as stocks distinct from the larger population of the U.S. Atlantic coast. Although the hard clam occupies a virtually continuous range from Florida to Massachusetts, its distribution north of Cape Cod becomes disjunct. Here, we use protein electrophoresis to determine gene frequencies a seven polymorphic enzyme loci in clam populations from Maine, USA, and New Brunswick and Prince Edward Island, Canada, and compare our results with previously published data from Massachusetts. The fit to the Hardy-Weinberg expectation within populations was very good. The Maine population showed small but statistically significant divergence from its putative source population to the south at most loci, with an apparent loss of two rare alleles. Both Canadian populations showed larger levels of divergence, with the loss of 6-12 alleles and significant reductions in overall heterozygosity. The recognition of a St. Lawrence stock of hard clams at Prince Edward Island may have important implications for the fishery.