Allozyme variation and gene flow between cockle Cerastoderma edule populations in southern United Kingdom

Cockles, Cerastoderma edule, were sampled from eight sites on the western coast (Isle of Man, Anglesey, Dee estuary, Dwyryd estuary and Burry Inlet) and the eastern coast (the Wash, Southend and East Barrow) of the UK. Genotypes were scored at nine polymorphic allozyme loci. Significant deficiencies of heterozygotes were noted al several loci in most populations and these were possibly caused by the presence of null alleles, or by selection against heterozygotes, at these loci, Eastern populations were genetically homogeneous at all but one locus (Odh) while western populations showed significant allele frequency variation at seven of the nine loci. The major contributor to the variation in the western populations was the Bully inlet population and its exclusion from the group reduced the significant allele frequency differences to a single locus (Gsr). The estimated average number of migrants per generation between populations was 39.94 (range = 22.5-83.1), The results provide little or no support for the existence of genetically isolated beds of cockles, except perhaps at Burry Inlet, Wales, but it is argued that allozyme data showing relatively limited genetic heterogeneity do not prove that gene flow between populations is a common occurrence.