Genetic evidence supports recolonisation by Mya arenaria of western Europe from North America

Abstract

Mya arenaria (L.), the softshell clam, is currently widespread on the east and west coasts of North America. This bivalve also occurs on western European shores where the post Pleistocene origin of the species, whether introduced or relict, has been debated. In this study 320 M. arenaria from eight locations in Europe and North America were collected. Eightyfour clams from seven of the locations were examined for mitochondrial DNA variation by sequencing a section of the cytochrome oxidase I (COXI) gene. These were analysed together with 212 sequences, sourced from Genbank, from the same gene from 12 additional locations, chiefly from eastern North America but one site each from western North America and one from western Europe. Ten microsatellite loci were also investigated in all 320 clams. Nuclear markers showed reduced levels of variation in certain European samples. The same common COXI haplotypes and microsatellite alleles were present throughout the range of M. arenaria, though significant differences were identified in haplotypic and allelic composition between many samples, particularly those from the two continents, Europe and North America. These findings support the hypothesis of post Pleistocene colonisation of European shores from eastern North America (and the recorded human transfer of clams from the east to the west coast of North America in the 19 th Century).