The cockle Cerastoderma edule at Northeast Atlantic shores: genetic signatures of glacial refugia

The cockle Cerastoderma (Cardium) edule (L.) ranges from NW Africa to northern Scandinavia. Abundance in shallow coastal sediment is often high, and it attracts commercial harvest. In this study, a complex genetic pattern has been revealed by mitochondrial DNA in 383 individuals from 19 sampling sites. Parsimony network analysis of 79 haplotypes identified two dominant central haplotypes separated by low divergence. One is characteristic for a homogenous southwestern group of populations from Africa to the British Isles. The other is characteristic for a heterogeneous northern group with a deviant Arctic population. At the entrance of the Baltic Sea, a mixture zone of the dominant haplotypes was found. The estimated population expansion time for the northern haplotype group predates that of the southwestern one, suggesting northern glacial refugia and a subsequent southern expansion of C. edule populations. Introduction Quaternary glaciation cycles and corresponding extinctions, contractions into glacial refugia and interglacial expansions of individual species have laid the foundation for present species assemblages at temperate and high latitudes (Hewitt 2000; Martin and McKay 2004; Hoffmann and Sgrò 2011). Large-scale population genetic analysis may detect signatures of these developments. In the simplest scenario with a single glacial refugium below the glaciated area, a linear decrease in genetic diversity toward higher latitudes can be expected. In the Northeast Atlantic, there are, however, several examples where marine species do not conform to this. Two species of seagrass, for example, show highest allelic diversity in the North Sea region (Coyer et al. 2004; Olsen et al. 2004). Several species of benthic macroalgae and invertebrates seem to have survived glaciations in periglacial or cryptic northern refugia (Roman and Palumbi 2004; Hoarau et al. 2007; Maggs et al. 2008; Luttikhuizen et al. 2008; Olsen et al. 2010; Coyer et al. 2011). In benthic bivalves, no clear evidence for northern glacial refugia has been found so far. However, trans-Arctic pathways from Pacific coasts and dispersal across the northern Atlantic have been inferred from genetic analyses for bivalves. Notable examples are the circumpolar Macoma balthica complex (Luttikhuizen et al. 2003; Nikula et al. 2007) and the closely related mussels Mytilus edulis, M. trossulus and M. galloprovincialis (Riginos and Henzler 2008; Brannock and Hilbish 2010; Väinölä and Strelkov 2011). Patterns in the latter may have been further complicated by human-mediated translocations. Such pathways have been genetically reconstructed for various introduced alien species (e.g., Darling et al. 2008; Reitzel et al. 2008; Reusch et al. 2010; Lejeusne et al. 2011). In the lagoon Communicated by S. Uthicke. Electronic supplementary material The online version of this article (doi:10.1007/s00227-011-1802-8) contains supplementary material, which is available to authorized users. M. Krakau S. Jacobsen K. Reise (&) Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstr. 43, 25992 List, Germany e-mail: [email protected] K. T. Jensen Marine Ecology, Department of Biological Sciences, University of Aarhus, Ole Worms Allé 1, Bld. 1135, 8000 Aarhus, Denmark 123 Mar Biol (2012) 159:221–230 DOI 10.1007/s00227-011-1802-8