Species status and population structure of mussels (Mollusca: Bivalvia: Mytilus spp.) in the Wadden Sea of Lower Saxony (Germany)

Three species of mussel (genus Mytilus) occur in Europe: M. edulis (Linnaeus 1758), M. galloprovincialis (Lamarck 1819) and M. trossulus (Gould, Boston Society of Natural History 3: 343–348, 1850). Although these species are indigenous to the North Sea, the Mediterranean and the Baltic Sea, respectively, they form an extended patchy species complex along the coasts of Europe (“the Mytilus edulis complex”) and are able to hybridize where their distributions overlap. Recent studies examining the taxonomic status and genetic composition of Mytilus populations in the Netherlands and the British Isles have revealed introgressive hybridization processes within this species complex, with hints of an invasion of nonindigenous M. galloprovincialis into the North Sea. Furthermore, an extensive international mussel fishery industry in Europe (i.e., Great Britain, the Netherlands, Denmark, and Germany) is also in discussion for a possibly anthropogenically induced bioinvasion of nonindigenous Mytilus traits into the Wadden Sea area. Although it is assumed that the Wadden Sea of Germany comprises M. edulis only, this has never been confirmed in a molecular genetic study. To assess the situation for the Wadden Sea of Lower Saxony, we conducted the first molecular study of the Mytilus genus in the region. Taxonomic identification of 504 mussels from 13 intertidal mussel banks using the nDNA marker Me15/16 revealed a population composition of 99% M. edulis and 1% M. edulis X M. galloprovincialis hybrids. Hence, the Wadden Sea population is unaffected by range expansion of nonindigenous Mytilus traits. The genetic structure of the M. edulis populations was investigated using the phylogenetic and population genetics analyses of the mitochondrial DNA cytochrome-c-oxidase subunit I (COI) and the first variable domain of the control region (VD1), which were sequenced for >120 female individuals. These results showed a heterogeneous, panmictic population due to unrestricted gene flow. This can be attributed to extensive larval dispersal linked to the tidal circulation system in the back barrier basins of the Wadden Sea.