Three representatives of the mussel genus Mytilus are found along the European coasts. The Mediterranean Sea and parts of the Spanish and French Atlantic coasts are the habitat of M. galloprovincialis. The Baltic Sea is considered the refuge of M. trossulus. The best-known representative, M. edulis, colonises the majority of European coasts and seas: from the Atlantic, via the North Sea, to the European North Sea. In habitats with sympatric occurrences of these mussel taxa, hybrid zones very frequently develop.

Despite numerous phylo- and population-genetic studies of European Mytilus populations, the molecular-genetic status of the blue mussel in the Wadden Sea of Lower Saxony is completely unknown. For this reason, the aim of my work is a first molecular-taxonomic determination of the species composition of Mytilus populations. For this purpose, a taxonomic differentiation was carried out on the basis of species-specific fragment lengths of the sequence of the adhesive protein of the byssus filaments (according to Inoue et al. 1995).

Furthermore, the relationships of the M. edulis population in Lower Saxony were analysed in detail for the first time using a variety of phylogenetic and population genetic analysis methods. Based on two mitochondrial sequence fragments (cytochrome c oxidase subunit 1 and a section of the control region), maximum likelihood haplotype phylogenetic trees (Figure 1) and haplotype networks (Figure 2) were created, and an analysis of molecular variance (AMOVA) was carried out.

A total of 13 mussel beds (Figure 3) were sampled between March and July 2009 in the back tidal flats of the Lower Saxon Wadden Sea National Park. The sampling area extends from the mussel beds off Borkum to the Wurster coast beyond the Weser estuary and thus covers almost the entire length of the coastal waters of Lower Saxony.

As a result of this study, I was able to identify M. edulis / galloprovincialis hybrids within mussel populations in Lower Saxony for the first time. At 1% hybrids versus 99% M. edulis, the percentage is very low and corresponds to the results of Dutch studies (Luttikhuizen et al. 2002, Kijewski et al. 2009). The phylo- and population genetic analyses reveal a panmictic mussel population whose unrestricted gene flow is most likely due to the dynamic flow regime in the Wadden Sea and the associated planktonic larval phase within the water column.

Project staff: Georg Steinert, Olaf Bininda-Emonds
Funding: EWE Foundation

Three species of the blue mussel, Mytilus, occur in the North Sea region: M. edulis, M. galloprovincialis and M. trossulus. Hybridisation is likely to occur in areas where their distributions overlap. The blue mussel(M. edulis) is supposed to be the indigenous species in the Lower Saxonian Wadden Sea. Recent studies examined the taxonomic status and genetic composition of Mytilus populations in the Netherlands, Great Britain and the Baltic Sea in general. However, to my knowledge comparable genetic studies in the German Wadden Sea of this genus were never carried out.

A total of 504 mussels from 15 sample areas were assayed for the Nuclear DNA gentic marker Me 15/16. I analysed the mussels genetic structure using phylo- and populationgenetic methods. Therefore two fragments of maternal transmitted mitochondrial DNA, comprising parts of cytochrome c oxidase subunit one and the first variable domain of the control region were sequenced from >120 individuals. The mussels were identified as 99% M. edulis and 1% M. edulis / galloprovincialis hybrids composed of two mixed genotypes. Overall, genetic composition results showed a heterogeneous population structure due to unrestricted gene flow within the sampling area.

This is the first record of the presence of hybrid Mytilus mussels in the North Saxonian Wadden Sea. Additionally, this is the first genetic demonstration of a M. edulis panmictic population structure of the North Saxonian intertidal. The observed heterogenous population can be attributed to extensive larval dispersal linked to the prevailing hydrodynamic conditions in the intertidal back barrier basins of the Wadden Sea.

Project participants: Gerog Steinert, Olaf Bininda-Emonds
Funding: EWE-Foundation

Inoue, K., J.H. Waite, M. Matsuoka, S. Odo, S. Harayama (1995). Interspecific variations in adhesive protein gene squencies of Mytilus edulis, M. galloprovincialis and M. trossulus. Biological Bulletin 189:370-375.

Kijewski, T., J. W. M. Wijsman, et al. (2009). Genetic composition of cultured and wild mussels Mytilus from the Netherlands and transfers from Ireland and Great Britain. Aquaculture 287(3-4): 292-296.

Luttikhuizen, P.C., A. Koolhaas, et al. (2002). Mytilus galloprovincialis-type foot-protein-1 alleles occur at low frequency among mussels in the Dutch Wadden Sea. Journal of Sea Research 48(3): 241-245.