Samples of Mytilus were collected at eight sites located in and around Bergen, Norway, and analysed by starch gel electrophoresis for the two highly polymorphic loci PGM* and PGI*. The genotype distribution and allele frequencies varied significantly among samples from the different locations. The variations were most significant between localities with full strength seawater and brackish water, and this difference was so large that it indicated the presence of two populations, possibly representing two species. The brackish water mussels may represent the species Mytilus trossulus, while the species Mytilus edulis may be distributed on the outer shores where salinity is normally around 30‰. Differential survival, as a result of specific adaptation to different salinities, may be the mechanism that maintains the populations (or species) and prevents gene flow between them.

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