title: Mytilus trossulusin Northern Europe.
abstract: From data on allozyme, nuclear DNA and mitochondrial DNA markers, we show that the originally North Pacific/Northwest Atlantic musselMytilus trossulusis widespread on North European coasts, earliM. trossuluser thought to be inhabited only byMytilus edulis. Several local occurrences of , interspersed with a dominantM. edulis, were recorded on the North Sea, Norwegian Sea and Barents Sea coasts of Norway and the Barents and White Sea coasts of Kola Peninsula in Russia. The proportion ofM. trossulusgenetic background observed at any one site varied from 0 to 95%. These new occurrences are not related to the previously known, introgressedM. trossuluspopulation that occupies the Baltic Sea. The new northern occurrences retain both the F and MM. trossulusmitochondria, which have been lost from the Baltic stock. While hybridization takes place whereverM. trossulusandM. edulismeet, the extent of hybrization varies between the different contact areas. Hybrids are rare, and the hybrid zones are bimodal in the northern areas; more interbreeding has taken place further south in Norway, but even there genotypic disequilibria are higher than those in the steep transition zone between the Baltic mussel andM. edulis: there is no evidence of a collapse toward a hybrid swarm unlike in the Baltic. The Barents and White SeaM. trossulusare genetically slightly closer to the NW Atlantic than NE Pacific populations, while the Baltic mussel has unique features distinguishing it from the others. We postulate that the presence ofM. trossulusin Northern Europe is a result of repeated independent inter- or transoceanic cryptic invasions of various ages, up to recent times.
authors.affiliation: Finnish Museum of Natural History, University of Helsinki, POB 17, 00014 Helsinki, Finland.
authors.forename: Risto
authors.initials: R
authors.lastname: Väinölä
authors.affiliation: Finnish Museum of Natural History, University of Helsinki, POB 17, 00014 Helsinki, Finland ; Department of Ichthyology and Hydrobiology, St Petersburg State University, 16 Line, 29, Vasilevsky Island, St Petersburg, 199178 Russia.
authors.forename: Petr
authors.initials: P
authors.lastname: Strelkov