In a temporal comparison over 18 years, we documented changes in the position and structure of the North European blue mussel hybrid zone in the Öresund
strait, between Mytilus edulis of the marine Kattegat and Mytilus trossulus of the brackish Baltic Sea. In 1987 the midpoint of the 140-km wide multilocus allozyme cline in shallow-water populations was estimated to be located halfway
along the strait. In 2005, it was shifted 25 km towards the Baltic end of the Öresund, and was located near the fxed link (bridge) that was built across the strait meanwhile in the 1990s. The cline also appeared to have become narrower and the extent of hybridity among individuals decreased. Factors that theoretically can control the position and shape of a clinal hybrid zone involve environmental gradients between habitats that diferentially favor the two hybridizing taxa, or barriers to geographical dispersal of the organism. We
consider two alternative hypotheses to explain the movement of the mussel hybrid zone. (1) Environmental change related to climate warming: the more stenothermal M. trossulus was pushed out from the Öresund towards the cool Baltic by elevated temperatures. (2) Change of dispersal dynamics: the construction of the fxed link locally afected mussel dispersion which attracted the zone. We raise the question whether similar changes have taken place also in the other euryhaline taxa where genetic clines between Baltic vs. Kattegat
populations occur.