What determines the distribution of coastal hydrobiid mudsnails within north-western Europe?

The three north-west European species of Hydrobia are often partially segregated along salinity gradients and/or sympatric in the nontidal eastern Danish waters and Baltic Sea, but are rarely sympatric and are confined either to nontidal coastal lagoons (Hydrobia ventrosa and Hydrobia neglecta) or to the marine/estuarine intertidal zone (Hydrobia ulvae) along the macrotidal Atlantic and North Sea coasts. Significant interspecific competition has been found in Danish waters but not elsewhere in the Atlantic and North-Sea Europe. The two nontidal species and H. ulvae also possess contrasting reproductive strategies. The roles of life-history strategy, interspecific competition and between-habitat dispersal in restricting these mudsnails to their differing habitats are evaluated in the light of these and other findings. It is concluded that all are likely to be relevant, especially dispersal in maintaining and interspecific competition in opposing, mixed populations in the Kattegat-Baltic, and life-history strategy in maintaining the lagoonal versus intertidal dichotomy in the rest of north-west Europe. This dichotomy prevents segregation of species along common salinity gradients outside the Baltic region and restricts potential interspecific competition to within lagoons, where H. ulvae is at a recruitment disadvantage.