Dispersal-mediated coexistence of mud snails (Hydrobiidae) in an estuary

The distribution of the mud snails Hydrobia ventrosa (Montagu), H. neglecta Muus, and Potamopyrgus jenkinsi (Smith) was studied in the Kysing Fjord estuary, Denmark, from November 1977 to February 1979. The hydrobiids show habitat selection with regard to salinity but coexist in large areas of the estuary: P. jenkinsi at the innermost parts of the estuary together with H. ventrosa, with the latter species extending its distribution further to the mouth of the estuary, where H. neglecta dominates. H. ventrosa and H. neglecta are the only species that have permanent populations in the estuary. P. jenkinsi dies out in the estuary in the winter, but recolonizes there from a small river in the spring and builds up a population in the innermost parts of the estuary during the summer. H. ventrosa and H. neglecta coexist in the middle part of the estuary due to continued dispersal into this area.