The invasive polychaete worm Marenzelleria cf. viridis spread into various coastal habitats in the Baltic Sea. The estimated limits regarding its salinity tolerance obtained from different laboratory experiments alone only rarely explain the actual species boundaries in nature, e.g. in the Curonian lagoon. Therefore, a field study was carried out aimed to define, how a population maintains itself in a dynamic estuarine habitat with rapid and irregular changing salinity (annual mean 3 PSU, range of variation from 0 to 7.5 PSU). Under these conditions the species females reached maturity. However, during the final reproduction phase their spawning was delayed and oosorption started. It was estimated that roughly 0.05% of the nearby sea population offspring was transported to the estuary but most of the pelagic larvae were not able to develop beyond the 10 segments stage. It seems that the estuarine benthic population is maintained by migration of the species benthic stages. This mode probably is more efficient in comparison to dispersal by pelagic larvae when a species colonises stressed and dynamic environments.
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