Species strategy near its boundary: the Marenzelleria cf. viridis (Polychaeta, Spionidae) case in the south-eastern Baltic Sea

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