Coastal ecosystems have highly valuable goods and services for human society and provide important ecological functions (such as primary and secondary productions and nutrient cycling).
Invertebrates living in those areas, such as worms, play an important role by linking organic material with predators, providing food to fish for instance. However, in the last decades, these areas have been experiencing increasing threats, mainly due to human activities. They are particularly vulnerable to species introductions. The Baltic Sea is a very special environment with its brackish water, young ecological age and low number of species. This contributes to nonnative species establishment.
The number of worm species is very small in the northern Baltic. Consequently, the introductions of two worms, Boccardiella ligerica and Marenzelleria spp., represent a substantial increase in polychaete diversity. Usually, species invasions are considered as a major threat to ecosystem functioning. However, given Boccardiella abundance in some brackish habitats, this species may play an important role in estuarine food webs, as a consumer of phytoplankton and detritus, and as food for small fishes and predatory invertebrates in low salinity regions. Regarding Marenzelleria worms, they have a different way to move in the sediment than the local species. They could then be potentially positive for the ecosystem by bringing new functions in the Baltic. In consequences, predators could also benefit from their presence.
