New records of non-indigenous polychaeta Boccardiella ligerica (Ferronnière, 1898) (Spionidae) in the southern Baltic Sea

The non-indigenous Boccardiella ligerica is a polychaete introduced to the Baltic Sea. Although the species has been recorded in the Baltic Sea since the 1960s, this is the first time we have reported the repeated occurrence of B. ligerica in various new locations along the Polish coast. Between 2009 and 2018, the species was recorded in the Vistula Lagoon, the Gulf of Gdańsk and Puck Bay. Samples of the species were collected from hard substrates and bottom sediments at depths ranging from approximately 1.0 m to 13.3 m using a range of sampling gear, including Van Veen and Ekman-Birge grab samplers, a HAPS corer and settlement plates, as well as by scraping vertical surfaces during diving. The highest densities, reaching up to 1689 ind. m−2 and 414 ind. m−2, were recorded in the Gulf of Gdańsk and the Vistula Lagoon. The lowest abundance (13 ind. m−2) of this polychaete was recorded in Puck Bay. The results obtained contribute to the understanding of the dynamics of this non-indigenous species in the brackish environments of the Baltic Sea. They provide a basis for further research on this species, considering that B. ligerica may play an important role in food webs, as it feeds on phytoplankton and detritus, and serves as food for small fish and invertebrates.