You have a Boccardiella ligerica (Horst, 1920) – a polychaete worm (mudworm)
Synonyms: Boccardia ligerica; Boccardia redeki (Horst, 1920);Polydora redeki; (see taxonomic note).
Common names: Mudworm
Taxonomic note: Boccardia redeki is presently considered a synonym of B. ligerica. However, this is not accepted by all specialists. WoRMS (http://www.marinespecies.org/index.php) now consider them synonymous.
http://www.frammandearter.se/5arter/pdf/Boccardia_redeki.pdf
Native distribution: It has been assumed that this is a native species in the North Sea (Kirkegaard, 1996; Främmande Arter, 2008).
Introduced distribution: First record in Britain in the 1970s, Severn estuary (Boyden et al., 1977). In Finland it was first found in the 1960s, and in 1979 it was found in Åland Islands in the Baltic (Bonsdorff, 1981).
Boccardiella ligerica is considered a brackish water species. It is often associated with Ficopomatus enigmaticus (WoRMS: http://www.marinespecies.org/aphia.php?p=taxdetails&id=131114). It was listed as Endangered in the Red List for the Wadden Sea ( Petersen et al., 1996).
Boccardia proboscidea and Boccardiella hamata are marine species. They are usually found associated with man-made structures and/or shellfish culture, particularly Crassostrea gigas, but it has not been possible to determine whether aquaculture or shipping has been the vector of introduction (Kerckhof & Faasse, 2014).
None known for Boccardiella ligerica. Boccardia proboscidea can form dense mats and smother other invertebrates. Boccardiella hamata is known to bore into shells of cultured oysters and abalone (Kerckhof & Faasse, 2014).
Bonsdorff, E. 1981. Notes on the occurrence of Polychaeta (Annelida) in the archipelago of Åland, SW Finland. Memorando Societas pro Fauna et Flora Fennica 57: 141-146.
Boyden, C.R., Crothers, J.H., Little, C. and Mettam, C. 1977. The intertidal invertebrate fauna of the Severn estuary. Field Studies 4: 477-554.
Gollasch, S. and Nehring, S. 2006. National checklist for aquatic alien species in Germany. Aquatic Invasions 1(4): 245-269.
Kerckhof, F. and Faasse, M.A. 2014. Boccardia proboscidea and Boccardiella hamata (Polychaeta: Spionidae: Polydorinae), introduced mud worms new for the North Sea and Europe, respectively. Marine Biodiversity Records 7: e76.
Kirkegaard, J.B. 1996. Havbørsteorme II. Sedentaria. Danmarks Fauna 86: 1-451.
Petersen, G.H., Madsen, P.B., Jensen, K.T., van Bernem, K.H., Harms, J., Heiber, W., Kröncke, I., Michaelis, H., Rachor, E., Reise, K., Dekker, R., Visser, G.J.M. and Wolff, W.J. 1996. V. Red list of macrofaunal benthic invertebrates of the Wadden Sea. Helgoländer Meeresuntersuchungen 50 (Supplement): 69-76.