You have a Marenzelleria ssp.
Marenzelleria viridis (Verrill, 1873) and other introduced species of Marenzelleria – Red gilled mud worm
Synonyms: See taxonomic note
Common names: (Marenzelleria spp.) Red gilled mud worm (UK); Amerikanmonisukasmato (FI); Rogoza amerykanska (PL).
Taxonomic note: There has been considerable confusion about the identity of the species of Marenzelleria introduced to European waters. At the present time 3 species occur in Nordic waters, but because of the confused identity, it is very difficult to reconstruct which species have been used in the various publication, especially from the Baltic where all 3 species occur. Marenzelleria viridis was the first to be found in Europe in 1979 when it was identified as M. wireni Augener, 1913. In 1985 the first specimens were found in the Baltic, where it was identified as M. viridis and thought to be identical to the North Sea species. However, it was soon realized that these species were different. The first one was then re-identified as M. cf. wireni and the Baltic species as M. cf. viridis (Bick & Zettler, 1997). When molecular studies were included it was concluded that the North Sea species was M. viridis and the Baltic species an undescribed one from North America. The original description of this species, M. negelcta Sikorsky & Bick, 2004, used the Baltic Sea as type locality(!). Shortly after it was realized that a third species was present in the Baltic, M. arctia (Chamberlin, 1920).
The species of Marenzelleria can only be identified to species level with certainty by using molecular methods. There are a few morphological differences, but these require a microscope, complete undamaged worms, and apparently also some experience. The links below refer mostly to the Baltic invader, M. neglecta.
Native distribution: East coast of North America
Introduced distribution: Currently M. viridis occurs in the waters of Germany, both North Sea and Baltic coasts (Gollasch & Nehring, 2006), Netherlands (Wolff, 2005), Belgium (Kerckhof et al., 2007), United Kingdom (Eno et al., 1997), Denmark (Jensen & Knudsen, 2005) and Sweden (Göransson et al., 2005; Strömberg & Persson, 2005). In Danish waters it seems to be spreading to all localities with suitable conditions (Dr. Gary Banta, pers. comm.). In Norway it is considered cryptogenic (Hopkins, 2001). It occurs along all coasts except the southermost Oslo Fjord (as M. wireni) (Brattegard & Holthe, 1997). Recently molecular studies have shown that it also occurs in Poland (Blank et al., 2008).
M. neglecta ocurs in Germany, Baltic and North Sea coasts (Gollasch & Nehring, 2006), Sweden (Främmande Arter, 2006), Poland (Gruszka, 1991; Zmudzinski, 1996; Warzocha et al., 2005), Russian parts of Baltic coast (Gusev & Starikova, 2005; Maximov, 2009), Finland (Norkko et al., 1993; Stigzelius et al., 1997), Lithuania (Daunys et al., 2000), Estonia (Simm & Pollumae, 2006; Blank et al., 2008).
M. arctia so far has been found only on the Baltic coast of Sweden and Finland (Blank et al., 2008).