Fig. 1

Marenzelleria spp. Map of the Baltic Sea and the eastern North Sea showing the sampling locations. Ems estuary (EM); Elbe river estuary (E); Ringkøbing Fjord (RK); Barsebäck (BB); Öresund (ÖS, includes four sampling sites); Isle of Langenwerder (LW); Warnemünde (W); Warnow-Breitling (B); Darss-Zingst Bodden Chain (D); Isle of Rügen (RG); Pomeranian Bight (OB, includes 19 sampling sites); Peene Estuary (PE); Isle of Askö, 30 m depth (AK); Isle of Askö, 20 m depth (IA); Isle of Askö, shallow water (BA); Sandviken (SV); Himmerfjärden (HJ); Åland Sea (AS); Åland Sea (AE); Söderhamn (SÖ); Bothnian Sea (BS); Bothnian Sea (BE); Tvärminne (TV); Gulf of Finland (GF); Puck Bay (PU); Vistula Lagoon (FH); Pärnu (PÄ); Gulf of Riga (GR); Riga (RI); Klaipeda (KL). Asterisks denote samples from earlier sampling dates (Table 1), indicating M. neglecta populations. Dots denote the different species M. viridis (white), M. neglecta (grey) and M. arctia (black). Sympatric sampling sites are designated by dichromic dots. Bisect dichromic dots show sites with well-balanced species proportions, dots with small corner pieces represent sites which are dominated by one species and where the sympatric species is found only in low abundance