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Proceedings of the 13th Symposium of the Baltic Marine Biologists: 127-129, 1996 Expansion of the Spionid Polychaete Marenzelleria viridis in the Southern Part of the Baltic Sea L. Zmudzinski (1, 2), S. Chubarova-Solovjeva (3), Z. Dobrowolski (2), P. Gruszka (4), S. Olenin (5), N. Wolnomiejski (6) 1. Marine Biology Centre, Gdynia, Poland; 2. University of Education, Slupsk, Poland; 3. Lithuanian Marine Research Centre, Klaipeda, Lithuania; 4. Academy of Agriculture, Szczecin, Poland; 5. Klaipeda University, Lithuania; 6. Sea Fisheries Institute, Swinoujscie, Poland. ABSTRACT. The North American polychaete Marenzelleria viridis (Verril 1873) was first observed in the south- western Baltic in 1985, that is three years after the species had been found in Europe in the North Sea. In the end of the eighties it was recorded in several areas along the Polish, Russian (Kalinin- grad district) and Lithuanian coasts. This paper is a co-operative report on further expansion of the polychaete into the southern Baltic, including its coastal lakes and lagoons. In some areas M. viridis has became more numerous than the authochtonal polychaetes. The rapid expansion of this species is most likely due to a long persistence of its larvae in the plankton. INTRODUCTION The North American polychaete Marenzelleria viridis (Verril 1873) was first recorded in Europe in the Forth estuary in 1982 (McLusky et al. 1993) and in 1983 at the European mainland coast in the Ems estuary (Essink, Kleef 1988). Two years later this species was found in the south-western Baltic in the German coastal zone (Bick, Burckhardt 1989). Since 1986 it has been observed in Polish waters (Z. Dob- rowolski and J. Maslowski, unpublished data; Gruzska 1991a), and in 1989-1990 in the Russian (Kali- ningrad district) and Lithuanian inshore areas and lagoons (Olenin and Chubarova 1992). In 1990 M. vi- ridis was found near Karlskruna, Sweden (Persson 1991). In 1992 the polychaete was found in Finnish coastal waters (Norkko et al. 1993), where now it occurs from Kotka in the east up to the Bothnian Bay (M. Enberg, pers. comm.). This paper gives an overview of the recent distribution of Marenzelleria in the large area from the Po- meranian Bay at the west to the Curonian Lagoon in the east. The authors would like to thank Dr. T. Radziejewska and Mr. Okolotowicz for reviewing and discus- sion of the manuscript. MATERIAL AND METHODS The sampling and processing techniques used by different investigators were basically similar. The sam- ples were usually taken with about 200 cm2 Van Veen or Ekman grabs in the Szczecin Lagoon, Curonian Lagoon and Russian part of the Vistula Lagoon and about 0.1 m2 Van Veen or Petersen grabs in the Po- meranian Bay, Russian and Lithuanian inshore Baltic areas. In 1993, a Reineck box corer (225 cm2) was additionally used in the Pomeranian Bay. All these grabs penetrate the sediment down to about 20 cm. In the Polish coastal lakes and the Polish part of the Vistula Lagoon a 50 cm2 box corer penetrating down to 20-35 cm was used. The samples were sieved through sieves of similar mesh size (0.5-0.7 mm) and the sieve residue was fixed with 4% formaldehyde. In the laboratory, specimens of Marenzelleria viridis were counted and the density (ind. m-2) was calculated. Plankton was sampled with an 80 µm Apstein net. Vertical hauls through the entire water depth were made. The samples collected were fixed with 4% formaldehyde and examined for the presence of M. vi- ridis larvae in the laboratory. GRADUAL WEST-EAST EXPANSION In 1986, M. viridis was found in small numbers in two separate areas: the Szczecin Lagoon and the coas- tal Lake Leba (Figure 1). By the end of 1980s, the species was already known from the Polish lakes Res- ko and Jamno, and from the inshore areas near the outlets of the Szczecin and Vistula lagoons (Figure 2). In 1990, another Polish lake, the Bukowo was populated by Marenzelleria; also it was found at the larger number of stations in the Szczecin Lagoon, in the northern part of the Vistula Lagoon (Kaliningrad Bay), and in the northern part of the Curonian Lagoon (Figure 3). The most recent observations made in 1992- 93 show that M. viridis has become a common component of macrofauna in the southern Baltic coastal zone including coastal lakes and shallow lagoons (Figure 4). Figure 5 shows the gradual expansion of Marenzelleria and increase in its abundance in deeper areas of the Baltic Sea near the Lithuanian coast: in 1990, the species was present only at depths of about 20 m, while in 1992 it had reached depths of 46-50 m (Olenin and Chubarova 1992). The macrofaunal species most common in the Baltic, i.e. the polychaete nereis (=Hediste) diversicolor, bivalves Macoma baltica and Mytilus edulisare known in the polyhaline seas (e.g. the North Sea) to inhabit intertidal areas only. In the Baltic, however, they extend their depth range, a process known as basal "submergence" (Remane 1971). The expansion of M. viridis from shallow to deeper areas off the southern Baltic coast, discussed also by Gruszka (1991a), gives the most recent example of the submergence process. INCREASE IN ABUNDANCE In the initial period of the expansion only single juveniles and adults of M. viridis were found in Szczec- in Lagoon or Lake Leba (also in the Russian and Lithuanian waters) and the abundance of the species did not exceeded 100 ind. m-2 (Figures 1 and 2). In the early nineties, the abundance of the introduced poly- chaete has increased considerably, reaching several thousand ind. m-2 in some areas. At present, the densest population of Marenzelleria (5000-7000 ind. m-2) occurs inside the Vistula La- goon and in the vicinity of its outlet (Figure 4). There had been no autochthonous polychaetes in the inner part of the Lagoon until the M. viridis appeared, and at some stations in the lagoons and some in- shore Baltic areas the species became quantitatively dominant contributing up to 60-90 % of total abun- dance of zoobenthos. Off the Lithuanian coast the maximum abundance of M. viridis reaches 1100 ind. m-2; while numbers of up to 750 ind. m-2 were recorded in the Pomeranian Bay affected by the Szczecin Lagoon outflow. In the strongly freshened Polish coastal lakes and the Curonian Lagoon the abundance of the new polychaete is considerably lower (70-100 ind. m-2). The numbers exceed some hundred ind. m-2 only in the narrow channels connecting these water bodies with the Sea. ENVIRONMENTAL PREFERENCES Presently M. viridis inhabits the following areas within the larger southern Baltic region: 1. Coastal lakes connected with the Baltic Sea (the lakes Leba, Resko, Jamno, Bukowo); 2. Semi-enclosed shallow brackish-to-fresh water lagoons (Szczecin, Vistula, Curonian Lagoons); 3. Coastal zone in the vicinity of rivers/lagoons outlets (Pomeranian Bay, the southern and eastern parts of the Gulf of Gdansk; the area off the Curonian Lagoon); 4.Coastal zone not directly affected by river outflow; 5. Deeper areas beneath the coastal zone (down to 50 m). Thus, Marenzelleria occurs in areas exhibiting a wide range of environmental conditions: from a very shallow freshwater lagoon or a coastal lake to a marine area of 50 m depth where the salinity reaches 8 ‰. The M. viridis population is abundant in the oligohaline waters of the Vistula Lagoon (salinity 2-3 ‰) as well as in the offshore area near its outlet (salinity 5-7 ‰). M. viridis was found in the Szczecin Lagoon only in spring and early summer when the salinity of the Lagoon's water was higher. Salinity may be the reason why the polychaete is a temporary inhabitant of this water body. The Marenzelleria population in the Szczecin Lagoon was most abundant in 1990-1991, accompanying the inflow of more saline waters into the Lagoon. M. viridis inhabits soft bottom habitats of various types, from coarse sand to a mud. However, it is quite evident that this species prefers sediments enriched with organic matter, typically found off river mouths. PROPAGATION MODE AS A FACTOR FACILITATING DISPERSAL The dispersal of Marenzeileria viridis is associated with its propagation mode by plankton development stages, i.e. pelagic eggs and larvae which occur in extremely high densities in the water in autumn (Sep- tember) till spring inclusive (Gruszka 1991b). The long presence of its early development stages contributes to the dispersal of the polychaete not only by ships ballast waters but also by sea currents. M. viridis eggs and/or larvae occur in the greatest numbers in the Pomeranian Bay, but they are also observed along the Baltic open coasts (off Ustka, Wla- dislawowo and Klaipeda) and off Hel and Sopot in the Gulf of Gdansk (Figure 4). In the Szczecin Lagoon early larval stages have been recorded since 1987 (Gruszka 1991b). Thus the larvae of Maren- zeileria viridis are good indicators of the species dispersal in the Baltic Sea. REFERENCES Bick, A., Burckhardt, R. (1989). First record of Marenzelleria viridis (Polychaeta, Spionidae) in the Baltic Sea, with a key to the Spionidae of the Baltic Sea. Mitt. Zool. Mus. Berl., 65, 2: 237-247 Essink, K., Kleef, H.L. (1988). Marenzelleria viridis (Polychaeta, Spionidae) - a new record from the Ems estuary (the Netherlands/Federal Republic of Germany). Zool. Bijdr. Leiden, 38: 1-13 Gruszka, P. (1991a). Marenzelleria viridis (Verril, 1873) (Polychaeta, Spionidae) - a new component of shallow water benthic community in the Southern Baltic. Acta Ichthyol. Pisc., 21, Suppl.: 57-65 Gruszka, P. (1991b). Marenzelleria viridis keeps conquering the Baltic Sea. -12th Baltic Marine Biologists Sympo- sium. August 23-30, 1991. Helsingor, Denmark.

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