Review of recent immigrants
Dikerogammarus haemobaphes (Eichwald, 1841)
For the first time in the Baltic Sea basin, the species was recorded in Poland in 1997 (Konopacka 1998). The range expansion of this Ponto-Caspian invader in European waters and in Poland was already presented by Jazdzewski & Konopacka (2000). Recently, we discovered large population of D. haemobaphes in the Vistula Lagoon, Gruszka (2000) and Müller et al. (2001) found this species in the lower Oder river.
Dikerogammarus villosus (Sowinsky, 1894)
The species was recently recorded in the Oder river, in 1999 (Gruszka 2001, Müller et al. 2001, Jazdzewski and Konopacka 2002) downstream of the canal connecting the Oder river with the Elbe basin. The penetration of D. villosus into the Oder basin is especially interesting since it has used first the so-called southern corridor, i.e. Danube river, for westward range expansion (Bij de Vaate et al., 2002). In the upper reach of Danube river D. villosus was first recorded by Tittizer et al. (1994) in 1992, and soon it has penetrated into the Rhine river via the Main – Danube canal (Bij de Vaate & Klink 1995). From the Rhine river D. villosus continued range expansion eastward by using the Mittelland-canal joining the Rhine, Weser, Elbe and Oder basins (Grabow et al. 1998, Zettler 1998, Rudolph 2000). In the lower Oder river, the species co-occurrs with other alien gammarids, like D. haemobaphes, P. robustoides and G. tigrinus (Müller et al. 2001, own unpubl. data).
Pontogammarus robustoides (G.O. Sars, 1894)
First records of P. robustoides come from north-western Poland. Gruszka (1999) found this species in the Szczecin Lagoon and the lower Oder river in 1988. The species was also reported from the lower Vistula river (Konopacka 1998) and from the Vistula Lagoon (Jazdzewski & Konopacka 2000). Pontogammarus robustoides reached the Vistula and Oder deltaic systems possibly with ballast waters via Baltic Sea from the Neman river system and Curonian Lagoon, where it was introduced in the 1960s (Gasjunas 1972, Arbaciauskas, 2002). However, the species could also have entered the Vistula Lagoon through the Pregola river system connecting the Vistula river delta with the Curonian Lagoon. More details on the origins and distribution routes of the mentioned species in Europe can be found in Jazdzewski & Konopacka (2000).
Obesogammarus crassus (G.O. Sars, 1894)
This species is most recently discovered in Polish waters, namely in the Vistula Lagoon and in the Dead Vistula in 1998 (Konopacka & Jazdzewski, 2002). Original distribution areas of O. crassus encompassed offshore Caspian Sea waters and lower courses of rivers emptying to this water body; in the Volga river the species penetrated upstream as far as to Volgograd (Mordukhaj-Boltovskoj 1979). In the Black Sea system O. crassus occurred originally in brackish lagoons and in the lower courses of large rivers (Dedju 1980, Jazdzewski 1980). In the Danube river it was noted as far upstream, as in its Yugoslavian sector (Dudich 1967). Like P. robustoides, O. crassus was transplanted in early 1960s into the Kaunas artificial reservoir on the Neman river in Lithuania and from there, after acclimatisation, it entered the Curonian Lagoon (Gasjunas 1972, Arbaciauskas, 2002). Subsequently it entered the Vistula Lagoon, most probably via the Pregel river system. However, due to its comparatively high euryhalinity, O. crassus could have also dispersed south-westwards along the Baltic Sea shores, with the average salinity of 7 PSU in this region.
Gammarus tigrinus Sexton, 1939
This North American euryhaline species was observed in waters with salinities ranging from 1 to 25 PSU (Bousfield 1973). Information on its introduction and distribution routes in Europe have been summarised by Jazdzewski & Konopacka (2000). First observations of G. tigrinus in Polish waters were done in 1988 in the Szczecin Lagoon (Gruszka 1995, 1999, Wawrzyniak-Wydrowska & Gruszka 2001). Recent survey of the entire Oder river (unpublished data) proved that G. tigrinus entered this river upstream as far as to the city of Opole. The localities of the species in the Vistula Lagoon are by now the easternmost ones in Europe.