Alien Crustacea in Polish waters – Amphipoda

Among ca. 750 species of Crustacea recorded from Poland, 18 representatives of 5 orders of macro-crustaceans have been identified as alien species that have invaded or have been introduced to Polish waters. Out of 44 freshwater, brackishwater and semiterrestrial species of Amphipoda occurring in Poland (not counting several stygobiotic species), 8 species from three families may be included in this group. They are Corophiidae: Chelicorophium curvispinum (=Corophium curvispinum), Gammaridae: Gammarus roeselii, G. tigrinus, Chaetogammarus ischnus (=Echinogammarus ischnus), Pontogammaridae: Dikerogammarus haemobaphes, D. villosus, Pontogammarus robustoides and Obesogammarus crassus. It is noticeable that most of them (C. curvispinum, C. ischnus, D. haemobaphes, D. villosus, P. robustoides, O. crassus) are of Ponto-Caspian origin, one species was introduced from North America (G. tigrinus), and one from south-eastern Europe (G. roeselii). All the species listed above have spread widely in Poland, in large rivers and artifical reservoirs (Chelicorophium curvispinum, Dikerogammarus villosus, D. haemobaphes, Pontogammarus robustoides, Gammarus tigrinus) or in medium sized rivers (Gammarus roeselii), in brackish coastal waters (Obesogammarus crassus) or both in fresh and brackish waters (Gammarus tigrinus, Pontogammarus robustoides). In most places they successfully outnumber or even completely replace native amphipod species. This paper presents data on biogeography, history, biology and ecology of alien species, as well as the consequences of their invasion in Poland with an extended bibliography and references to other European countries.


Introduction
To date, ca.750 species of Crustacea have been recorded in Poland (Razowski 1997, Jażdżewski et al. 2002).From this number 18 species, belonging to 5 orders of macrocrustaceans, have been identified as alien species that have entered Polish waters in historical times.This paper, dedicated to alien amphipod crustaceans, is a continuation of work by Grabowski et al. (2005), which described the geographical sources and migration routes for alien Crustacea entering Polish waters, and summarised the invasion history and progress of alien Decapoda in Poland.
The aim of this work is to summarize the historical changes in the distribution of alien Amphipoda in Polish waters, including a bibliography and extensive references to neighbouring countries.In addition, to give an update on the very recent invasion progress of these species, to describe their biology and to discuss the ecological consequences of these invasion events.

Species of Ponto-Caspian origin
Chelicorophium curvispinum (G.O.Sars, 1895) (Figure 2) This is the oldest amphipod invader of Ponto-Caspian origin noted in Polish waters.Natively it occurs in large rivers (e.g.Volga, Dniester, Danube) of the Black and the Caspian seas.The earliest record of C. curvispinum outside its native range comes from Wundsch (1912).He found it in the Spree-Havel system near Berlin in Germany.Apparently, the species had arrived into the Baltic Sea and the North Sea drainage systems through the river connections defined as the central corridor (Bij de Vaate et al. 2002).The species evidently had crossed the Polish territory and perhaps had been present in the Middle Vistula and the Noteć rivers well before it was discovered in Poland in 1920s (Kulmatycki 1930, Wolski 1930).By now it has Quite recently, the species has begun a rapid expansion in the Lower Rhine system (Den Hartog et al. 1992, Van den Brink et al. 1989, Van den Brink et al. 1993and Van der Velde et al. 2000).Now in Poland, the species is very common and abundant in numerous localities along Vistula, Oder, Bug, Narew and Noteć (Jażdżewski 1980, Jażdżewski and Konopacka 1990, 2000, unpublished data).C. curvispinum is associated with clumps of another Ponto-Caspian invader, Dreissena polymorpha (Pallas, 1771)in the lower Vistula dredged colonies of the zebra mussel were always accompanied by C. curvispinum (Jażdżewski and Konopacka 2002).This phenomenon can be also observed in other invaded areas in Western Europe (Devin et al. 2003), as well as in the species' native occurrence, i.e. in the Dnieper river.Apparently the shells offer a good solid base for building silttubes, which are inhabited by this amphipod.Colonisation success of this species in the Rhine was discussed by den Hartog et al. (1992), and explained by its opportunistic characters, rapid growth, early maturity, ability to produce several generations per year, and high fecundity.As shown recently, the species is now one of the most important primary consumers, and is an important food source for fish in the above river, determining its functional diversity and food web structure (Van Riel at al. 2006).Distribution and probable migration routes of this species in Poland are presented in Figure 3. Chaetogammarus ischnus (Stebbing, 1899) This is also an older amphipod immigrant to Polish waters.The native range of this species consists of the Black and Caspian sea drainages, including large Ponto-Caspian rivers, lagoons (limans), and dam reservoirs where it was intentionally introduced (Jażdżewski 1980).Its first record outside the Ponto-Caspian region was from the Vistula River between Warsaw and Tczew (Jarocki and Demianowicz 1931).Jażdżewski (1975) found this species in the Vistula mouth.A very interesting locality is also the Licheńskie Lake, an artificially heated water body belonging to a power plant cooling system.The species was found there by Jażdżewski and Konopacka (1990), Konopacka and Jesionowska (1995) and it still occurs in the lake (unpublished data).Samples collected in 1998 by Jażdżewski and Konopacka (2000) confirmed the occurrence of this species in the middle and lower Vistula and in its largest tributary -Bug.However, although it still occurred there, C. ischnus was not common and was evidently outnumbered by new Ponto-Caspian gammarid invaders.This is possibly an example of an invader, whose population had stabilised after an initial phase of rapid population growth.Interestingly, the species has been recently observed more frequently in both previous and new localities.Jażdżewski (2003) found an abundant population inhabiting the Narew River near Nowogród.The species co-occurred there with another alien D. haemobaphes and two native species: Gammarus fossarum Koch, in Panzer, 1836, and G. varsoviensis Jażdżewski, 1975. In 2001, C. ischnus was also found in the Noteć River (unpublished data).The river is an important element of the so called central migration corridor (Bij de Vaate et al. 2002).The river empties to the Warta River, which is a tributary of the Oder River, but at the same time Noteć is connected to Vistula through the Bydgoski canal (Bij de Vaate et al. 2002, Grabowski et al. 2005).However, our very recent findings (unpublished data) have not indicated the presence of C. ischnus in the Oder, but this species has been previously found further west in the 1970s, in canals joining the Elbe, Weser and Ems (Herhaus 1978, Herbst 1982), and later in some Mecklenburgian lakes (Waterstraat and Köhn 1989).Quite recently, the species has also become an invader in the Great Lakes in North America (Witt et al. 1997).Life history of the species in Central and Western Europe was studied by Konopacka and Jesionowska (1995) as well as by Kley and Maier (2005).Distribution and probable migration routes of this species in Poland are presented in Figure 4.In the 1960s, it was successfully naturalised in many Ukrainian, Lithuanian and Caucasian lakes and artificial reservoirs.In Lithuania it was also introduced to the Neman drainage system, including the Curonian Lagoon.Thus, the species reached the Baltic Sea basin (Gasjunas 1968, Jażdżewski 1980, Arbaciauskas 2002).Some years ago P. robustoides was also reported from Mecklenburg (Rudolph 1997, Zettler 1998).Very recent records of this species come from Belarus, where it is present in Dnieper, wher it could migrate from Lithuania (Mastitsky and Makarevich 2007).In Poland the species was found for the first time in 1988 in the Oder estuary -Szczecin Lagoon, by Gruszka (1999).Our most recent sampling (unpublished data) has also indicated the occurrence of P. robustoides in the lower Oder River.Further discoveries come from Konopacka (1998), Jażdżewski and Konopacka (2000) and Jażdżewski et al. ( 2004), who found P. robustoides in the lower Vistula and in the Vistula Lagoon.Most probably this species invaded the Vistula and Oder deltaic systems from Lithuanian waters, through the Pregel (Pregoła) river system connecting the Kuronskij and the Vistula Lagoon (Jażdżewski et al. 2002, Grabowski et al. 2003).In addition, it could have migrated through the mesohaline coastal Baltic waters.Our findings (unpublished data) indicate also that the species prefers lentic parts of large rivers of very slow current or stagnant water bodies rich in nutrients (e.g.Zegrzyński Reservoir, Włocławski Reservoir, Szczecin Lagoon, Vistula Lagoon), where it is usually found in higher densities than in running water sections.A very recent and surprising finding of this species comes from the Lucieńskie Lake located in the Vistula valley (Grabowski and Bącela 2005).The lake is a channel valley lake of mesotrophic characteruntil now P. robustoides was usually found in large, warm, eutrophic rivers, lagoons or dam reservoirs.In the lake, this species co-occurs with an alien D. haemobaphes and native amphipod Gammarus lacustris G.O. Sars, 1863.Most probably the species was accidentally introduced to that lake with boat traffic from the Vistula River.In laboratory conditions, the species appears to show some predatory behaviour, hunting for fly larvae (unpublished data).Bącela and Konopacka (2005) studied the life cycle of P. robustoides in Poland, and showed that this species is one of the most fecund amphipods (particularly if compared to native species) inhabiting Polish waters.That, combined with early sexual maturity, body size, euryoeciousness and eurytopicity, may be one of features enhancing the invasive potential of this species.
Distribution and probable migration routes of this species in Poland are presented in Figure 6.
Obesogammarus crassus (G.O. Sars, 1894) Original distribution of this species includes coastal Caspian Sea and lower courses of its rivers.In the Black Sea it lives in lagoons, entering the lower sections of the Kuban, Don, Dnieper, Ingulec, Boh, Dniester and the Danube as far as Serbia (Dudich 1947, Jażdżewski 1980).As for P. robustoides, it was intentionally introduced and naturalised in the 1960s to the Neman river system in Lithuania, namely to the Kaunas Reservoir and from there transferred, both naturally and through human activity, to Curonian Lagoon (Gasjunas 1972, Arbaciauskas 2002).In 1999and 2000, Konopacka and Jażdżewski (2002) found O. crassus in the littoral zone of the Vistula Lagoon and of the Dead Vistula (Martwa Wisła), both brackish, with salinities between 1 and 6 PSU.Our recent studies (Jażdżewski et al. 2004(Jażdżewski et al. , 2005) ) prove occurrence of the species only in such oligohaline waterbodies in Poland.Evidently, the species invaded there from Lithuanian waters, most probably through the Pregel (Pregoła) river system connecting the Curonian and the Vistula Lagoons.In addition, it could have migrated through the mesohaline coastal Baltic waters.The latter is highly probable as in 2003 this species was found in the Szczecin Lagoon (Konopacka 2003), which is a brackish part of the Oder River delta.Soon after, Eggers and Anlauf (2005) recorded O. crassus in the Elbe River in Germany, which is now the westernmost limit of its distribution in Europe.Distribution and probable migration routes of this species in Poland are presented in Figure 7.
Dikerogammarus haemobaphes (Eichwald, 1841) (Figure 8) This species occurs naturally in the lower and middle courses of the Black and Caspian Sea basin rivers and brackish lagoons, down to the Sea of Marmara.It is a species with a very wide ecological tolerance, e.g. it occurs in salinities from freshwater up to 8 PSU (Ponomareva 1975) and in temperatures ranging from 6 to 30 o C (Kititsyna 1980).Accounting for the above  1993, Schleuter et al. (1994) ) observed the species in the Main-Danube Canal, followed by its entrance to the North Sea basin through the Rhine River (Schöll et al. 1995).In Poland, D. haemobaphes was found in 1997 by Konopacka (1998) in the middle Vistula River.Further research made by our team in the middle and lower sections of the river showed that this species was definitely a dominant gammarid, followed by two other Ponto-Caspian invaders: abundant P. robustoides and scarce C. ischnus.In the Vistula River, upstream of Warsaw as far as Sandomierz, it was the only gammarid present (Jażdżewski et al. 2002).We also found this species in the Vistula's main tributary -Bug as well as in Noteć, Warta and Oder (Odra) (Jażdżewski and Konopacka 2000, unpublished data).Furthermore, we have found the gammarid to be abundant along the coasts of the Vistula Lagoon (Jażdżewski et al. 2004).At the same time Gruszka ( 2000 Dikerogammarus villosus (Sowinsky, 1894) This is a very recent gammarid invader to Polish waters.Similar to other Ponto-Caspian invaders it is naturally distributed in the lower courses of large rivers in the Black and Caspian Sea basins (Mordukhai-Boltovskoi 1969).The original dispersal route of D. villosus was the southern migration corridor (Bij de Vaate et al. 2002).Outside its natural range, it was first recorded from the upper Danube in 1992, outcompeting D. haemobaphes -the earlier invader (Nesemann et al. 1995).Soon after, it was recorded in high abundance in the lower Rhine River (Bij de Vaate andKlink 1995, Bij de Vaate et al. 2002).From there it moved eastwards through the Mittelland canal to the Elbe (Grabow et al. 1998, Zettler 1999, Rudolph 2000).From the Elbe, it most probably dispersed through the above mentioned Havel-Spree system, to the Oder River where it was recorded for the first time downstream of the canal which connects this river to Havel-Spree system (Müller et al. 2001, Jażdżewski et al. 2002).At the moment this species has spread almost along the entire stretch of the Oder within Polish borders (unpublished data), including the brackish Szczecin Lagoon in the river's delta (Jażdżewski et al. 2005).
Surprisingly, in 2003 this species was also discovered in the Bug River (Konopacka 2004), where it migrated along the central migration corridor, through the Pripet-Bug connection.This is supported by the recent records of D. villosus species in the Dnieper in Belarus (Mastitsky and Makarevich 2007).In the Bug River, this species occurs from this connection down to the Zegrzyński Reservoir, although it has not yet reached the Vistula River (unpublished data).In this case, we are seeing two independent invasion events of this species into Polish waters.
This species appears to be a very versatile feeder, either acting as a filter-feeder exploiting micro-algae (Platvoet et al. 2006)    deteriorative effect on local macroinvertebrate and fish populations and due to its high fertility and fast growth rate it may become a cosmopolitan species that creates problems in many parts of the world (Devin et al. 2003).Distribution and probable migration routes of this species in Poland are presented in Figure 10.

Gammarus tigrinus Sexton, 1939
The invasion history of this species in Europe is one of the best documented so far.This gammarid occurs natively in brackish waters, along the Atlantic shores of North America from southern Labrador to Florida.It was reported to tolerate salinities between 1 and 25 PSU (Bousfield 1973).It is unknown when G. tigrinus was first introduced to Great Britain (Gledhill et al. 1976), but it was transplanted to the rivers Werra and Weser in Germany in 1957 (Schmitz 1960, Fries andTesch 1965).Then, Bulnheim (1976) noted the occurrence of G. tigrinus in the Baltic Sea drainage system (Schlei estuary).Simultaneously, this species dispersed into the Netherlands, probably from Iljsselmeer and soon occupied the brackish and fresh waters of this region (Nijssen and Stock 1966, Dieleman and Pinkster 1977, Pinkster et al. 1977, 1980, 1992).Further eastward steps of G. tigrinus along the Baltic Sea shores and inland localities in NE Germany were documented by Bulnheim (1985), Rudolph (1994Rudolph ( a, b, 2000) ) and Zettler (1995,1998).In Poland, this species was first recorded by Gruszka (1995Gruszka ( , 1999) ) in the Szczecin Lagoon in 1988.Our recent survey of the entire Oder River flow (unpublished data) proved that G. tigrinus entered this river upstream as high as the city of Kędzierzyn-Koźle, where it occurs in freshwater conditions.Thus this species has considerably extended its European range southeastwards.In the years 1998-2000, we found this species dominating some sections of the Vistula River mouth, including the Dead Vistula and the Vistula Lagoon (Jażdżewski and Konopacka 2000, Jażdżewski et al. 2002, 2004, Grabowski et al. 2006).Interestingly, in the Vistula deltaic system this species distribution was limited exclusively to its brackishwater parts.Also our survey of coastal Malacostraca performed in 2004 along the entire stretch of the Polish Baltic shoreline, revealed that G. tigrinus is present along the entire studied area, occurring both in coastal Baltic waters and in the estuaries of small rivers, however, it did not disperse further upstream (Jażdżewski et al. 2005).Most recently, G. tigrinus was found further east, along the Finnish coast to the Gulf of Finland (Pienimäki et al. 2004), in the brackish Curonian Lagoon in Lithuania (Daunys and Zettler 2006) and in the Gulf of Riga (Kotta 2005).Gammarus tigrinus is well known for its high invasive potential.In most colonised habitats, the species has replaced native invertebrates, including other gammarid species.In Watch Lane Flash (UK) it has eliminated the native G. duebeni (Savage 1982).In Northern Ireland, this species was shown to predate upon native possum shrimp, Mysis relicta Loven, 1862, however juvenile G. tigrinus were also under predation pressure by the mysid (Bailey et al. 2006).In Germany this species has eliminated many other macroinvertebrates (Fries and Tesch 1965).In the Netherlands, it was recorded that besides replacing native G. duebeni and G. zaddachi Sexton, 1912, it was even injuring trapped fish (Pinkster et al. 1977).In Poland, G. tigrinus successfully colonized oligohaline Baltic lagoons, eventually replacing not only native brackishwater gammarids, G. duebeni and G. zaddachi, but also dominating other aliens: P. robustoides, O. crassus and D. haemobaphes (Grabowski et al. 2006).In Puck Bay, the species is abundant and dominates the coastal zone, while in the deeper parts native species are still the main component of the amphipod community.Similarly, along the Baltic coast, G. tigrinus dominates in estuaries and coastal tigrinus, is able to reproduce throughout the year with the exception of the coldest months when water temperatures are close to 0ºC.Also, the natives cease reproduction in lowered salinities, which is not the case for G. tigrinus.All that, combined with higher fecundity, more generations per year and the ability to thrive in heavily polluted waters makes this species one of the most successful amphipod invaders in Europe (Pinkster et al. 1977).Distribution and probable migration routes of this species in Poland are presented in Figure 11.

Gammarus roeselii Gervais, 1835
This species is often treated as a native to our fauna, but has been recognised as a species of Balkan origin (Karaman and Pinkster 1977, Jażdżewski 1980, Jażdżewski and Roux 1988).Occurring naturally in the freshwaters of the Balkan, Peloponesian and Anatolian peninsulas, it entered (possibly starting from XIX century) western and northern Europe through the southern invasion corridor, namely the Danube system.The Rhine and Danube systems were first connected by the Ludwigskanal in 1845, however G. roeselii was already recorded and described from the vicinity of Paris in 1835 (Karaman and Pinkster 1977).Therefore, Jażdżewski (1980) suggests that this species could have crossed the watershed with unintentional human help, e.g. with aquatic plants.Currently it is distributed throughout the whole southern, middle and western continental Europe, excluding the Apennine and Iberian peninsulas and Atlantic coasts.G. roeselii could have reached the territory of Poland in two different ways.Firstly, from the west using the Oder-Spree and Oder-Havel canal systems, secondly -from the south, crossing a rather low watershed between Oder and Morava (the Danube tributary) in the Czech Republic (Straškraba 1958).The distribution of this species in Polish waters is rather well known.It occurs in freshwaters -small and bigger rivers of slow current, in lakes and in artificial canals mainly in western Poland (Jażdżewski and Konopacka 1995).Its localities in the lower Vistula system (affluents of Elbląg Canal) are at the most northeastern limits of its distribution in Europe (Jażdżewski 1980).
The species does not spread quickly and does not exhibit a high invasive potential.Pöckl (1993Pöckl ( , 2003) ) studied the reproduction of G. roeselii and its ability to compete with Gammarus fossarum.Apparently the former prefers warmer waters, where it may mature earlier and give larger broods, and hence is able to dominate the latter, which performs better in colder temperatures.Generally, G. roeselii may be defined as an old alien coloniser, well established but not invasive.
Distribution and probable migration routes of this species in Poland are presented in Figure 12.

Conclusions
In summary, the alien amphipods constitute a significant part of the benthic communities in large Polish rivers and in coastal Baltic waters.In lower sections of rivers such as the Bug, Vistula, Oder, and also in Szczecin and Vistula lagoons, the amphipod fauna is now limited exclusively to alien species (Jażdżewski 2003, Jażdżewski et al. 2002, 2004, 2005, Grabowski et al. 2006, unpublished data).The retreat of native species is a serious problem in many places colonised by alien amphipods not only in Poland but across Europe.The possible reasons behind this phenomenon include: a) pollution of aquatic ecosystems which are intolerable to the indigenous gammarids, b) predation pressure from the newcomers, and c) greater reproductive capacity of the alien species.
Among the interactions between the alien invaders and local species, trophic relationships should be particularly studied as they may alter the functional structure of entire ecosystems.Of the above invaders, one -D.villosus -is known to be a voracious predator exterminating local fauna, including other gammarids, in newly invaded areas.We also observed predatory behaviour of P. robustoides in laboratory conditions (unpublished data).These two species, taking into account their size, can become lethal for the local benthic communities in Polish waters.
Predatory behaviour was also shown in the case of G. tigrinus.From the other side, the newcomers can become an important food source for local species.Even the same species can be both: a predator and a sought after prey item, which is true for both mentioned pontogammarids.In many artificial reservoirs in the Ukraine and Lithuania, these alien species have been introduced as a productive food source for commercial fishes.We have frequently observed C. curvispinum, P. robustoides and D. haemobaphes individuals in fish stomachs; interestingly these species are an important food source for Ponto-Caspian fish invaders: racer goby (Neogobius gymnotrachelus (Kessler, 1857)) and monkey goby (Neogobius fluviatilis (Pallas, 1814)) which commonly occur in the Bug and Vistula rivers (Kostrzewa andGrabowski 2003, Grabowska andGrabowski 2005, unpublished data).This brings another possible threat for local fauna -an infection by alien plagues or parasites, e.g.P. robustoides is known to be a vector for one species of microsporidia and two gregarines coming from the Ponto-Caspian region (Ovcharenko et al. 2006).
Greater reproductive capacity of alien species compared to natives has been proven for almost all alien amphipod species in European fresh and brackish waters.Combined with high tolerance to warm water temperatures, heavy pollution and the above mentioned predatory behaviour, it makes the alien amphipods extremely successful invaders.
Another effect of invasion is the formation of new biotic communities in local habitats -an "Invasional Meltdown" phenomenon.It occurs when species co-evolving in their native range facilitate the invasion through mutual interaction of newly colonized areas.A good example may be the Ponto-Caspian C. curvispinum using colonies of the bivalve D. polymorpha (of the same geographic origin) as a substrate for building silt-tubes (Jażdżewski and Konopacka 2002), when other suitable substrates are lacking.The same relationship may be observed in both species' native area of occurrence.
We can expect further successful invasions, particularly from the Ponto-Caspian region.The first reason for this is the present lack of isolation between this basin and the Baltic drainage system.Secondly, there are numerous naturalisations of Ponto-Caspian species in former Soviet Union countries, close to Polish territory, which may facilitate the expansion of the alien species by establishing new source populations.Thirdly, a bit more complex, is a set of physio-ecological features shared by the species coming from the Ponto-Caspian region.All these species are euryoecious and rather euryhaline, occurring from freshwater to, in some cases, mesohaline conditions (Bij de Vaate et al. 2002, Konopacka 2004).Besides crustaceans, successful invasions in European waters are also known for Ponto-Caspian gobiid fishes, molluscs, triclads, hydrozoans, leeches and water-mites (Bij de Vaate et al. 2002, Jażdżewski andKonopacka 2002).It is presumed that the next Ponto-Caspian amphipod to enter Polish waters may be Chaetogammarus warpachowskii (G.O.Sars, 1894), which occurs in the Curonian Lagoon (Konopacka 2004).It is likely that it may follow the route already taken by P. robustoides and O. crassus.
To conclude, the amphipod fauna of Poland has undergone a very dynamic change.However, the exact nature of the processes, particularly the biological features of each species, physiological tolerance to environmental stress and trophic relationships with local species remain largely unknown.It will, therefore, be a great challenge for scholars in the near future.
The data presented in this paper come from activity supported financially by the Polish State Committee of Scientific Research (KBN), grant No. 2 P04G 076 026 p01, as well as from the internal grants and funds from the University of Łódź.Authors are indebted to Ewa Janowska and Karolina Bącela for their extensive help in the fieldwork.Many thanks are due also to Dr Elizabeth Cook, Dunstaffnage Marine Laboratory, Oban, UK for improving the language of this paper.

Figure 1 .
Figure 1.Contribution of alien species to amphipod fauna in Poland

Figure 4 .Figure 5 .
Figure 4. Distribution and possible migration routes in Poland of Chaetogammarus ischnus.Legend as in Figure 1

Figure 6 .
Figure 6.Distribution and possible migration routes in Poland of Pontogammarus robustoides.Legend as in Figure 1

Figure 7 .Figure 8 .
Figure 7. Distribution and possible migration routes in Poland of Obesogammarus crassus.Legend as in Figure 1

Figure 9 .
Figure 9. Distribution and possible migration routes in Poland of Dikerogammarus haemobaphes.Legend as in Figure 1 native G. pulex(Linnaeus, 1758).The same pattern was observed very recently in the Odra river, where the former coloniser, G. tigrinus, is now being replaced by D. villosus (unpublished data).In the Rhine River it has become one of the key secondary consumers occupying high trophic levels comparable to fish(Van Riel et al.  2006).For the above reasons, it is of key importance to monitor further expansion and naturalisation processes of this species in Polish waters.Life history of D. villosus in Western Europe was previously studied byKley and  Maier (2003), Piscart et al. (2003), Devin et al.  (2004)  and byPöckl (2007).Generally, this particular invader may have an extremely

Figure 10 .
Figure 10.Distribution and possible migration routes in Poland of Dikerogammarus villosus.Legend as in Figure 1

Figure 11 .
Figure 11.Distribution and possible migration routes in Poland of Gammarus tigrinus.Legend as in Figure 1