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The ‘killer shrimp’, Dikerogammarus villosus, has been recognised as one of the 100 worst alien species in Europe, in terms of negative impacts on the biodiversity and functioning of invaded ecosystems. During the last twenty years, this Ponto-Caspian amphipod crustacean has rapidly spread throughout Europe’s freshwaters and its invasion and continued range expansion represents a major conservation management problem. Although a great deal of research has focussed on this almost ‘perfect’ invader as its damaging impacts, realised and potential, have become evident, we now present the first comprehensive review of D. villosus taxonomy, morphology, distribution, community impacts, parasites, life history, physiological tolerance and finally, possible eradication methods. We show the direct and indirect ecosystem impacts of this invader can be profound, as it is a top predator, capable of engaging in a diverse array of other feeding modes. It can quickly dominate resident macroinvertebrate communities in terms of numbers and biomass, with subsequent large-scale reductions in local biodiversity and potentially altering energy cycling, such as leaf litter processing. This damaging European invader has the potential to become a key invader on a global scale as it may be capable of reaching North American freshwaters, such as the Great Lakes. One positive aspect of this invader’s spread and impact is increased interest in alien species research generally, from decision-makers, stakeholders and the general public. This has resulted in greater financial support to study invasion mechanisms, preventative measures to stop invasion spread and ways to minimise damaging impacts. Our review provides a specific example, that studies identifying management strategies that mitigate against a potential invader’s spread should be undertaken at the earliest possible opportunity in order to minimise potentially irreversible ecosystem damage and biodiversity loss.

Gammarus varsoviensis is morphologically close to G. lacustris, with which it is often misidentified. Geographic range of G. varsoviensis includes Germany, Poland, Lithuania, Latvia, Belarus and Ukraine. Such a distribution pattern led us to the assumption that the species might have originated in the Black Sea drainage area. From there, as early as the nineteenth century, it could possibly have migrated to the Baltic basin through the Pripyat-Bug canal. Thus, the goals of this study are: (1) to indicate the level of genetic divergence of G. varsoviensis from the morphologically closest species – G. lacustris and (2) to investigate the possibility of the Pontic origin of G. varsoviensis and its range expansion across the Black Sea/Baltic Sea watershed to Central Europe through the artificial canal network. Altogether 128 partial 16S rDNA sequences of Gammarus varsoviensis from 19 localities were gained. They were analysed in conjunction with a sequence of G. lacustris obtained in this study from the Dnieper system and sequences of G. lacustris, G. pulex and G. fossarum available in GenBank in order to estimate the relationships among the species. GenBank accession numbers for all the haplotypes defined within this study are: G. varsoviensis – from JN641868 to JN641875; G. lacustris – JN641876. The genetic distance within and between the species was calculated, as well as phylogenetic relationships among haplotypes, which were inferred with Neighbor-Joining method. The haplotype relationships were analysed with the Minimum Spanning Network. Also mismatch distribution of the haplotypes were tested under sudden expansion model and sequence deviations from selective neutrality. Neighbor-Joining analysis revealed that G. varsoviensis and G. lacustris haplotypes formed separate well defined clades. Mean genetic diversity between the two species was ca. 15× higher than the intraspecific distance and similar to the value obtained for G. fossarum/G. pulex species pair. Therefore, G. varsoviensis can be definitely considered as a distinct species from G. lacustris. Among 128 sequences obtained from G. varsoviensis, 8 haplotypes were identified and grouped into two clades: one found only in the lower Dnieper (two haplotypes) and the second one encompassing the rest of haplotypes observed in the upper Dnieper and Baltic Sea basin. Only one haplotype was found in the Baltic Sea drainage area. A mismatch distribution curve as well as selective neutrality tests demonstrated sudden expansion model. Our findings suggest that G. varsoviensis is an alien gammarid that originated in the Pontic area. Its expansion in Central Europe apparently started soon after the opening of the artificial waterways joining the Black and the Baltic Sea drainage basins.

Dikerogammarus villosus (Sowinsky, 1894) has been recorded for the first time in the Vistula River, the biggest river in Poland and in the whole Baltic drainage system. Until now the species has been found in the Bug River that is a tributary of the Vistula. It has migrated there from the Ponto-Caspian basin through the central invasion corridor (namely Pripet-Bug canal). Another population of that species has been found in the Oder River. It came there via the Mittelland Canal from the Rhine River that had been colonised from the Black Sea basin through the southern invasion corridor. In the Vistula River, D. villosus co-occurs only with other Ponto-Caspian gammarids.

The Baltic palaemonid fauna comprises four species: Palaemonetes varians, Palaemon adspersus and two newcomers, P. elegans and P. longirostris. The first three species have been reported from Polish waters. This paper presents the history of faunal change associated with P. elegans recent colonization along the Polish Baltic coast, its estuaries, coastal lakes and lagoons. The oldest record of P. elegans comes from the Vistula deltaic system collected in 2000. Presumably moving eastwards from the Atlantic, the species colonized and formed a vivid, reproducing population all along the studied part of the Baltic shores. In many places it has replaced the native P. adspersus and it has became an abundant element of the palaemonid community in the Gulf of Gdansk and in the Vistula delta, still accompanied by the two other species.

Orchestia cavimana, a semiterrestrial amphipod of presumably Mediterranean origin was found for the first time in a freshwater habitat in Poland in October 2009, in the main Vistula channel at Świbno. Most probably the species reached this locality from the nearby Baltic coast by natural spread up the river. Taking into account its high abundance at the site and presence of immature individuals, the species has probably established a permanent population in this new habitat in Poland.

The present work reports the findings of a new alien annelid, Branchiura sowerbyi, in freshwaters of mainland Greece. The species was found in August 2008 in Strymonas River near Neo Petritsi in Serres Prefecture, and in Aliakmonas River near Niselli in Imathia Prefecture.