, 2006, Blank et al., 2008 and Maximov, 2011). Since the end of the 1980s several new species have been observed for the first time in the southern

part of the Baltic Sea, like Gammarus tigrinus Sexton, 1939 and Palaemon elegans Rathke, 1837 ( Gruszka, 2002, Janas et al., 2004a and Wawrzyniak-Wydrowska Topoisomerase inhibitor and Gruszka, 2005). The invasion of these two species and the retreat of native species in the coastal water bodies of the southern Baltic has been documented in gammarids ( Jażdżewski et al., 2004, Szaniawska et al., 2005 and Surowiec and Dobrzycka-Krahel, 2008) and palaemonids ( Grabowski 2006). The areas most likely to be colonised by new species are coastal lagoons and river mouths, where the broad

diversity of habitats and low salinity allow the co-existence of species of both freshwater and marine origin (Paavola et al., click here 2005 and Zaiko et al., 2007). One such area is Puck Bay, where eleven non-indigenous benthic species have settled. The studies carried out so far on the benthic communities of Puck Bay, dealing with species composition, density and biomass, have covered solely the non-indigenous species already present in these waters for several decades (e.g. Legeżyńska and Wiktor, 1981, Wenne and Wiktor, 1982 and Kotwicki et al., 1993). An exception is the paper by Kotwicki (1997), which supplies information on the density and biomass of Marenzelleria spp. Species of benthic fauna new to this area have usually been treated in separate articles ( Szaniawska et al., 2003, Janas et

al., 2004a and Janas and Wysocki, 2005), or at most they have been compared to other species from the same family (e.g. Jażdżewski et al., 2005, Spicer and Janas, 2006, Szaniawska et al., 2005, Grabowski, 2006 and Packalén et al., 2008). There are no papers, however, on the present-day occurrence of alien species forming benthic communities with other species, or on their proportions in the abundance of the entire macrozoobenthos. Such data are also scarce with respect to the whole Baltic Sea ( Ezhova et al., 2005 and Daunys and Zettler, 2006). Moreover, only fragmentary data are available on the preferred habitats of non-indigenous species and on the relationships between native and non-native species ( Zaiko et al. 2007). The objective also of this research was therefore to seek answers to the following questions: 1. What is the species composition, distribution and percentage share of non-indigenous species in the total number, abundance and biomass of benthic species in Puck Bay? Alien species are considered to be one of the most serious threats to coastal ecosystems (Gray 1997). Information on distribution, abundance, biomass and habitat preferences are of crucial importance in developing permanent monitoring programmes for alien species or designing a suitable mechanism for managing coastal ecosystems.