2008). Since then, the species has managed to colonise not only the Gulf of Finland but also waters farther west (the Gulf of Riga). Its abundance has been gradually increasing – ca ten-fold in six years. The maximum abundance was observed in 2004 (157 indiv. m− 3) ( Rodionova & Panov 2006), and in 2006 it was 120 indiv. m− 3 ( Põllupüü et al. 2008).

The selleck inhibitor results of the present study demonstrate the further expansion of E. anonyx. This Ponto-Caspian species was found in the Gulf of Gdańsk for the first time in July 2006. It was observed in the eastern and western Gulf of Gdańsk down to a maximum depth of 20 m until the second half of August in rather low densities, not exceeding 2 and 6 indiv. m− 3 in July and August respectively. A similarly low abundance, less than 1 indiv. m− 3, and the occurrence of the species in shallow waters was recorded at the beginning of the invasion of E. anonyx in the

Gulf of Finland ( Rodionova and Panov, 2006 and Põllupüü et al., 2008). We consider it probable that a period of seven years elapsed between the first record of E. anonyx in the Baltic Sea and the first one in the Gulf of Gdańsk. This is the same period of time as in the case Thiazovivin molecular weight of the expansion of another cladoceran alien to the Baltic Sea, Cercopagis pengoi, which appeared for the first time in the Gulf of Riga in 1992 ( Ojaveer & Lumberg 1995) and in the Gulf of Gdańsk in 1999 ( Bielecka et al., 2000 and Duriš et al., 2000). In addition, at the beginning of its expansion into the Gulf of Gdańsk the E. anonyx population seemed to be Elongation factor 2 kinase rather unevenly distributed – it was not recorded at three stations: So2, K2 and K4. A similar trend was observed in the case of C. pengoi – initially the species was recorded rather irregularly (Bielecka & Mudrak 2000). In the eastern Baltic Sea E. anonyx is most abundant in summer, in June and July ( Rodionova and Panov, 2006 and Põllupüü et al., 2008). Generally, E. anonyx was recorded in the eastern Gulf of Finland at a water temperature of 11–24.5° C and a salinity of 1–3

PSU, with the first specimens appearing at 17–18° C ( Rodionova & Panov 2006). However, other results were obtained by Põllupüü et al. (2008), who investigated a larger part of the Gulf of Finland (Tallinn Bay and Narva Bay) and the Gulf of Riga (Pärnu Bay). These authors found that E. anonyx was constantly present in the plankton when the water temperature reached 15 °C, with its maximum density being recorded at 16–20° C and 12–13 PSU ( Põllupüü et al. 2008). In the Gulf of Gdańsk E. anonyx was observed somewhat later in the year. It appeared at the beginning of July when the water temperature was 11.4–23.6 °C and was present for a shorter period of time – only a month and a half. Its abundance was correlated positively with water temperature. The ranges of water temperature and salinity during the occurrence of this cladoceran were 4.2–23.6° C and 4.6–7.5 PSU respectively.