The absence of this ciliate in the zebra mussels examined by Raabe (1956) is more difficult to interpret as very little is currently known about its ecology and biology. The levels of infection of D. polymorpha with C. acuminatus and Ophryoglena sp. recorded in our study are comparable to those in other European Smad inhibitor water bodies ( Molloy et al., 1997, Mastitsky, 2004 and Karatayev et al., 2007). The quantitative dominance of C. acuminatus over Ophryoglena sp. observed in our samples is also consistent

with previous studies. Such a dominant position of C. acuminatus is generally explained by its commensal relationship with D. polymorpha, allowing the ciliate to reach high numbers without causing any significant harm to its host ( Molloy et al., 1997 and Karatayev et al., 2007). In contrast, Ophryoglena sp. is a true parasite ( Molloy et al., 1997 and Karatayev et al., 2002), whose levels of infection are likely to be inversely related to the fitness of its host. The numbers of C. acuminatus and Ophryoglena sp. in zebra mussels were significantly positively associated with temperature ( Tables 1, 2). Whereas such a positive relationship has been well documented in previous works for C. acuminatus ( Karatayev et al., 2000a and Karatayev et al., 2003), the existing data for Ophryoglena sp. are controversial. As in our results ( Figure 4), ABT-199 order the highest levels

of the prevalence and intensity of Ophryoglena sp. infection in D. polymorpha from the Dnieper-Bug Canal, Belarus, were observed in summer months ( Karatayev et al. 2002). However, considerably Methamphetamine lower levels of the Ophryoglena sp. infection in zebra mussels were recorded in summer than in winter months in the Drozdy Reservoir, Belarus ( Karatayev et al. 2003) and in the River Meuse, NE France ( Minguez & Giambirini 2012). Additional investigations would help to better understand the seasonal dynamics of this parasitic ciliate in D. polymorpha and the role of temperature and other environmental factors in this process. In his study in the Vistula Lagoon, Raabe (1956) found C. acuminatus to be less tolerant to salinity than its host D. polymorpha, so that the prevalence of infection declined to 0% with increasing

salinity. Neither C. acuminatus nor Ophryoglena sp. demonstrated such a pattern in the Curonian Lagoon. This, however, could be explained by the relatively low average monthly salinities we observed (≤ 4.5 PSU most of the time), preventing confident statistical inference. In addition to the ciliates, we found D. polymorpha to be infected with unidentified nematodes. Several studies conducted in freshwater lakes in Europe ( Karatayev et al., 2003, Mastitsky and Gagarin, 2004 and Mastitsky et al., 2008) suggest that these worms were probably free-living species typically inhabiting periphyton. The most common species of nematodes documented thus far in zebra mussels are oxyphilic representatives of the family Chromadoridae ( Mastitsky and Gagarin, 2004 and Mastitsky et al., 2008).