(1997)) was moderately but statistically significantly correlated with shell length of D. polymorpha (Spearman r = 0.421, n = 240, p < 0.001 for C. acuminatus and Spearman r = 0.318, n = 240, p < 0.001 for Ophryoglena Pexidartinib mw sp.). As concluded by the corresponding Poisson log-linear models, the numbers of ciliates were also positively associated with water temperature, but not salinity ( Table 1 and Table 2). In addition to the host-specific C. acuminatus and Ophryoglena sp., we occasionally encountered zebra mussels whose mantle cavities contained live nematodes. These unidentified

worms were observed in D. polymorpha collected from August to October, and were consistently found only in molluscs with shell length > 15 mm. The number of nematodes in infected zebra mussels never exceeded 1, with the prevalence of infection being 10% in August and September, and 15% in October. Although Dreissena polymorpha has been present in the Curonian Lagoon for about 200 years ( Leppäkoski & Olenin 2000), our study is the first report of endosymbionts in the mollusc from this part of the Baltic Sea, and also the first record of the ciliates Conchophthirus acuminatus and Ophryoglena sp. in

Lithuanian populations of zebra mussels. There have been occasional studies of the parasites of D. polymorpha in Lithuania related to the cytogenetics of the trematodes Phyllodistomum folium Olfers, 1817 and Bucephalus polymorphus Baer, 1826, hosted by the mollusc in freshwater lakes; however, no data on the levels of check details infection have been reported for these parasites ( Petkevičiūtė et al., 2003 and Stunžėnas et al., 2004). In the 1950s, a study similar to ours was conducted by Raabe also (1956) in the brackish Vistula Lagoon (0.5–6.5 PSU (Chubarenko & Margonski 2008)) of the Baltic Sea, Poland. The author found two species of ciliates infecting

zebra mussels, i.e. C. acuminatus and Hypocomagalma dreissenae Jarocki & Raabe, 1932. The presence of C. acuminatus in both the Curonian and Vistula Lagoons is not surprising and in line with the ubiquitous distribution of this protozoan in European populations of D. polymorpha ( Molloy et al., 1997, Karatayev et al., 2007 and Mastitsky et al., 2008). There could be two major reasons for the absence of H. dreissenae in our samples, the first one being the ecology of this ciliate. H. dreissenae prefers saline waters ( Raabe, 1956 and Jankowski, 2001), so that the rather low salinity levels in the central part of the Curonian Lagoon ( Figure 2) as compared to the truly brackish Vistula Lagoon ( Raabe, 1956 and Rolbiecki and Rokicki, 2008) could have prevented H. dreissenae from developing a detectable population. The second reason could be associated with the dissection technique used in our study: H. dreissenae are of rather small size (length 32–50 μm; Molloy et al. 1997), which makes it difficult to detect this ciliate without histological analysis. In contrast, Ophryoglena sp.