| Summary: | We report the first results of a parasitological study of<i>Dreissena polymorpha</i> (zebra mussels) from the brackishCuronian Lagoon, SE Baltic Sea. Zebra mussels were collected monthlyfrom May to October 2011 from a site near the mouth of the River Nemunas.Three types of endosymbionts were found in the mantle cavity andvisceral mass of the molluscs during dissections, i.e. thecommensal ciliate <i>Conchophthirus acuminatus</i> and parasitic ciliate<i>Ophryoglena</i> sp., and rarely encountered, unidentified nematodes.The abundances of <i>C. cuminatus</i> and <i>Ophryoglena</i> sp.were positively associated with host shell length and watertemperature, but no effect of water salinity was detected.As the endosymbionts are either highly host-specific to zebra mussels(<i>C. acuminatus</i> and <i>Ophryoglena</i> sp.) or are probablyfree-living organisms that inadvertently infect the molluscs (nematodes),we conclude that the presence of <i>D. polymorpha</i> in theCuronian Lagoon does not pose any serious parasitologicalrisk to native biota. We emphasize, however, that this conclusionshould be treated with caution as it is based on a study conductedonly at a single location. Our work extends the currentlyscarce records of <i>D. polymorpha</i> parasites and commensals frombrackish waters, and adds to a better understanding of the ecologicalimpact this highly invasive mollusc causes in the areas it has invaded.
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