ALEXANDER Y. KARATAYEV 1, *, LYUBOV E. BURLAKOVA 1 , DANIEL P. MOLLOY 2, ** and LYUDMILA K. VOLKOVA 1 1 General Ecology Department, Belarussian State University, 4 Skoryna Ave., Minsk, Belarus 220050. e-mail: karataev@geo.bsu.unibel.by 2 Biological Survey, New York State Museum, Cultural Education Center, Albany, NY 12230, USA. e-mail: dmolloy@mail.nysed.gov Endosymbionts of Dreissena polymorpha (P ALLAS) in Belarus key words: Dreissena polymorpha, endosymbionts, Conchophthirus acuminatus, trematodes Abstract Dreissena polymorpha were dissected and examined for endosymbionts from 17 waterbodies in Belarus – the country through whose waterways zebra mussels invaded Western Europe nearly two centuries ago. Fourteen types of parasites and other symbionts were observed within the mantle cavity and/or associated with internal tissues, including ciliates (Conchophthirus acuminatus, Ancistrumina limnica, and Ophryoglena sp.), trematodes (Echinostomatidae, Phyllodistomum, Bucephalus polymor- phus, and Aspidogaster), nematodes, oligochaetes, mites, chironomids, and leeches. Species composi- tion of endosymbionts differed among river basins and lake systems. The most common endosymbiont was the ciliate C. acuminatus. Its mean infection intensity varied significantly among waterbodies from 67 ± 6 to 3,324 ± 556 ciliates/mussel. 1. Introduction Zebra mussels, Dreissena polymorpha, have been spreading throughout European water- bodies since the beginning of the 19th century causing significant ecological impacts in the process (MACISAAC, 1996; KARATAYEV et al., 1997). Only a little more than a decade ago, Dreissena spp. were introduced into North American waters. Within a few years after the discovery of D. polymorpha in Lake St. Clair in 1988 (HEBERT et al., 1989), these mussels rapidly spread across much of North America (JOHNSON and PADILLA, 1996), causing hundreds of millions of dollars in damage and increased operating expenses at raw water- dependent infrastructures (O’NEILL, 1996, 1997). The extent of the ecological and economic impact of zebra mussels is directly related to their density within a habitat, and it is well documented that population densities of zebra mussels are not stable and can fluctuate widely (RAMCHARAN et al., 1992). Defining the role endosymbionts play in these density fluctuations is a critical step toward a comprehensive understanding of the population dynamics of these pest mussels. Although the first paper on zebra mussel endosymbionts appeared almost 150 years ago (CLAPARÈDE and LACHMANN, 1858), little is still known about their biology, infection prev- alence (i.e., percent of mussels with endosymbionts), and infection intensity (i.e., number of endosymbionts per infected mussel). Compared to the wide diversity of virulent parasites known from other bivalves, particularly commercially valuable marine species (LAUCKNER, 1983; SPARKS, 1985; SINDERMANN, 1990), zebra mussels appear to have relatively few * This author has also published under ALEXANDER YU.KARATAEV ** author to whom the correspondence should be addressed Internat. Rev. Hydrobiol. 85 2000 5–6 543–559