A wide diversity of endosymbiotic organisms have been found within the mantle cavity or within the organs/tissues of Dreissena. Molloy et al. (1997) discussed 34 species as parasites — all of which were reported from attached mussels (i.e., none from pelagic larvae). Their definition of “parasites,” however, was intentionally broad and included a number of organisms whose symbiotic relationships with Dreissena still remain unclear (e.g., possibly mutualism or commensalism).
In nature, some parasites exhibit a broad host range, and thus it was expected that some indigenous North American parasites would be capable of infecting Dreissena. This appears to be occurring, as the few obligate parasites that have been found thus far in North American Dreissena are indigenous organisms whose hosts are North American native bivalves (i.e., aspidogastrid and plagiorchiid trematodes, Toews et al. 1993).
In European zebra mussels, species of obligate, strictly host specific ciliates in the orders Scuticociliatida (Conchophthirusacuminatus and Conchophthirusklimentinus) and Rhynchodida (Hypocomagalmadreissenae, Sphenophryadreissenae, and Sphenophryanaumiana) are known from the mantle cavity and at least one species in the order Hymenostomatida (suborder Ophryoglenina) from the digestive gland (Molloy et al. 1997).
The nature of the symbiotic relationships of these ciliates appears to range from commensalism (scuticociliatid ciliate species) to parasitism (rhynchodid and hymenostomatid ciliate species). In a personal communication from S. Kazubski appearing in Stanczykowska (1977), the hymenostomatid ciliates were reported as naturally causing lethal infections in Dreissena.
In Europe, seven genera of trematodes have been reported as parasites of Dreissena spp. (Molloy et al. 1997). In their life cycles, Dreissena can serve as the first intermediate host (e.g., for Bucephaluspolymorphus and Phyllodistomum spp.), second intermediate host (e.g., for Echinoparyphiumrecurvatum), or the only host (e.g., for Aspidogaster spp.). Those trematode species that use Dreissena as an intermediate host usually subsequently develop in fish or waterfowl.
Aspidogaster spp.
Whereas host-specific ciliates and trematodes are the most commonly reported European parasites, a variety of other pathogenic organisms have been recorded from field samples, including suspected bacterial and haplosporidan (ascetosporan) infections (Molloy et al. 1997). Mites, nematodes, leeches, chironomids, and oligochaetes have also been observed to be associated symbiotically within the mantle cavity, but with little to no adverse effect.
How pathogenic are the parasites reported from Dreissena? The Eurasian trematode Bucephaluspolymorphus has been well-documented as being seriously debilitating to Dreissena (i.e., it sterilizes Dreissena by destroying their gonads). A putatively lethal infection with haplosporidan (ascetosporan) protozoans has been reported from D. polymorpha populations in the Netherlands (Bowmer and van der Meer 1991, de Kock and Bowmer 1993). Infections were noted over a
3-year period in several rivers. The entire blood system – including the blood spaces of the digestive gland and gonad, the filamentary blood vessels of the gills, and the mantle – were observed to contain plasmodia, sporocysts, and spores. Infection often resulted in little or no remaining functional tissue in the digestive gland. Compared to the wide diversity of virulent parasites known from other bivalves, particularly commercially valuable marine species (Lauckner 1983, Sparks 1985, Sindermann 1990), the list of seriously debilitating Dreissena parasites is currently a short one. Prior to their arrival in North America, however, relatively little attention was paid to their diseases (Molloy 1992). Eurasian parasite records were almost all reports of “large” organisms easily detected during dissection (trematodes, ciliates, etc.). An intensive research effort employing histological techniques to detect diseases at the cellular level is now under way both in Europe and North America (author, unpublished data), and this is likely to reveal a much broader assemblage of parasites, particularly pathogenic, intracellular, microbial species.