A wide diversity of endosymbiotic organisms can be found within the mantle cavity or within the organs/tissues of Dreissena, but which have potential as biocontrol agents? 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).
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).
Ciliate species that have been recorded only from Dreissena spp.
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. Thus, these host-specific parasitic ciliates may well have potential as biocontrol agents if they can be economically mass-produced.
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.).
White branching sporocyst stage of the trematode Bucephalus polymorphus within the gonads.
Dozens of Phyllodistomum sporocysts within a gill demibranch
The cysts are typically found embedded in tissues lining the mantle cavity
Adults of two Aspidogaster spp. known to parasitize zebra mussels
Those trematode species that use Dreissena as an intermediate host usually subsequently develop in fish or waterfowl. Because of the potential adverse impact on these subsequent hosts, these trematode species have little potential as biocontrol candidates.
Whereas ciliates and trematodes are the most commonly reported parasites, a variety of other pathogenic organisms have been recorded from field samples, including suspected bacterial and 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. In laboratory trials, Toews et al. (1993) reported initiating lethal infections in D. polymorpha with Gram negative bacteria.
Host specificity of a parasite is a key feature for selection of a biocontrol candidate. 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 native bivalves (i.e., aspidogastrid and plagiorchiid trematodes, Toews et al. 1993). The broad host range of these parasites essentially eliminates them from consideration as biocontrol agents. In contrast, many other parasites could be particularly safe control agents since they have been fine-tuned through evolution to be strictly host-specific. But such host-specific parasites, however, can also have limitations as control agents. They often have complex growth requirements and elaborate life cycles, and these characteristics could represent obstacles toward their economical, invitro mass production (in vivo mass production of parasites would likely be more expensive, thereby hindering commercialization).
High host pathogenicity is another feature desirable in a parasitic biocontrol agent. The Eurasian trematode Bucephaluspolymorphus has been well-documented as being seriously debilitating to Dreissena (i.e., it sterilizes them by destroying their gonads), but its use is out of the question since fish are also infected as part of its life cycle. A putatively lethal infection with 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. Using parasites to debilitate Dreissena in its veliger stage (i.e., prior to settlement) would be an ideal control strategy, and a special effort should be made to detect the parasites (likely all intracellular) present in these pelagic larvae.
Comprehensive investigations to identify host-specific, quick-killing parasites suitable for invitro mass production are also a research priority. Research to date has confirmed that many of the organisms living within the bodies of Dreissena do tend to be extremely host-specific. The above-mentioned six species of ciliates, for example, have never been found in any other bivalve other than Dreissena. Research leading to the commercial development of a parasite as a control agent is typically a long-term process requiring exhaustive experimentation to verify host specificity prior to parasite release for control purposes. Commercially available parasite products, although a small component of global pesticides, do exist for control of other invertebrate pests, primarily insects, and include bacteria, fungi, nematodes, protozoans, and viruses (Rodgers 1993). Because many Dreissena parasites are strictly host-specific, the future use of parasites as biocontrol agents is not an unrealistic possibility.
The most effective use of parasites as biocontrol agents may be in an inundative fashion (Debach and Rosen 1991) where Dreissena spp. are simply overwhelmed by exposure to abnormally high concentrations of a parasite. The effect of a disease on a host (i.e., ranging from benign to lethal) is governed by a complex of factors, such as host physiological state, environmental stress, and parasite density within the host (Lauckner 1983). A parasite that normally causes a disease of low pathogenicity could actually induce a lethal infection if, for example, its density within the host was abnormally high. Thus, if parasites that are strictly host-specific can be economically mass-produced, they could be used to induce artificially high, intense infections, resulting in serious host debilitation and death. For this reason, parasites that are strictly host-specific to Dreissena, yet which in nature have low pathogenicity, should not be excluded from consideration as potential biocontrol agents.
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