Is there a link between shell morphology and parasites of zebra mussels?

The shell morphology of zebra mussels, Dreissena polymorpha, was analyzed to determine if alterations in shell shape and asymmetry between valves were related to its infection status, i.e. infected or not by microparasites like ciliates Ophryoglena spp. or intracellular bacteria Rickettsiales-like organisms (RLOs), and by macroparasites like trematodes Phyllodistomum folium and Bucephalus polymorphus. For microparasites, two groups of mussels were observed depending on shell measurements. Mussels with the more concave shells were the most parasitized by ciliates. This could be more a consequence than a cause and we hypothesized that a modification of the water flow through the mantle cavity could promote the infection with a ciliate. There were more RLOs present in the most symmetrical individuals. A potential explanation involved a canalization of the left-right asymmetry as a by-product of the parasite infection. Trematode infections were associated with different responses in valve width. Females infected by P. folium displayed significantly higher symmetry in valve width compared with non-infected congeners, whereas the infection involved an opposite pattern in males. B. polymorphus was also linked to a decrease in valve width asymmetry. This study suggested that a relationship exists between parasitism and shell morphology through the physiological condition of host zebra mussels.     

Haplosporidium raabei n. sp. (Haplosporidia): a parasite of zebra mussels, Dreissena polymorpha (Pallas, 1771)

Extensive connective tissue lysis is a common outcome of haplosporidian infection. Although such infections in marine invertebrates are well documented, they are relatively rarely observed in freshwater invertebrates. Herein, we report a field study using a comprehensive series of methodologies (histology, dissection, electron microscopy, gene sequence analysis, and molecular phylogenetics) to investigate the morphology, taxonomy, systematics, geographical distribution, pathogenicity, and seasonal and annual prevalence of a haplosporidian observed in zebra mussels, Dreissena polymorpha. Based on its genetic sequence, morphology, and host, we describe Haplosporidium raabei n. sp. from D. polymorpha - the first haplosporidian species from a freshwater bivalve. Haplosporidium raabei is rare as we observed it in histological sections in only 0·7% of the zebra mussels collected from 43 water bodies across 11 European countries and in none that were collected from 10 water bodies in the United States. In contrast to its low prevalences, disease intensities were quite high with 79·5% of infections advanced to sporogenesis.     

Comparison of rDNA sequences from colchicine treated and untreated sporocysts of Phyllodistomum folium and Bucephalus polymorphus (Digenea)

The most frequently used antimitotic agent in cytogenetic studies is colchicine. We investigated whether the initial treatment of trematodes for karyological analysis with colchicine would have mutagenic or degradational effect on rDNA sequences. Dreissena polymorpha is the intermediate host of Phyllodistomum folium and Bucephalus polymorphus, and the sporocyst stage of these trematode species develop, respectively, in the gills and gonads of this mussel. Sporocysts of P. folium and B. polymorphus were obtained from D. polymorpha collected from waterbodies in Belarus and in Lithuania. 5.8S and 28S rDNA genes, ITS1 and ITS2 of P folium and B. polymorphus were sequenced and compared, and no nucleotide sequence differences between colchicine treated and untreated trematodes were found. Based on these results, we conclude that colchicine treatment for 3-5 h has no mutagenic or degradational effect on rDNA sequences. During the course of this investigation, two genetically different P. folium samples were noted in Belarus.     

Field and laboratory studies of Ophryoglena sp. (Ciliata: Ophryoglenidae) infection in zebra mussels,Dreissena polymorpha (Bivalvia: Dreissenidae)

This study, conducted in the Dnieper-Bug Canal in Belarus, is the first to monitor the seasonal (June-November) dynamics of infection with the parasitic ciliate Ophryoglena sp. in a zebra mussel (Dreissena polymorpha) population. Mean population prevalence and intensity of infection varied, respectively, from 11 to 62% and from 0.9 to 24.1 ciliates/mussel. Mean prevalence was highly correlated with mussel length in mussels <20 mm (R(2)=0.97) and was lower in larger mussels. Mean infection intensity in mussels 1-25 mm long was similarly correlated with their size (R=0.98), reached a maximum in the 20-25 mm size-class, and then sharply decreased, thus providing evidence, albeit limited, that high intensity of infection might be lethal. Transinfection of zebra mussels by Ophryoglena sp. was achieved in the laboratory-a first for a protozoan parasite of D. polymorpha; from an initial complete lack of infection, mean prevalence and intensity rose, respectively, to 86.7% and 8.3 ciliates/mussel.     

Involvement of Apoptosis in Host-Parasite Interactions in the Zebra Mussel

The question of whether cell death by apoptosis plays a biological function during infection is key to understanding host-parasite interactions. We investigated the involvement of apoptosis in several host-parasite systems, using zebra mussels Dreissena polymorpha as test organisms and their micro- and macroparasites. As a stress response associated with parasitism, heat shock proteins (Hsp) can be induced. In this protein family, Hsp70 are known to be apoptosis inhibitors. Mussels were diagnosed for their respective infections by standard histological methods; apoptosis was detected using the TUNEL methods on paraffin sections and Hsp70 by immunohistochemistry on cryosections. Circulating hemocytes were the main cells observed in apoptosis whereas infected tissues displayed no or few apoptotic cells. Parasitism by intracellular bacteria Rickettsiales-like and the trematode Bucephalus polymorphus were associated with the inhibition of apoptosis whereas ciliates Ophryoglena spp. or the trematode Phyllodistomum folium did not involve significant differences in apoptosis. Even if some parasites were able to modulate apoptosis in zebra mussels, we did not see evidence of any involvement of Hsp70 on this mechanism.     

Phyllodistomum folium (Trematoda: Gorgoderidae) infecting zebra mussels (Dreissena polymorpha) in the Ebro River, Spain

Zebra mussels (Dreissena polymorpha) were first found in the Ebro River (Spain) in Ribaroja reservoir, in the summer of 2001. This paper reports a study to detect parasites in this bivalve species. From September 2003 to August 2004, a total of 1380 zebra mussels were collected and dissected or sectioned in paraffin and haematoxylin and eosin staining. We observed the presence of Phyllodistomum folium (Olfers, 1816) in two hosts (prevalence 0.14%). Sporocysts containing metacercariae were located within the gill lamellae. One of the mussels was collected in January and the other one in July. In both cases the shell length was >2 cm. P. folium had not been previously reported in Spain and D. polymorpha is its only known intermediate host. It represents a new invasive species in this river basin, presumably introduced together with the zebra mussels.     

Predation of zebra mussels by round gobies, Neogobius melanostomus

Preliminary gut analysis of a recent Great Lakes invader, the round goby, Neogobius melanostomus (7.0–8.4 cm), collected from the Detroit River, showed that they ate zebra mussels (58%), snails (6%), and other invertebrates (36%), including aquatic insects (Hexagenia), softshelled crayfish, and zooplankton. Because zebra mussels, Dreissena polymorpha, predominated as prey, we investigated the ability of round gobies to consume different size classes of zebra mussels. In laboratory experiments, we examined feeding preferences of three size classes of round gobies (5.5–6.9 cm; 7.0–8.4 cm; 8.5–10.3 cm standard length) on four different size classes of zebra mussels (6.0–9.9 mm, 10.0–12.9 mm, 13.0–15.9 mm, 16.0–18.9 mm). All sizes of round gobies ate zebra mussels < 10.0 mm. Only the largest size class of round gobies ate larger zebra mussels (10.0–12.9 mm) when all prey sizes were presented. The association between the total mass of zebra mussels available and the amount consumed by round gobies increased positively up to about 6.5 g of available mussels and then levelled off. Round gobies consumed an average of 1.0 g of mussels in 24 h. There was a significant positive relationship between gape size and standard length of round gobies. Although larger round gobies (over the size range of fish in our study) are able to consume larger zebra mussels, small mussels were preferred. Our findings suggest that the preference of small zebra mussels by round gobies has the potential to alter the size structure of zebra mussel populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Factors affecting trematode infection rates in freshwater mussels

Mussels are intermediate hosts of digenean trematodes, but determinants of these infections remain unknown. To address this problem, we collected duck mussels Anodonta anatina in eighteen lakes from northeastern Poland and examined how mussel age, sex, and the encrustation with zebra mussels Dreissena polymorpha and environmental conditions in lakes influenced infection rates. We also assessed parasitic preferences to host gonads and hepatopancreas and the impact of parasites on female fertility. Mussels were infected with Rhipidocotyle campanula and Phyllodistomum sp. Infection rates were higher in older and female mussels but were unrelated to the biomass of encrusting D. polymorpha and the trophy, thermal conditions, and Ca²⁺ availability. Parasites occupied gonads more often than hepatopancreas. Infected females were less likely to carry glochidia and incubated fewer glochidia. We suggest that the risk of infection by digenean trematodes increases with the amount of water processed by filter-feeding hosts and/or that parasites actively seek hosts which can provide them with abundant resources. This mechanism explains why parasites more often occupied older and female mussels and targeted their gonads. Future research on trematode-mussel interactions should integrate knowledge on different elements of the complex trematode life cycles, including effects of higher-order hosts such as fish.     

Northern map turtles (Graptemys geographica) derive energy from the pelagic pathway through predation on zebra mussels (Dreissena polymorpha)

1. Zebra mussels ( Dreissena polymorpha ) derive their energy from the pelagic energy pathway by filtering plankton. Because zebra mussels occur in high densities in littoral habitats, they potentially constitute an important trophic link between littoral consumers and pelagic energy sources. Northern map turtles ( Graptemys geographica ) are widespread in North America and consume zebra mussels.
2. We used stable isotopes analyses to quantify the flow of energy from the pelagic pathway to northern map turtles and to infer the contribution of zebra mussels to map turtle biomass. We then built a bioenergetic model to estimate the annual intake of zebra mussels by northern map turtles in Lake Opinicon, Ontario, Canada.
3. Stable isotopes analyses indicated that zebra mussels constitute between 0% and 14% of the diet of males and between 4% and 36% of the diet of females. Assuming that zebra mussels account for all of the pelagic contribution, we estimated that map turtles consume 3200 kg of zebra mussels annually. Because female map turtles are much larger than males and consume more zebra mussels, they are responsible for 95% of the zebra mussel biomass ingested annually.
4. The pelagic pathway supports an important part of the standing crop biomass of map turtles in Lake Opinicon. We highlight the importance of freshwater turtles in lake ecosystems. Unravelling the trophic interactions mediated by freshwater turtles will lead to a more integrated picture of lake ecosystems.     

Occurrence and parasite fauna of Dressena bugensis in the Rybinsk reservoir

The zebra mussel Dreissena bugensis (Andrusov), a representative of mollusc fauna in the Dnieper-Bug estuary, have been found for the first time in the Rybinsk reservoir (the Volga reach, Shumorovka River). In pericardial and mantle cavities and inner organs of molluscs the following parsites have been found: Aspidogaster limacoides, Caspiobdella fadejevi, Helobdella stagnalis, eggs of the water mites of the genus Unionicola, and representatives of nonspecific saprotrophic fungi of the genus Acremonium. It is shown that composition of parasites and free-living organisms in D. bugensis is similar to that in D. polymorpha.     

Ciliary activity of cells of gill and leg glimmeral epithelium of Unionidae invaded by trematodes of Aspidogaster conchicola and Bucephalus polymorphus

The paper provides data concerning the influence of the parasitic worms Aspidogaster conchicola and Bucephalus polymorphus on the rate and duration of ciliary beating of gill and leg in 7 species of Unionidae (Unio conus borysthenicus, U. tumidus falcatulus, U. rostratus rostratus, U. limosus graniger, U. pictorum ponderosus, Colletopterum piscinale falcatum, C. ponderosum rumanicum). The high level of infection of molluscs with B. polymorphus oppresses the glimmeral epithelium activity of gill and leg (by 4.2-32.3%). The weak and moderate levels of infection of molluscs with B. polymorphus do not decrease the glimmeral epithelium activity and sometimes even rise it (by 3.4-8.1%). The presence of few A. conchicola (1-3 individuals) in the organism of mollusc does not change the functioning of glimmeral epithelium.     

Predicting zebra mussel fouling on native mussels from physicochemical variables

1. Predictive models of impact are needed for the risk assessment of invasive species. One such species is the Eurasian zebra mussel ( Dreissena polymorpha ), a fouling bivalve that overgrows and kills native mussels (Unionidae) in many North American lakes and rivers. The level of mortality in a native mussel population increases with the fouling intensity – i.e. the mean number of zebra mussels attached to each native mussel.
2. We conducted a multi-site survey within the St Lawrence and Richelieu rivers (Quebec, Canada) to determine whether zebra mussel fouling intensity can be predicted from environmental variables. We found fouling intensity to be positively correlated with calcium concentration [Ca²⁺] and negatively correlated with sediment size, but not affected by local macrophyte cover.
3. A multiple regression model that includes calcium concentration and sediment size explains 86% of variation in fouling intensity across all sites.
4. Analysis of data from invaded sites in North America and Europe revealed a nonlinear relationship in which fouling intensity increases with calcium concentration up to an asymptotic threshold of 24 mg L⁻¹ Ca²⁺.
5. Our results suggest that the community-level impacts of zebra mussels are mediated by abiotic environmental variables and gradients in these variables may provide local habitat refugia for native mussels.     

Infection status of pond smelts, Hypomesus olidus, and other freshwater fishes with trematode metacercariae in 6 large lakes

In order to determine the current infection status of pond smelts, Hypomesus olidus, and other freshwater fishes with trematode metacercariae, a total of 4,861 pond smelts and 18 other freshwater fishes, collected from the Soyang and Uiam Lakes in Gangwon-do, the Unam Lake in Jeollabuk-do, the Jangseong Lake in Jeollanam-do, the Uirim-ji (lake) in Chungcheongbuk-do, and the Andong Lake in Gyeongsangbuk-do, were individually digested with 1% pepsin-HCl and examined under a dissecting microscope. In all pond smelts caught from the 6 lakes, we were unable to detect any known human infectious trematode metacercariae in Korea. However, in other freshwater fishes, such as, Squalidus japonicus coreanus (Unam Lake), and Zacco platypus (Jangseong Lake) and Opsariichthys uncirostris amurensis (Jangseong Lake), metacercariae of human-infecting trematodes, i.e., Clonorchis sinensis and Metagonimus sp. were detected, respectively.     

Different host exploitation strategies in two zebra mussel-trematode systems: adjustments of host life history traits

The zebra mussel is the intermediate host for two digenean trematodes, Phyllodistomum folium and Bucephalus polymorphus, infecting gills and the gonad respectively. Many gray areas exist relating to the host physiological disturbances associated with these infections, and the strategies used by these parasites to exploit their host without killing it. The aim of this study was to examine the host exploitation strategies of these trematodes and the associated host physiological disturbances. We hypothesized that these two parasite species, by infecting two different organs (gills or gonads), do not induce the same physiological changes. Four cellular responses (lysosomal and peroxisomal defence systems, lipidic peroxidation and lipidic reserves) in the host digestive gland were studied by histochemistry and stereology, as well as the energetic reserves available in gonads. Moreover, two indices were calculated related to the reproductive status and the physiological condition of the organisms. Both parasites induced adjustments of zebra mussel life history traits. The host-exploitation strategy adopted by P. folium would occur during a short-term period due to gill deformation, and could be defined as "virulent." Moreover, this parasite had significant host gender-dependent effects: infected males displayed a slowed-down metabolism and energetic reserves more allocated to growth, whereas females displayed better defences and would allocate more energy to reproduction and maintenance. In contrast, B. polymorphus would be a more "prudent" parasite, exploiting its host during a long-term period through the consumption of reserves allocated to reproduction.     

Effects of the benthic suspension feeder Dreissena polymorpha on zooplankton in a large river

1. We conducted a series of in situ enclosure experiments to assess the impact of zebra mussels ( Dreissena polymorpha ) on the plankton of the Ohio River. Adult mussels were suspended in pelagic enclosures ('potamocorrals') at three densities (0, 1000, 2500 mussels per corral) and incubated for 6 days with daily plankton and physiochemical sampling.
2. The presence of adult zebra mussels was correlated with a shift in composition of the phytoplankton community and a severe reduction in some rotifers. The effects of zebra mussels on the larger zooplankton were taxon-dependent, but bacterial density showed no trend among treatments.
3. Zebra mussels may have significant negative impacts on zooplankton, which may in turn alter riverine food webs.     

The effect of seasonal variation in selective feeding by zebra mussels (Dreissena polymorpha) on phytoplankton community composition

1. To investigate the impact of zebra mussels ( Dreissena polymorpha ) on phytoplankton community composition, temporal variability in selective feeding by the mussels was determined from April to November 2005 in a natural lake using Delayed Fluorescence (DF) excitation spectroscopy.
2. Selective grazing by zebra mussels varied in relation to seasonal phytoplankton dynamics; mussels showed a consistent preference for cryptophytes and avoidance of chlorophytes and cyanobacteria. Diatoms, chrysophytes and dinoflagellates responded differentially to zebra mussel grazing depending on their size. Analysis of excreted products of the zebra mussels revealed that in addition to chlorophytes and cyanobacteria, phytoplankton >50  μ m and very small phytoplankton (≤7  μ m) were largely expelled in pseudofaeces.
3. The zebra mussel is a selective filter-feeder that alters its feeding behaviour in relation to phytoplankton composition to capture and ingest high quality phytoplankton, especially when phytoplankton occur in preferred size ranges. Flexibility of zebra mussel feeding behaviour and variation in susceptibility among phytoplankton groups to mussel ingestion indicate that invading zebra mussels could alter phytoplankton community composition of lakes and have important ecosystem consequences.     

Microsatellite loci for dreissenid mussels (Mollusca: Bivalvia: Dreissenidae) and relatives: markers for assessing exotic and native populations

We developed and tested 14 new polymorphic microsatellite loci for dreissenid mussels, including the two species that have invaded many freshwater habitats in Eurasia and North America, where they cause serious industrial fouling damage and ecological alterations. These new loci will aid our understanding of their genetic patterns in invasive populations as well as throughout their native Ponto-Caspian distributions. Eight new loci for the zebra mussel Dreissena polymorpha polymorpha and six for the quagga mussel D. rostriformis bugensis were compared with new results from six previously published loci to generate a robust molecular toolkit for dreissenid mussels and their relatives. Taxa tested include D. p. polymorpha, D. r. bugensis, D. r. grimmi, D. presbensis, the 'living fossil'Congeria kusceri, and the dark false mussel Mytilopsis leucophaeata (the latter also is invasive). Overall, most of the 24 zebra mussel (N = 583) and 13 quagga mussel (N = 269) population samples conformed to Hardy-Weinberg equilibrium expectations for the new loci following sequential Bonferroni correction. The 11 loci (eight new, three previously published) evaluated for D. p. polymorpha averaged 35.1 alleles and 0.72 mean observed heterozygosity per locus, and 25.3 and 0.75 for the nine loci (six new, three previously published) developed for D. r. bugensis. All but three of these loci successfully amplified the other species of Dreissena, and all but one also amplified Congeria and Mytilopsis. All species and populations tested were significantly divergent using the microsatellite data, with neighbour-joining trees reflecting their evolutionary relationships; our results reveal broad utility for resolving their biogeographic, evolutionary, population and ecological patterns.     

Mortality of zebra mussel, Dreissena polymorpha, veligers during downstream transport

1. Streams flowing from lakes which contain zebra mussels, Dreissena polymorpha , provide apparently suitable habitats for mussel colonization and downstream range expansion, yet most such streams contain few adult mussels. We postulated that mussel veligers experience high mortality during dispersal via downstream transport. They tested this hypothesis in Christiana Creek, a lake-outlet stream in south-western Michigan, U.S.A., in which adult mussel density declined exponentially with distance downstream.
2. A staining technique using neutral red was developed and tested to distinguish quickly live and dead veligers. Live and dead veligers were distinguishable after an exposure of fresh samples to 13.3 mg L⁻⁻¹ of neutral red for 3 h.
3. Neutral red was used to determine the proportion of live veligers in samples taken longitudinally along Christiana Creek. The proportion of live veligers (mean ± SE) declined from 90 ± 3% at the lake outlet to 40 ± 8% 18 km downstream.
4. Veligers appear to be highly susceptible to damage by physical forces (e.g. shear), and therefore, mortality in turbulent streams could be an important mechanism limiting zebra mussel dispersal to downstream reaches. Predictions of zebra mussel spread and population growth should consider lake-stream linkages and high mortality in running waters.     

Zebra mussels mediate benthic–pelagic coupling by biodeposition and changing detrital stoichiometry

1. The zebra mussel ( Dreissena polymorpha ) is one of the most successful invasive species; it has colonised many aquatic systems in Europe and North America with strong impacts on various ecosystem processes. The effect of D. polymorpha filtration on pelagic seston concentrations has been quantified in several studies, but the magnitude and stoichiometry of the transfer of sestonic biomass into benthic detritus by D. polymorpha and the accompanying enrichment of the benthic habitat is still under-investigated.
2. We studied biodeposition by zebra mussels in two series of laboratory experiments with the food algae Cryptomonas erosa and Scenedesmus obliquus . We also measured the year-round biodeposition rate under natural conditions in the oligotrophic Lake Constance.
3. In all experiments, zebra mussel biodeposition was linearly related to seston concentration. In the field, the relationship changed with a seasonal shift in algal composition and lower biodeposition rates during the spring algal bloom.
4. For both algal species in laboratory experiments, biodeposited material was depleted in phosphorous at an algal concentration ≤0.6 mg ash-free dry mass L⁻¹, but not at higher concentrations. This effect was not observed in the field, probably because of high variation in C : N : P stoichiometry.
5. By mediating the transfer of pelagic resources into the benthos zebra mussels provide a sufficient amount of detritus for benthic invertebrates, especially during summer. Thus, material biodeposited by the mussels might increase benthic secondary production from pelagic resources, and zebra mussels are important mediators of this flux of organic matter from the pelagic zone into the benthos.     

Zebra mussel adhesion: structure of the byssal adhesive apparatus in the freshwater mussel, Dreissena polymorpha

The freshwater zebra mussel (Dreissena polymorpha) owes a large part of its success as an invasive species to its ability to attach to a wide variety of substrates. As in marine mussels, this attachment is achieved by a proteinaceous byssus, a series of threads joined at a stem that connect the mussel to adhesive plaques secreted onto the substrate. Although the zebra mussel byssus is superficially similar to marine mussels, significant structural and compositional differences suggest that further investigation of the adhesion mechanisms in this freshwater species is warranted. Here we present an ultrastructural examination of the zebra mussel byssus, with emphasis on interfaces that are critical to its adhesive function. By examining the attached plaques, we show that adhesion is mediated by a uniform electron dense layer on the underside of the plaque. This layer is only 10-20 nm thick and makes direct and continuous contact with the substrate. The plaque itself is fibrous, and curiously can exhibit either a dense or porous morphology. In zebra mussels, a graded interface between the animal and the substrate mussels is achieved by interdigitation of uniform threads with the stem, in contrast to marine mussels, where the threads themselves are non-uniform. Our observations of several novel aspects of zebra mussel byssal ultrastructure may have important implications not only for preventing biofouling by the zebra mussel, but for the development of new bioadhesives as well.