Byssal proteins of the freshwater zebra mussel,Dreissena polymorpha

The freshwater zebra mussel (Dreissena polymorpha) is a notorious biofouling organism. It adheres to a variety of substrata underwater by means of a proteinaceous structure called the byssus, which consists of a number of threads with adhesive plaques at the tips. The byssal proteins are difficult to characterize due to extensive cross-linking of 3,4-dihydroxyphenylalanine (DOPA), which renders the mature structure largely resistant to protein extraction and immunolocalization. By inducing secretion of fresh threads and plaques in which cross-linking is minimized, three novel zebra mussel byssal proteins were identified following extraction and separation by gel electrophoresis. Peptide fragment fingerprinting was used to match tryptic digests of several gel bands against a cDNA library of genes expressed uniquely in the mussel foot, the organ which secretes the byssus. This allowed identification of a more complete sequence of Dpfp2 (D. polymorpha foot protein 2), a known DOPA-containing byssal protein, and a partial sequence of Dpfp5, a novel protein with several typical characteristics of mussel adhesive proteins.     

Particle transport in the zebra mussel, Dreissena polymorpha (Pallas)

The capture, transport, and sorting of particles by the gills and labial palps of the freshwater mussel Dreissena polymorpha were examined by endoscopy and video image analysis. More specifically, the morphology of the feeding organs in living zebra mussels was described; the mode and speeds of particle transport on the feeding organs was measured; and the sites of particle selection in the pallial cavity were identified. Particle velocities (outer demibranch lamellae, 90 microm s(-1); inner demibranch lamellae, 129 microm s(-1); marginal food groove of inner demibranchs, 156 microm s(-1); dorsal ciliated tracts, 152 microm s(-1)), as well as the movement of particles on the ctenidia and labial palps of D. polymorpha, are consistent with mucociliary, rather than hydrodynamic, transport. Particles can be sorted on the ctenidia of zebra mussels, resulting in a two-layer transport at the marginal food groove of the inner demibranch. That is: preferred particles are transported inside the marginal groove proper, whereas particles destined for rejection are carried superficially in a string of mucus. Sorting also occurs at the ventral margin of the outer demibranch; desirable particles are retained on the outer demibranch, whereas unacceptable particles are transferred to the inner demibranch and ultimately excluded from ingestion. We suggest that the structure of homorhabdic ctenidia does not preclude particle sorting, and that some ecosystem modifications attributed to zebra mussels may ultimately be due to ctenidial sorting mechanisms.     

Indigenous microflora and opportunistic pathogens of the freshwater zebra mussel, Dreissena polymorpha

Freshwater fouling invertebrate zebra mussels (Dreissena polymorpha) harbor a diverse population of microorganisms in the Great Lakes of North America. Among the indigenous microorganisms, selective species are opportunistic pathogens to zebra mussels. Pathogenicity to zebra mussels by opportunistic bacteria isolated from the mussels was investigated in this study. Among the more than 30 bacteria isolated from temperature-stressed mussels, Aeromonas media, A. veronii, A. salmonicida subsp. salmonicida, and Shewanella putrefaciens are virulent pathogens to juvenile zebra mussels. Inoculation of a bacterial concentration of A. media, A. salmonicida subsp. salmonicida and S. putrefaciens at 10⁷ cells per zebra mussel resulted in 100% mortality within 5 days, and only 64.9% for A. veronii. In contrast, mortality was less than 12.3% following inoculation of a sterile phosphate buffer solution as a control. In addition, mortality was dependent on the size of the pathogen population used in inoculation and the incubation temperature, indicating the close relationship between the bacterial population and subsequent death. On the mussel tissue, a dense microbial population was evident from the moribund mussels viewed with Scanning Electron Microscope (SEM). Opportunistic bacteria invaded and destroyed the D. polymorpha tissue after 7 days of incubation when the bacterial inoculation was larger than 10⁵ per zebra mussel. Our results suggest that mussels are reservoirs of opportunistic pathogenic microorganisms to aquatic organisms and humans and a better understanding of the microbial ecology of the mussels will provide insights to the possible health hazards from these microorganisms.     

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.     

Respiratory response to temperature and hypoxia in the zebra mussel Dreissena polymorpha

The effects of temperature acclimation, acute temperature variation and progressive hypoxia on oxygen consumption rates (VO2) were determined for the zebra mussel Dreissena polymorpha. In the first experiment, after acclimation to 5, 15 or 25 degrees C for at least 2 weeks, VO2 was determined at 5 degrees C increments from 5 to 45 degrees C. VO2 increased in all three acclimation groups from 5 to 30 degrees C, corresponding to the normal ambient temperature range for this species. Mussels displayed imperfect temperature compensation at temperatures above 15 degrees C, but exhibited little acclimatory ability below 15 degrees C. In the hypoxia experiment, VO2 was determined over the course of progressive hypoxia, from full saturation (oxygen tension [PO2]=160 Torr [21.3 kPa]) to a PO2 at which oxygen uptake ceased (<10 Torr [1.3 kPa]). Mussels were acclimated to either 5, 15 or 25 degrees C for at least 2 weeks and their respiratory response to progressive hypoxia was measured at three test temperatures (5, 15 and 25 degrees C). The degree of oxygen regulation increased with increasing test temperature, particularly from 5 to 15 degrees C, but decreased with increasing acclimation temperature. The decreased metabolic rate observed for warm-acclimated animals, particularly in the upper portion of the temperature range of the zebra mussel, may allow for conservation of organic energy stores during warm summer months. Compared to other freshwater bivalves, D. polymorpha is a relatively poor oxygen regulator, corresponding with its preference for well-oxygenated aquatic habitats. In addition, a new quantitative method for determining the degree of oxygen regulation is presented.     

Serotonergic mechanisms mediating spawning and oocyte maturation in the zebra mussel,Dreissena polymorpha

Summary

The zebra mussel, Dreissena polymorpha, is a freshwater biofouling bivalve unintentionally introduced in the 1980s into North America from Europe. Oocyte maturation (germinal vesicle breakdown, GVBD) and spawning of the zebra mussel can be triggered with serotonin (5-hydroxytryptamine, 5-HT). In pharmacological experiments to characterize the receptor mediating spawning, the serotonin receptor agonists 8-OH-DPAT, TFMPP, and 1-(1-naphthyl)piperazine were effective at stimulating spawning; whereas, 2-methylserotonin and alpha-methylserotonin had no effect. In experiments with antagonists of serotonin receptors ketanserin and propranolol had no effect; mianserin, NAN-190, and cyproheptadine had partial inhibitory effects; and methiothepin was a very effective antagonist. Metergoline had mixed agonist/antagonist properties. Ergotamine was the most effective activator of spawning in females. Compared to serotonergic receptors in other organisms, the receptors that activate spawning in zebra mussels resemble 5HTlym, 5HTdro2 and human 5HT1Dβ, which are receptors that may act both by inhibiting adenylyl cyclase and by activating phospholipase C. In zebra mussels, 5-HT and 8-OH-DPAT activate GVBD in gonad fragments, a process also initiated by manual dissection of gonad fragments. GVBD can be inhibited by pre-treatment of ovaries with forskolin and theophylline, suggesting an inhibitory role for cyclic AMP. The Ca²⁺ ionophore A23187 can trigger GVBD and polar body formation. Thus, oocyte maturation in zebra mussels may be initiated via serotonergic receptors simultaneously inhibiting adenylyl cyclase and activating Ca²⁺ mechanisms.     

Effects of deionized water on viability of the zebra mussel,Dreissena polymorpha

1. Zebra mussels exposed to deionized water (DW, changed daily) begin to die within several days. More than half the animals die in DW within a week.2. Animals allowed to reattach themselves prior to exposure to DW live somewhat longer than detached animals exposed to DW but are still sensitive to the toxic effects of DW.3. Small animals die in response to DW faster than large animals.4. The toxic effect of DW can be suppressed by addition of 0.5 mM NaCl, 0.25 mM MgSO₄, or small amounts of aquarium water.5. KCl is about 10 times more toxic to zebra mussels when applied in DW than when applied in aquarium water.6. The toxic effects of 37 mg KCl/1 DW are not prevented by the addition of 0.5 mM NaCl or 0.25 mM MgSO₄.7. Stimulation of potassium transport mechanisms by DW may account for the increased toxicity of KCl in DW. Toxic effects of DW and increased toxicity of potassium in DW may have applications in controlling zebra mussel infestations.     

Modelling the local dynamics of the zebra mussel (Dreissena polymorpha)

1. The bivalve Dreissena polymorpha has invaded many freshwater ecosystems worldwide in recent decades. Because of their high fecundity and ability to settle on almost any solid substratum, zebra mussels usually outcompete the resident species and cause severe damage to waterworks. Time series of D. polymorpha densities display a variety of dynamical patterns, including very irregular behaviours. Unfortunately, there is a lack of mathematical modelling that could explain these patterns. 2. Here, we propose a very simple discrete‐time population model with age structure and density dependence that can generate realistic dynamics. Most of the model parameters can be derived from existing data on D. polymorpha. Some of them are quite variable: with respect to these we perform a sensitivity analysis of the model behaviour and verify that non‐equilibrial regimes (either periodic or chaotic) are the rule rather than the exception. 3. Even in circumstances where the model dynamics are aperiodic it is possible to predict total density peaks from previous peaks. This turns out to be true also in the presence of environmental stochasticity. 4. Using the stochastic model we explore the effects of age‐selective predation. Quite surprisingly, larger removal rates of adults do not always result in smaller population densities and mussel biomasses. Moreover, non‐selective predation can result in skewed size‐frequency distributions which, therefore, are not necessarily the footprint of predators’ preference for larger or smaller zebra mussels.     

Variation in growth rates of the zebra mussel, Dreissena polymorpha, within Lake Wawasee

1. Field experiments conducted in Lake Wawasee in 1995 and 1996 measured the response of shell growth of Dreissena polymorpha to environmental gradients.
2. Shell growth decreased with initial shell length in four mussel size classes ranging between 8 and 22 mm, and decreased with depth, with mussels in shallow water (<4 m) having growth rates nearly twice those of mussels in deeper water (4–7 m).
3. Growth occurred early in the spring–summer period (May–June) with relatively little shell added later in the summer (July–September), and varied significantly among sites within Lake Wawasee, but not between the 2 years of this study.
4. Rank order of sites was consistent for both years implying that environmental conditions responsible for variation in shell growth were stable within Lake Wawasee.
5. Cage design did not have a significant effect on mussel shell growth nor did the distance of growth cages above the bottom (0.5–0.75 m above the bottom versus directly on the bottom).
6. This study demonstrates the sensitivity of adult mussel growth to subtle variation in environmental conditions occurring within and among lakes, with potential consequences for mussel population dynamics and community structure and function.     

Long-term demography of a zebra mussel (Dreissena polymorpha) population

1. We used long‐term data and a simulation model to investigate temporal fluctuations in zebra mussel populations, which govern the ecological and economic impacts of this pest species. 2. The size of the zebra mussel (Dreissena polymorpha) population in the Hudson River estuary fluctuated approximately 11‐fold across a 13‐year period, following a cycle with a 2–4 year period. 3. This cycling was caused by low recruitment during years of high adult population size, rapid somatic growth of settled animals, and adult survivorship of 50% per year. 4. Adult growth and body condition were weakly correlated with phytoplankton biomass. 5. The habitat distribution of the Hudson's population changed over the 13‐year period, with an increasing proportion of the population spreading onto soft sediments over time. The character of soft‐sediment habitats in the Hudson changed because of large amounts (mean = 34 g DM m^?2) of empty zebra mussel shells now in the sediments. 6. Simulation models show that zebra mussel populations can show a range of long‐term trajectories, depending on the balance between adult space limitation, larval food limitation, and disturbance. 7. Effective understanding and management of the effects of zebra mussels and other alien species depend on understanding of their long‐term demography, which may vary across ecosystems.     

Food quality controls reproduction of the zebra mussel (Dreissena polymorpha)

Species such as Dreissena polymorpha sometimes contribute substantially in the transfer of primary to secondary production. During the ontogenetic cycle, the reproductive investment of adult mussels is one of the main parameters that affect recruitment success. We studied how food quality and temperature affect the reproductive investment in term of egg mass of D. polymorpha in a lake by sampling mussels monthly from 4 m and 15 m depths. Temperature affected reproduction directly and also indirectly through the food. To assess whether temperature and/or food conditions led to the differences observed in mussels sampled from the two depths, mussels were reared in the laboratory under two different temperature regimes for 3 months, simulating the temperature of the lake at 4 m and 15 m depth. Possible effects of food quality were tested at each temperature using four diets differing in fatty acid composition. Temperature played an important role as a trigger for spawning, and the type of diet clearly affected the reproductive investment. When the heterokont chromophyte alga Nannochloropsis limnetica, which is rich in polyunsaturated fatty acids (PUFAs) and long-chained PUFAs (>C18), was fed to mussels, an increased egg mass was obtained. This result was in contrast to that found when the green alga Scenedesmus obliquus and the cyanobacterium Aphanothece sp., both of which are deficient in long-chained PUFAs, were offered as food to the mussels. Such a PUFA-dependent food quality may affect reproduction in lakes. Food quality effects vary seasonally in a lake and may be most important in summer, when low-food-quality green algae and cyanobacteria are abundant. The low biochemical quality of these blooms may affect at least the later period of gametogenesis of D. polymorpha, which reproduces from June to August.     

Genetic variability and phylogeography of the invasive zebra mussel, Dreissena polymorpha (Pallas)

The zebra mussel, Dreissena polymorpha (Pallas), a bivalve species originally native to the Black and Caspian seas, has invaded Ireland in the last decade. Five microsatellite loci were used to investigate genetic diversity and population structure in 10 populations across Europe (Ireland, UK, the Netherlands and Romania) and the Great Lakes (Lake Ontario and Lake St Clair). Levels of allelic diversity and mean expected heterozygosity were high for all populations (mean number of alleles/locus and H(E) were 10-15.2 and 0.79-0.89, respectively). High levels of polymorphism observed in Irish populations suggest that the Irish founder population(s) were large and/or several introductions took place after foundation. Significant deficits of heterozygotes were recorded for all populations, and null alleles were the most probable factor contributing to these deficits. Pairwise comparisons using Fisher exact tests and F(ST) values revealed little genetic differentiation between Irish populations. The UK sample was not significantly differentiated from the Irish samples, most probably reflecting an English origin for Irish zebra mussels. No significant differentiation was detected between the two Great Lakes populations. Our data support a northwest rather than a central or east European source for North American zebra mussels.     

Histological changes and micronucleus induction in the zebra mussel Dreissena polymorpha after paraquat exposure

The herbicide paraquat (PQ), still widely used in developing countries, represents a serious risk factor for human and environmental health. To test the sublethal effects of PQ on the freshwater bivalve Dreissena polymorpha, mussels were exposed to 0.125, 0.250, 0.500 mg/L for 7 and 14 days and histologically screened. PQ's genotoxic potential was also determined in haemocytes by the micronucleus, MN, assay. At concentrations > or = 0.250 mg/L, severe lesions, such as cellular vacuolation, lysis and thinness of the germinative epithelia were observed in the digestive gland and testis. A positive trend between the number of granulocytes and all PQ concentrations was observed in both gonads and digestive glands, addressing the inflammatory capacity of this herbicide on these tissues. Mussels exposed to PQ also exhibited a significant MN induction. The spontaneous MN frequencies ranged from 2.75 to 0.425 per cent, while PQ-induced MN rates in treated mussels were between 3.50 and 1.250 per cent. The histopathological effects on the digestive and reproductive systems, as well as the MN induction in the haemocytes, confirmed the cytotoxic and genotoxic effects of PQ also in D. polymorpha.     

Ionic and acid-base consequences of exposure to increased salinity in the zebra mussel, Dreissena polymorpha

Dreissena polymorpha, an invasive freshwater bivalve, displays physiological characteristics that reflect its ancestry in brackish water, yet it has limited ability to withstand modest increases in salinity. We examined changes in hemolymph ion concentrations and acid-base variables in mussels transferred to and incubated in 10% artificial seawater (ASW) for 7 days and then returned to pondwater (PW) for a further 7 days. Hemolymph was sampled (10 animals per sample period) every 4 h for the first 24-h incubation and at 72 h and 168 h for both the transfer to 10% ASW and the transfer back to PW. The initial response to transfer to 10% ASW was a rapid attainment of an apparent isoosmotic steady state, with most hemolymph ion concentrations rising and attaining steady state within 12 h. Hemolymph magnesium rose more slowly, and hemolymph calcium declined despite an increase in its concentration in the bathing medium. Hemolymph pH rose significantly during the first 24 h, from 7.96 to 8.25, as a result of increases in bicarbonate; pH subsequently returned to normal through increases in PCO2. When animals were returned to PW after 7 days' incubation in ASW, the response of the major hemolymph ions was largely the reverse of that effected by the transfer to ASW. Hemolymph pH was not altered significantly until after 72 h in PW, when declines in bicarbonate lowered the pH to 7.73. Strong ion difference (SID) was related significantly to hemolymph pH. Hemolymph calcium and magnesium showed a reciprocal relationship throughout both transfer and incubation. Solubility interactions between sulfate and calcium and magnesium may be important in determining calcium availability in solution. The Na/K ratio in hemolymph was maintained within relatively narrow bounds throughout the procedure and may contribute to the mussels' ability to volume-regulate during an osmotic challenge. Overall, the responses of D. polymorpha to modest changes in salinity were largely the result of passive processes.     

Grazing on colonial and filamentous, toxic and non-toxic cyanobacteria by the zebra mussel Dreissena polymorpha

Colony forming and toxic cyanobacteria form a problem in surfacewaters of shallow lakes, both for recreation and wildlife. Zebramussels, Dreissena polymorpha, have been employed to help torestore shallow lakes in the Netherlands, dominated by cyanobacteria,to their former clear state. Zebra mussels have been presentin these lakes since they were created in the 19th century bythe excavation of peat and are usually not considered to bean invasive species. Most grazing experiments using Dreissenahave been performed with uni-cellular phytoplankton laboratorystrains and information on grazing of larger phytoplankton taxahardly exists. To gain more insight in to whether D. polymorphais indeed able to decrease cyanobacteria in the phytoplankton,we therefore performed grazing experiments with zebra musselsand two species of cyanobacteria, that greatly differ in shape:colony forming strains of Microcystis aeruginosa and the filamentousspecies Planktothrix agardhii. For both species a toxic anda non-toxic strain was selected. We found that zebra musselscleared toxic Planktothrix at a higher rate than non-toxic Planktothrix,toxic or non-toxic Microcystis. Clearance rates between theother strains were not significantly different. Both phytoplanktonspecies, regardless of toxicity, size and shape, were foundin equal amounts (based on chlorophyll concentrations) in theexcreted products of the mussels (pseudofaeces). The resultsshow that zebra mussels are capable of removing colonial andfilamentous cyanobacteria from the water, regardless of whetherthe cyanobacteria are toxic or not. This implies that the musselsmay be used as a biofilter for the removal of harmful cyanobacterialblooms in shallow (Dutch) lakes where the mussels are alreadypresent and not a nuisance. Providing more suitable substratefor zebra mussel attachment may lead to appropriate mussel densitiescapable of filtering large quantities of cyanobacteria.     

Development of a molecular diagnostic system to discriminate Dreissena polymorpha (zebra mussel) and Dreissena bugensis (quagga mussel)

A 3-primer PCR system was developed to discriminate invasive zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussel. The system is based on: 1) universal primers that amplifies a region of the nuclear 28s rDNA gene from both species and 2) a species-specific primer complementary to either zebra or quagga mussel. The species-specific primers bind to sequences between the binding sites for the universal primers resulting in the amplification of two products from the target species and one product from the nontarget species. Therefore, nontarget products are positive amplification controls. The 3-primer system accurately discriminated zebra and quagga mussels from seven geographically distinct populations.     

Trinucleotide microsatellite loci for the zebra mussel Dreissena polymorpha,an invasive species in Europe and North America

The high mutation rate at microsatellite loci can supply important demographic information on founder events and range expansion in an invasive species such as the zebra mussel Dreissena polymorpha, following its initial introduction. In order to facilitate studies into the colonization patterns of this species in new habitats in Europe and North America, five trinucleotide microsatellite loci were isolated from a partial DNA library. Allelic diversity at all described loci was high, ranging from 20 to 35 alleles per locus. Homologous loci were not amplified in a second related invasive species, Dreisenna bugensis, using the primers developed here.