Sub-chronic effects of AgNPs and AuNPs on Gammarus fossarum (Crustacea Amphipoda): From molecular to behavioural responses.

The aim of the present study was the assessment of the sub-chronic effects of silver (AgNPs) and gold nanoparticles (AuNPs) of 40 nm primary size either stabilised with citrate (CIT) or coated with polyethylene glycol (PEG) on the freshwater invertebrate Gammarus fossarum. Silver nitrate (AgNO3) was used as a positive control in order to study the contribution of silver ions potentially released from AgNPs on the observed effects. A multibiomarker approach was used to assess the long-term effects of AgNPs and AuNPs 40 nm on molecular, cellular, physiological and behavioural responses of G. fossarum. Specimen of G. fossarum were exposed for 15 days to 0.5 and 5 µgL-1 of CIT and PEG AgNPs and AuNPs 40 nm in the presence of food. A significant uptake of both Ag and Au was observed in exposed animals but was under the toxic threshold leading to mortality of G. fossarum. Silver nanoparticles (CIT-AgNPs and PEG-AgNPs 40 nm) led to an up-regulation of Na+K+ATPase gene expression. An up-regulation of Catalse and Chitinase gene expressions due to exposure to PEG-AgNPs 40 nm was also observed. Gold nanoparticles (CIT and PEG-AuNPs 40 nm) led to an increase of CuZnSOD gene expression. Furthermore, both AgNPs and AuNPs led to a more developed digestive lysosomal system indicating a general stress response in G. fossarum. Both AgNPs and AuNPs 40 nm significantly affected locomotor activity of G. fossarum while no effects were observed on haemolymphatic ions and ventilation.

Profiling metal contamination from ultramafic sediments to biota along the Albanian shoreline of Lake Ohrid (Albania/Macedonia).

Ultramafic sediments exhibit high levels of geogenically-derived and potentially toxic metals, with Ni, Cr and Co often exceeding benchmark values. As yet, a holistic understanding of the bioavailability, mobility, potential ecotoxicity and trophic transfer of trace elements in both benthic and pelagic food chains in aquatic ultramafic environments (UME) is lacking. We investigated potential environmental health issues due to metal contamination by jointly implementing chemical, ecological and toxicological tools, along the Albanian shoreline of Lake Ohrid. It is an aquatic system of worldwide importance, representative of temperate UME with a legacy of Ni and Cr contamination from mining activities. Levels of Ni, Cr, Cd, Cu, Co, Fe, Mn and Zn were determined in waters, sediments and native biota. The potential environmental mobility of sediment-bound elements was further assessed using CaCl2, EDTA and acetic acid extractions. Whole-sediment ecotoxicity tests were also carried out using ostracods and chironomids, according to standardized procedures. Despite Ni and Cr concentrations above the sediment quality guidelines for probable effect levels, we did not observe ecotoxic effects in laboratory tests. However, these elements were bioavailable to native organisms under field conditions, especially to benthic primary producers in direct contact with sediments (up to 139 mg Cr kg-1 and 785 mg Ni kg-1). Although biomagnification was not observed, further investigations of metal translocation, metabolism and elemental trophic transfer along benthic food webs appears to be a general research priority in the management of temperate UME. The present study shows that proper management of temperate UME requires not only the integration of data from different lines of evidence, but also laboratory vs. field approaches to understand the subtler, long-term effects of increased elemental body burdens in native organisms.

Mode of action of Pseudomonas fluorescens strain CL145A, a lethal control agent of dreissenid mussels (Bivalvia: Dreissenidae).

Pseudomonas fluorescens strain CL145A (Pf-CL145A) has demonstrated promise as an efficacious and selective agent for the control of macrofouling Dreissena spp. mussels. Herein, we report trials to investigate the mode of action of this biocontrol agent against Dreissena polymorpha, the zebra mussel. Exposure to dead Pf-CL145A cells achieved the same temporal pattern and percentage mussel mortality as did live cells, thereby excluding infection as the possible lethal mode of action. Histological analysis revealed pathologies consistent with the cause of death being intoxicating natural products associated with Pf-CL145A cells. Irrespective of whether the mussels were exposed to live or dead Pf-CL145A cells, examination of tissues from histological sections revealed that: (1) at the end of the 24-h treatment period there was massive hemocyte infiltration into the lumina of both the digestive gland and stomach; and (2) mussel deaths occurred following lysis and necrosis of the digestive gland and sloughing of stomach epithelium. These trials provide strong evidence that the lethal mode of action of Pf-CL145A is intoxication.

Assessing the effects of silver nanoparticles on the ecophysiology of Gammarus roeseli.

Being part of the macrobenthic fauna, gammarids are efficient indicators of contamination of aquatic ecosystems by nanoparticles that are likely to sediment on the bottom. The present study investigates the effects of silver nanoparticles (nAg) on ecophysiological functions in Gammarus roeseli by using a realistic scenario of contamination. Indeed, an experiment was conducted during 72 h, assessing the effects of 5 silver nAg from 10 to 100 nm diluted at concentrations of maximum 5 µg L-1 in a natural water retrieved from a stream and supplemented with food. The measured endpoints in gammarids were survival, silver concentrations in tissues, consumption of oxygen and ventilation of gills. Additionally, a set of biomarkers of the energetic metabolism was measured. After a 72-h exposure, results showed a concentration-dependent increase of silver levels in G. roeseli that was significant for the smallest nAg size (10 nm). Ecophysiological responses in G. roeseli were affected and the most striking effect was a concentration-dependent increase in oxygen consumption especially for the smallest nAg (10 to 40 nm), whereas ventilation of gills by gammarids was not changed. The potential mechanisms underlying these findings are discussed. Thus, we demonstrated the very low exposure concentration of 0.5 µg L-1 for the small nAg size led to significant ecophysiological effects reinforcing the need to further investigate subtle effects on nanoparticles on aquatic organisms.

Spatial and seasonal use of biomarkers in dreissenids: implications for biomonitoring.

In addition to pollution, organisms are exposed to natural variations of the biotic and abiotic factors of their environment. A battery of sub-cellular biomarkers has been measured seasonally in several populations of both Dreissena polymorpha and Dreissena rostriformis bugensis. To observe and understand the variability associated with biomarker responses, water physicochemistry, sediment contamination, and internal concentrations of contaminants in soft tissues were also considered. Results evidenced seasonal, inter-specific, and inter-populational variability of the measured responses, highlighting the needs (1) to acquire long-term data on the studied populations and (2) to incorporate environmental parameters and contamination in the interpretation of biological responses. From a biomonitoring perspective, significant relationships were identified between biomarkers, internal concentrations of contaminants in soft tissues, and sediment contamination in D. r. bugensis and, to a lesser extent, in D. polymorpha. The detailed interpretation of each biomarker of the battery measured is complex, but a global analysis of all biomarkers at once allows to obtain this signature of the contamination of the studied sites.

Fate, subcellular distribution and biological effects of rare earth elements in a freshwater bivalve under complex exposure.

Rare earth elements (REE) are emerging contaminants due to their increased use in diverse applications including cutting-edge and green-technologies. Their environmental concerns and contradicting results concerning their biological effects require an extensive understanding of REE ecotoxicology. Thus, we have studied the fate, bioaccumulation and biological effects of three representative REE, neodymium (Nd), gadolinium (Gd) and ytterbium (Yb), individually and in mixture, using the freshwater bivalve Corbicula fluminea. The organisms were exposed for 96 h at 1 mg L-1 REE in the absence and presence of dissolved organic matter (DOM) reproducing an environmental contamination. Combined analysis of the fate, distribution and effects of REE at tissue and subcellular levels allowed a comprehensive understanding of their behaviour, which would help improving their environmental risk assessment. The bivalves accumulated significant concentrations of Nd, Gd and Yb, which were decreased in the presence of DOM likely due to the formation of REE-DOM complexes that reduced REE bioavailability. The accumulation of Nd, Gd and Yb differed between tissues, with gills > digestive gland ≥ rest of soft tissues > hemolymph. In the gills and in the digestive gland, Nd, Gd and Yb were mostly (>90 %) distributed among metal sensitive organelles, cellular debris and detoxified metal-rich granules. Gadolinium, Yb and especially Nd decreased lysosome size in the digestive gland and disturbed osmo- and iono-regulation of C. fluminea by decreasing Na concentrations in the hemolymph and Ca2+ ATPase activity in the gills. Individual and mixed Nd, Gd and Yb exhibited numerous similarities and some differences in terms of fate, accumulation and biological effects, possibly because they have common abiotic and biotic ligands but different affinities for the latter. In most cases, individual and mixed effects of Nd, Gd, Yb were similar suggesting that additivity approach is suitable for the environmental risk assessment of REE mixtures.

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.

Cross-effects of nickel contamination and parasitism on zebra mussel physiology.

Aquatic organisms are exposed to pollution which may make them more susceptible to infections and diseases. The present investigation evaluated effects of nickel contamination and parasitism (ciliates Ophryoglena spp. and intracellular bacteria Rickettsiales-like organisms), alone and in combination, on biological responses of the zebra mussel Dreissena polymorpha, and also the infestation abilities of parasites, under laboratory controlled conditions. Results showed that after 48 h, more organisms were infected in nickel-exposed groups, which could be related to weakening of their immune system. Acting separately, nickel contamination and infections were already stressful conditions; however, their combined action caused stronger biological responses in zebra mussels. Our data, therefore, confirm that the parasitism in D. polymorpha represents a potential confounding factor in ecotoxicological studies that involve this bivalve.

Implications of speciation on rare earth element toxicity: A focus on organic matter influence in Daphnia magna standard test.

Rare earth elements (REE) have become essential in high- and green-technologies. Their increasing use lead to the release of anthropogenic REE into the environment including aquatic systems. The limited data available on the aquatic ecotoxicology of REE indicate their biological effects are highly dependent on their speciation, posing challenges for a reliable environmental risk assessment (ERA). The current study assessed the influence of speciation on the toxicity of neodymium (Nd), gadolinium (Gd) and ytterbium (Yb) in the Daphnia magna mobility inhibition test (ISO 6341:2012). REE toxicity was assessed individually and in ternary mixture, in the absence and presence of dissolved organic matter (DOM). Speciation was predicted by modeling and REE bioaccumulation by D. magna was measured to better understand the relationship between REE speciation and toxicity. DOM decreased significantly the toxicity of Nd, Gd and the mixture towards this freshwater crustacean. This was explained by a lower REE bioaccumulation in the presence of DOM due to REE-DOM complexation, which reduced REE bioavailability. DOM effects on Yb toxicity and bioaccumulation were limited because of Yb precipitation. We show that the way of expressing EC50 values (based on nominal, measured or predicted REE concentrations in solution) drastically changed REE toxicity assessment and that these changes were influenced by REE speciation. This study demonstrates for the first time that REE speciation, and especially REE-DOM complexation, significantly influences REE bioaccumulation and toxicity towards D. magna. Our results have implications for the subsequent ERA of REE.

Early defense responses in the freshwater bivalve Corbicula fluminea exposed to copper and cadmium: Transcriptional and histochemical studies.

The aim of the present study was to measure the early effects of copper (10 and 50 µg L(-1)), cadmium (2, 10, and 50 µg L(-1)) and mixtures of these metals in the freshwater bivalve Corbicula fluminea exposed for 12 h in laboratory. Transcription levels of superoxide dismutase (SOD), catalase (CAT), selenium-dependent glutathione peroxidase (Se-GPx), pi-class glutathione S-transferase (pi-GST), metallothionein (MT) in digestive gland and gills, and response of lysosomal system and neutral lipids in digestive gland were determined after the exposure period. Results showed that lysosomal system, neutral lipids content, and mRNA levels were modified, suggesting their early response against oxidative stress and their important role in cell integrity. The integrated biomarker response was calculated and showed that the effects of the combinations of Cu and Cd on the biomarker responses are additive. MT and pi-GST mRNA expression correspond to the largest ranges of response. As efficient biomarkers should have an early warning capacity, SOD, CAT, Se-GPx, pi-GST, MT transcripts levels, lysosomal system, and neutral lipids could be used as biomarkers of metal contamination in the aquatic environment.

Spatial and temporal variations of biological responses to environmental pollution in the freshwater zebra mussel.

The validation of a suite of cellular biomarkers for biomonitoring studies necessitates a good knowledge of the meaning of these early responses to environmental stress in terms of individual health. This requires confirmation (i) of linkages between the cellular and higher levels of the biological organisation, (ii) of temporal persistence of the stress symptoms and (iii) of their reversibility after a return to more favourable conditions. Besides, (iv) the sensitivity of the biomarker suite towards subtle variations of environmental contamination has to be assessed. With this aim, field experiments were performed on deployed freshwater zebra mussels (Dreissena polymorpha) in the vicinity of the confluence of a small heavily anthropized stream with a larger river. We examined the persistence of the responses over a 90-day period and their reversibility after a depuration-transplantation. A second experiment was conducted later by adding a study site at an increased distance from the confluence. Decreased digestive lysosomal volume and neutral lipid contents, and lipofuscin accumulation preceded effects on the mussels' condition. The following experiment confirmed that the cellular biomarkers were more sensitive than both individual endpoints to reflect the effects of subtler variations of environmental contamination. Integration of the results with multivariate analysis and the Integrated Biomarker Response tended to confirm the relevance of the biomarker suite.

Interactions between parasitism and biological responses in zebra mussels (Dreissena polymorpha): Importance in ecotoxicological studies.

Given that virtually all organisms are hosts for parasites, the investigation of the combined effects of contamination and parasitism is important in the framework of aquatic bioindication procedures. To assess the impact of such multistresses at the host cellular level, we sampled parasitized zebra mussel (Dreissena polymorpha) populations from two sites in northeast France that presented different levels of contamination. Experimental groups were formed based on parasite species and host gender and tested by histochemistry and automated image analysis for biological responses, such as structural changes of the lysosomal system and neutral lipid accumulation. Infected organisms displayed smaller and more numerous lysosomes compared with uninfected congeners, and infection further elevated the effect of the chemical contamination on this biomarker. In contrast, co-infection of females with selected parasites did produce inverse results, i.e. a more developed lysosomal system and neutral lipid depletion. Our data, therefore, suggest that parasitism in zebra mussels represents a potential confounding factor in ecotoxicological studies and must be taken into account in environmental risk assessment studies.

Energy allocation in two dreissenid species under metal stress.

Measurements of biological responses on living organisms are essential in aquatic biomonitoring. In freshwaters, Dreissena polymorpha is an invasive bivalve commonly used in ecotoxicological studies and considered as a model organism. However, D. polymorpha abundances are declining while another species colonizes most of the freshwaters: Dreissena rostriformis bugensis. This species has already been studied in ecophysiology but there is still a lack of data concerning its responses to stressors before its use as a bioindicator of environmental pollution. This study aims to compare the responses of the two species exposed to metal stress. Responses at different levels of biological organization were targeted with measurement of sub-cellular and individual biomarkers following an exposure of up to 7 days to cadmium at 10 µg.L-1. At the individual level, the scope for growth (SFG) was measured. It corresponds to the energy allocated to growth and reproduction. D. polymorpha exhibits variations in biomarker measurements as well as in the SFG in presence of Cd. D. r. bugensis shows no variation in its responses at the different targeted levels. According to the present results, energy metabolism seems to have an essential role for these species when facing a metal stress. Different energy allocation strategies were evidenced between the two species, although the link with biochemical biomarkers is more evident for D. polymorpha than for D. r. bugensis.

A sub-individual multilevel approach for an integrative assessment of CuO nanoparticle effects on Corbicula fluminea.

Because they are widely used, copper oxide nanoparticles (CuO NPs) are likely to enter the aquatic environment and then reach the sediment. We have examined the effect of CuO NPs in the freshwater endobenthic bivalve Corbicula fluminea. Some previous studies have investigated effects at biochemical and physiological levels, but molecular endpoints are still poorly studied despite they are sensitive in early detection of NPs effect. In the present study, we have investigated short-term effects (96 h) of CuO NP (12, 30 nm; 0, 20 and 100 µg/L) using molecular endpoints as well as more conventional biochemical and physiological markers. The expression of antioxidant (CuZnSOD, MnSOD, Cat, Se-GPx, Trxr) and antitoxic (GST-Pi, HSP70, MT, Pgp, MRP1) related genes was measured at the mRNA level while antioxidant (SOD, TAC) and antitoxic (GST, ACP) defenses, energetic reserves and metabolism (ETS, Tri, LDH), and cellular damages (LPO) were assessed using a biochemical approach. The filtration rate measured at 96 h provided information at the physiological scale. Gene expression and filtration rate were responsive to CuO NPs but the effects differed according to the NP size. The results suggest that defense mechanisms may have been set up following 30 nm-NP exposure. The response to 12 nm-NP was lower but still showed that exposure to 12 nm-NP led to activation of cellular elimination mechanisms. The lowering of the filtration rate may have protected the organisms from the contamination. However, this raised the question of further repercussions on organism biology. Together, the results (i) indicate that CuO NP may exert effects at different levels even after a short-term exposure and (ii) point out the precocity of molecular response.

Corbicula fluminea gene expression modulated by CeO2 nanomaterials and salinity.

Cerium dioxide nanomaterials (CeO2 NMs) are used in different fields and incorporated in daily products. Several studies highlighted their effects on organism physiology, although molecular studies remain scarce. NM behavior is strongly dependent on the environment but few data are available using complex exposure media, raising the question of its environmental impacts. The aim of the present work was to assess the toxic potential of three CeO2 NMs in Corbicula fluminea at a molecular level by RT-qPCR under a more realistic scenario of exposure, in a multistress context at two different salinities (1.5 and 15 psu). C. fluminea was exposed for 28 days to pulses of the three selected NMs (reference, manufactured, and aged manufactured). In bivalves, the gills and digestive gland are two key organs used for ecotoxicological studies. The expression change of 12 genes was measured in control organisms after 28 days in both organs, allowing us to clearly separate the responses for both organs and salinities. As gills come in contact with the environment first, we monitored gene the expression at intermediate time points (7, 14, and 21 days) for this organ in order to highlight clams responses to NM and salinity. Two genes (Se-GPx, MnSOD) had a salinity-dependent level of expression. HSP70, Se-GPx, and Trxr mRNAs presented significant changes in their expressions in the presence of NM. This study was completed using an integrated statistical approach. The exposed organisms differed more from control at field salinity than those exposed to hyper-saline conditions. At 15 psu, salinity pressure seems to cause the first molecular impact. At 1.5 psu, gene expression patterns allowed the effect of each NM to separate clearly. These results confirmed the usefulness of gene expression studies. Moreover, we highlighted the necessity to assess the environmental toxicity of the different forms of manufactured NM.

Environmental transcriptomes of invasive dreissena, a model species in ecotoxicology and invasion biology.

Dreissenids are established model species for ecological and ecotoxicological studies, since they are sessile and filter feeder organisms and reflect in situ freshwater quality. Despite this strong interest for hydrosystem biomonitoring, omics data are still scarce. In the present study, we achieved full de novo assembly transcriptomes of digestive glands to gain insight into Dreissena polymorpha and D. rostriformis bugensis molecular knowledge. Transcriptomes were obtained by Illumina RNA sequencing of seventy-nine organisms issued from fifteen populations inhabiting sites that exhibits multiple freshwater contamination levels and different hydrosystem topographies (open or closed systems). Based on a recent de novo assembly algorithm, we carried out a complete, quality-checked and annotated transcriptomes. The power of the present study lies in the completeness of transcriptomes gathering multipopulational organisms sequencing and its full availability through an open access interface that gives a friendly and ready-to-use access to data. The use of such data for proteogenomic and targeted biological pathway investigations purpose is promising as they are first full transcriptomes for this two Dreissena species.

Changes in protein expression in mussels Mytilus galloprovincialis dietarily exposed to PVP/PEI coated silver nanoparticles at different seasons.

Potential toxic effects of Ag NPs ingested through the food web and depending on the season have not been addressed in marine bivalves. This work aimed to assess differences in protein expression in the digestive gland of female mussels after dietary exposure to Ag NPs in autumn and spring. Mussels were fed daily with microalgae previously exposed for 24 h to 10 µg/L of PVP/PEI coated 5 nm Ag NPs. After 21 days, mussels significantly accumulated Ag in both seasons and Ag NPs were found within digestive gland cells and gills. Two-dimensional electrophoresis distinguished 104 differentially expressed protein spots in autumn and 142 in spring. Among them, chitinase like protein-3, partial and glyceraldehyde-3-phosphate dehydrogenase, that are involved in amino sugar and nucleotide sugar metabolism, carbon metabolism, glycolysis/gluconeogenesis and the biosynthesis of amino acids KEGG pathways, were overexpressed in autumn but underexpressed in spring. In autumn, pyruvate metabolism, citrate cycle, cysteine and methionine metabolism and glyoxylate and dicarboxylate metabolism were altered, while in spring, proteins related to the formation of phagosomes and hydrogen peroxide metabolism were differentially expressed. Overall, protein expression signatures depended on season and Ag NPs exposure, suggesting that season significantly influences responses of mussels to NP exposure.

Is the freshwater gammarid, Dikerogammarus villosus, a suitable sentinel species for the implementation of histochemical biomarkers?

In order to enlarge the range of potential sentinel species for the implementation of a multiple biomarker approach, spatial and monthly morphological variations of four cellular compartments and contents were assessed during two years in the hepatopancreatic caeca of the freshwater gammarid, Dikerogammarus villosus (Crustacea, Amphipoda), using histochemistry coupled to image analysis. Among the three study sites, the second one, located in a reservoir receiving the overheated and copper-contaminated waters of a nuclear power plant, was the most anthropised. During this passive biomonitoring survey, unsaturated neutral lipids were more abundant, the surface densities of the lysosomal and peroxisomal systems were, respectively less and more important, and lipofuscin granules tended to accumulate in the amphipods from the second site compared to both others. Nonetheless, in this context, the present cellular biomarker suite, analysed through an integrative approach, was not powerful enough to highlight spatial heterogeneity significantly. This may partly result from particularities in the patterns of metal accumulation and copper physiological requirements of amphipods (haemocyanin synthesis). Nevertheless, we think that the use of this Ponto-Caspian non-indigenous species in biomonitoring surveys deserves further investigation, owing to its current position in freshwater and brackish European ecosystems, considering both biomass and functioning. Cellular responses could be studied in parallel with endpoints at other levels of the biological organization to compose a more powerful biomarker suite. Furthermore, comparing biological responses to environmental stress in this invasive species and less competitive autochthonous gammarids could be of great interest.

Impact of multiple stressors on biomarker responses in sympatric dreissenid populations.

Dreissenid mussels, well-known invaders of the northern hemisphere, also constitute good biomonitors for freshwater quality assessment. Whereas the oldest, Dreissena polymorpha, is relatively well-known, the new invasive, Dreissena rostriformis bugensis, has been very little characterized. The aim of this study was to compare subcellular biomarker responses within these species, by taking into account inter- and intra-specific variability in biomarker responses during a multi-stress assessment. Indeed, due to local environmental conditions and genetic background, biomarker responses can be different between populations of a given species, not only between species. To evaluate these differential responses, we sampled mussels on two sites where they are both present in sympatry (plus another D. r. bugensis population), and we exposed these populations in laboratory to different stressors: thermal stress (12 or 17 °C), dietary stress (fed or unfed), contamination (nickel at 0, 20 and 500 µg L-1) and exposure duration (0, 4 or 8 days). Results mainly evidenced strong inter-species and inter-population differences, underlining the need to discriminate correctly between the two species and to know well the populations used in biomonitoring. Results also evidenced thermal and food stress-related effects. The numerous data obtained during this multi-stress experiment also highlight the complexity of working on several stressors and analysing the associated results.

Differential tolerance to nickel between Dreissena polymorpha and Dreissena rostriformis bugensis populations.

Differential tolerance to stress is partly responsible for the heterogeneity of biomarker responses between populations of a sentinel species. Although currently used for freshwater biomonitoring, studies concerning inter-populational variability in tolerance to contaminants for the zebra mussel (Dreissena polymorpha) are scarce. Moreover, this well-known invader is currently replaced by another, the quagga mussel (Dreissena rostriformis bugensis). To evaluate the differential tolerance between dreissenids, several populations of both species were exposed to a high concentration of nickel. A LT50 (time when 50% of individuals were dead) was established for each population. Biomarker responses and internal nickel concentration were also measured, to link tolerance with physiological status. Results evidenced that D. polymorpha populations are more heterogeneous and more tolerant than D. r. bugensis ones. For D. polymorpha populations only, LT50 values were positively correlated with the nickel contamination in situ, with higher anti-oxidative defences and a higher Integrated Biomarker Response value in the field. Such findings may be explained by local adaptation and invasion dynamic within each species. The significance of this differential tolerance when using biomarker responses for biomonitoring purposes is thus discussed.