Reduction of growth and haemolymph Ca levels in the freshwater snail Lymnaea stagnalis chronically exposed to cobalt

Impact of aquatic humic substances on speciation and toxicity of arsenic and cobalt to Ceriodaphnia dubia

Humic substances (HS) interact with trace metals such as As and Co, affecting their mobility and availability in aquatic systems. However, their combined effects on toxicity to aquatic organisms are not totally understood. The objective of this study was to evaluate the toxicity of Co(II) and As(III) to the water flea Ceriodaphnia dubia in the presence of HS, considering element speciation. Toxicity assays were performed in the presence and absence of HS at two different concentrations of As(III) (10 and 20 μg/L) and Co(II) (50 and 100 μg/L). The free As(III) and Co(II) (< 1 kDa, fraction most potentially bioavailable) in the test solutions were determined via ultrafiltration. While free Co(II) decreased by approximately 80% in the presence of HS, free As(III) decreased just by 1%. Despite the higher percentage of As(III) potentially bioavailable, the presence of HS reduced significantly the toxicity of As at 20 μg/L (no toxicity was observed at 10 μg/L). This was attributed to direct effects of HS such as hormesis, hormone-like effects of HS and/or formation of protective coating. These effects also stimulated the reproduction, including in the assays in the absence of As and Co. HS reduced the toxicity of Co(II) at both test concentrations. The results of this investigation support that HS should be considered when safe limits for As and Co are defined.

What contributes to the metal-specific partitioning in the chub-acanthocephalan system?

Physiologically based pharmacokinetic (PBPK) models have been applied to simulate the absorption, distribution, metabolism, and elimination of various toxicants in fish. This approach allows for considering metal accumulation in intestinal parasites. Unlike "semi" physiologically-based models developed for metals, metal accumulation in fish was characterised based on metal-specific parameters (the fraction in blood plasma and the tissue-blood partition coefficient) and physiological characteristics of the fish (the blood flow and the tissue weight) in our PBPK model. In the model, intestinal parasites were considered a sink of metals from the host intestine. The model was calibrated with data for the system of the chub Squalius cephalus and the acanthocephalan Pomphorhynchus tereticolliis. Metal concentrations in this fish-parasite system were monitored in Ag and Co treatments in duplicate during a 48-day exposure phase (Ag and Co were added to tap water at concentrations of 1 and 2 µg/L, respectively) and a 51-day depuration phase. Their concentrations in the gills increased during the exposure phase and decreased in the depuration phase. A similar pattern was observed for Ag concentrations in other chub organs, while a relatively stable pattern for Co indicates regulations in the accumulation of essential metals by chubs. The metals were taken up by the acanthocephalans at similar rate constants. These results indicate that metal availability to parasites, which is determined by the internal distribution and fate, is critical to metal accumulation in the acanthocephalans. The high concentration of Ag in the liver as well as the high rate of Ag excretion from the liver to the intestine might contribute to higher concentrations of metals in the bile complexes in the intestine, which are available to the parasites, but not to the reabsorption by the host intestine. The opposite pattern might explain the lower availability of Co to the acanthocephalans.

Chronic Effects of Diazinon® Exposures Using Integrated Biomarker Responses in Freshwater Walking Catfish, Clarias batrachus

Diazinon exposures have been linked to the onset of toxic pathways and adverse outcomes in aquatic species, but the ecological implications on model species are not widely emphasized. The objective of this study was to determine how the organophosphate pesticide diazinon affected hematological (hemoglobin, total red blood count, total white blood count, and mean corpuscular hemoglobin), growth (condition factor, hepatosomatic index, specific growth rate), biochemical (total serum glucose, total serum protein), and endocrine (growth hormone, tri-iodothyronine, and thyroxine) parameters in Clarias batrachus after chronic exposure. Diazinon was administered at predefined exposure doses (0.64 and 1.28 mg/L) and monitored at 15, 30, and 45 days into the investigation. Observation for most biomarkers revealed patterns of decreasing values with increasing toxicant concentration and exposure duration. Correlation analysis highlighted a significant inverse relationship between variables (mean corpuscular hemoglobin, condition factor, specific growth rate, tri-iodothyronine, thyroxine, and total serum protein) and elevated chronic diazinon exposure concentrations. The integrated indices (IBR and BRI) indexes were used to provide visual and understandable depictions of toxicity effects and emphasized the relativity of biomarkers in terms of sensitivity and magnitude or severity of responses under graded toxicant exposures. The significant damage reflected by evaluated parameters in diazinon exposure groups compared to control portends risks to the health of local fish populations, including Clarias batrachus in aquatic systems adjacent to agrarian landscapes.

A Mystery Tale: Nickel Is Fickle When Snails Fail-Investigating the Variability in Ni Toxicity to the Great Pond Snail

Dissolved Ni concentrations inhibiting the growth of juvenile great pond snails (Lymnaea stagnalis) have been documented to vary from about 1 to 200 µg L^-1 Ni. This variability makes L. stagnalis either a moderately sensitive or the most sensitive freshwater species to chronic Ni exposure tested to date. Given the role of sensitive species in environmental risk assessment frameworks, it is particularly important to understand this variability, i.e., to characterize the factors that modulate Ni toxicity and that may confound toxicity test outcomes when uncontrolled. In the present study, we tested if this variability was due to analytical (growth calculation: biomass versus growth rate), environmental (water quality), lab-specific practices, and/or snail population differences among earlier studies. Specifically, we reanalyzed previously published Ni toxicity data and conducted additional measurements of Ni aqueous speciation, short-term Ni uptake, and chronic Ni toxicity with test waters and snail cultures used in previous studies. Corrections for Ni bioavailability and growth calculations explained a large degree of variability in the published literature. However, a residual 16-fold difference remained puzzling between 2 studies: Niyogi et al. (2014) (low ECxs) and Crémazy et al. (2018) (high ECxs). Indeed, differences in metal bioavailability due to water chemistry, lab-specific practices, and snail population sensitivity could not explain the large variation in Ni toxicity in these 2 very similar studies. Other potentially important toxicity-modifying factors were not directly evaluated in the present work: test duration, diet, snail holding conditions, and snail age at onset of testing. The present analysis highlights the need for further studies to elucidate 1) the mechanisms of growth inhibition in Ni-exposed L. stagnalis and 2) the important abiotic and biotic factors affecting this biological response. Until these processes are understood, substantial uncertainties will remain about inclusion of this species in Ni environmental risk assessment. Integr Environ Assess Manag 2020;16:983-997. © 2020 SETAC.

Shell damage leads to enhanced memory formation in Lymnaea

Ecologically relevant stressors alter the ability of the pond snail, Lymnaea stagnalis, to form long-term memory (LTM). Here, we show that an environmentally relevant stressor, shell damage, has a dramatic effect on the enhancement of LTM formation. Damage in the form of a shell clip 24 h before operant conditioning training resulted in long-term memory (LTM) formation following a single 0.5 h training session (TS). Typically, in these snails, two 0.5 h TSs with a 1 h interval between the sessions are required to cause LTM formation. We show here that even with a 72 h interval between shell clip and training, memory enhancement still occurred. The stress associated with shell clip could be mitigated by an ongoing high-Ca^{2 +} pond water environment, an injection of propranolol and a DNA methylation blocker. However, use of an anaesthetic (MgCl₂) during the clip or intermittent exposure to the high-Ca^{2 +} pond water environment did not mitigate the stress associated with the shell clip. Shell clip was also sufficient to cause juvenile snails, which neither learn nor form memory, to gain the capacity to form LTM. Together, the experiments demonstrate that shell clipping is an environmentally relevant stressor that can cause enhancement of LTM formation.

Lymnaea stagnalis as a freshwater model invertebrate for ecotoxicological studies

Lymnaea stagnalis, also referred to as great or common pond snail, is an abundant and widespread invertebrate species colonizing temperate limnic systems. Given the species importance, studies involving L. stagnalis have the potential to produce scientifically relevant information, leading to a better understanding of the damage caused by aquatic contamination, as well as the modes of action of toxicants. Lymnaea stagnalis individuals are easily maintained in laboratory conditions, with a lifespan of about two years. The snails are hermaphrodites and sexual maturity occurs about three months after egg laying. Importantly, they can produce a high number of offspring all year round and are considered well suited for use in investigations targeting the identification of developmental and reproductive impairments. The primary aims of this review were two-fold: i) to provide an updated and insightful compilation of established toxicological measures determined in both chronic and acute toxicity assays, as useful tool to the design and development of future research; and ii) to provide a state of the art related to direct toxicant exposure and its potentially negative effects on this species. Relevant and informative studies were analysed and discussed. Knowledge gaps in need to be addressed in the near future were further identified.

The Use of Mechanistic Population Models in Metal Risk Assessment: Combined Effects of Copper and Food Source on Lymnaea stagnalis Populations

Environmental risk assessment (ERA) of chemicals aims to protect populations, communities, and ecosystems. Population models are considered more frequent in ERA because they can bridge the gap between the individual and the population level. Lymnaea stagnalis (the great pond snail) is an organism that is particularly sensitive to various metals, including copper (Cu). In addition, the sensitivity of this species to Cu differs between food sources. The first goal of the present study was to investigate whether we could explain the variability in sensitivity between food sources (lettuce and fish flakes) at the individual level with a dynamic energy budget (DEB) model. By adapting an existing DEB model and calibrating it with Cu toxicity data, thereby combining information from 3 studies and 2 endpoints (growth and reproduction), we put forward inhibition of energy assimilation as the most plausible physiological mode of action (PMoA) of Cu. Furthermore, the variation in Cu sensitivity between both food sources was considerably lower at the PMoA level than at the individual level. Higher Cu sensitivity at individual level under conditions of lower food quality or availability appears to emerge from first DEB principles when inhibition of assimilation is the PMoA. This supports the idea that DEB explained Cu sensitivity variation between food sources. Our second goal was to investigate whether this food source effect propagated to the population level. By incorporating DEB in an individual-based model (IBM), population-level effects were predicted. Based on our simulations, the food source effect was still present at the population level, albeit less prominently. Finally, we compared predicted population-level effect concentration, x% (ECx) values with individual-level ECx values for different studies. Using the DEB-IBM, the range of effect concentrations decreased significantly: at the individual level, the difference in chronic EC10 values between studies was a factor of 70 (1.13-78 µg dissolved Cu/L), whereas at the population level the difference was a factor of 15 (2.9-44.6 µg dissolved Cu/L). To improve interstudy comparability, a bioavailability correction for differences in water chemistry was performed with a biotic ligand model. This further decreased the variation, down to a factor of 7.4. Applying the population model in combination with a bioavailability correction thus significantly decreased the variability of chronic effect concentrations of Cu for L. stagnalis. Overall, the results of the present study illustrate the potential usefulness of transitioning to a more modeling-based environmental risk assessment. Environ Toxicol Chem 2019;00:1-16. © 2019 SETAC.

Using the Biotic Ligand Model framework to investigate binary metal interactions on the uptake of Ag, Cd, Cu, Ni, Pb and Zn in the freshwater snail Lymnaea stagnalis

There is growing interest in the development of mechanistically-based models, such as the Biotic Ligand Model (BLM), for assessing the environmental risk of metal mixtures. However, the derivation of such models requires insights into the mechanisms of multimetal interactions that are often lacking for aquatic organisms. In the present study, we investigated how binary mixtures of six metals (Ag, Cd, Cu, Ni, Pb and Zn) interact for uptake in the great pond snail Lymnaea stagnalis, a freshwater species particularly sensitive to metals in chronic exposure. For each metal, short-term (2-3 h) uptake experiments on juvenile snails were performed with the metal alone and in combination with a second metal, at concentrations encompassing the chronic toxicity concentration range. These experiments showed significant binary metal interactions for 7 out of 15 mixtures. Most interactions were inhibitory in nature, not reciprocal and caused by either Ag or Cu. They led to relative changes of uptake that did not exceed 50% within the range of metal chronic toxicity. The BLM proved to be successful at explaining most of the interactions, via competitive inhibition. This study is in support of using bioavailability-based models, such as the BLM, to model metal mixture interactions in L. stagnalis.

Effects of LaCoO3 perovskite nanoparticle on Daphnia magna: accumulation, distribution and biomarker responses

Perovskite nanomaterials (PNMs) have been shown to be promising materials for the effective replacement of conventional energy source materials. With the increasing use of PNMs, they will inevitably enter aquatic environments, giving rise to concerns regarding the environmental impact of PNMs. To fill up the gap in information about the environmental effect of PNMs, Daphnia magna was exposed to a typical PNM LaCoO₃ for 48 h, to assess temporal patterns in PNM bioaccumulation and distribution. Synchrotron radiation based micro X-ray fluorescence spectroscopy (μ-XRF) was used to investigate the time dependent spatial distribution of LaCoO₃. Reactive oxygen species (ROS), superoxide dismutase (SOD) and Na⁺/K⁺-adenosine triphosphatase (ATPase) were measured as key biomarkers. The results showed that oxidative stress was observed at both LaCoO₃ concentrations and Na⁺/K⁺-ATPase was inhibited by high levels of LaCoO₃. The mode of action of LaCoO₃ was mainly dependent on the metal forms. At low LaCoO₃ levels, food ingestion was the main entry pathway into organisms and LaCoO₃ nanoparticle aggregates accumulated in the gut area. At high LaCoO₃ levels, both waterborne and dietary uptake was observed and the gut and thoracic limbs were the main target sites for LaCoO₃ nanoparticle aggregates and dissolved ions, respectively. LaCoO₃ was not found to translocate in daphnids during the 48 h exposure period at either concentration, suggesting that internalization did not occur. These findings help further our understanding of the fate of PNMs in aquatic organisms, as well as the associated biological responses to PNM exposure.

The instability of PNMs in water is of environmental concern. This study shows that in daphnids over 48 h, the mode of action of a representative PNM LaCoO₃ is dependent on Co species, which results in the differences in uptake, accumulation, distribution and toxicity.

Chronic Toxicity of Binary Mixtures of Six Metals (Ag, Cd, Cu, Ni, Pb, and Zn) to the Great Pond Snail Lymnaea stagnalis

Although metal-mixture toxicity has recently received increasing attention, there is still insufficient knowledge on joint effects occurring in chronic exposures to relatively low metal concentrations. We characterized the chronic toxicity of binary mixtures of six metals (Ag, Cd, Cu, Ni, Pb, and Zn) in 14 day growth tests with juveniles of the metal-sensitive freshwater snail Lymnaea stagnalis. Observations were compared with predictions from individual metals and from the two most frequently used mixture models: concentration addition (CA) and independent action (IA). Predictions based on measured total dissolved concentrations and on calculated free-ion activities did not differ greatly because multimetal geochemical interactions in the tests were limited. In around half of the tests, mixture toxicity was higher than the greatest effect caused by the individual metals, arguing in favor of considering joint effects. When the additive models were used, the great majority of interactions were either additive or less than additive (i.e., antagonism). In general, the IA model was the most accurate, while the CA model was the most conservative. Along with other studies, these findings suggest that, at least for binary combinations, the simple CA model may provide satisfactory protection from the chronic metal toxicity of metal mixtures to aquatic organisms.

Acidified seawater increases accumulation of cobalt but not cesium in manila clam Ruditapes philippinarum

The pH of seawater around the world is expected to continue its decline in the near future in response to ocean acidification that is driven by heightened atmospheric CO₂ emissions. Concomitantly, economically-important molluscs that live in coastal waters including estuaries and embayments, may be exposed to a wide assortment of contaminants, including trace metals and radionuclides. Seawater acidification may alter both the chemical speciation of select elements as well as the physiology of organisms, and may thus pose at risk to many shellfish species, including the manila clam Ruditapes philippinarum. The bioconcentration efficiency of two common radionuclides associated with the nuclear fuel cycle, ¹³⁴Cs and ⁵⁷Co, were investigated by exposing live clams to dissolved ¹³⁴Cs and ⁵⁷Co at control (pH = 8.1) and two lowered pH (pH = 7.8 and 7.5) levels using controlled aquaria. The uptake and depuration kinetics of the two radionuclides in the whole-body clam were followed for 21 and 35 days, respectively. At steady-state equilibrium, the concentration factor (CF_ss) for ⁵⁷Co increased as the pH decreased (i.e. 130 ± 5, 194 ± 6, and 258 ± 10 at pH levels 8.1, 7.8 and 7.5, respectively), whereas the ¹³⁴Cs uptake was not influenced by a change in pH conditions. During depuration, the lowest depuration rate constant of ⁵⁷Co by the manila clam was observed at the intermediate pH of 7.8. An increase in the accumulation of ⁵⁷Co at the intermediate pH value was thought to be caused mainly by the aragonitic shell of the clam, as well as the low salinity and alkalinity of seawater used in the experiment. Considering that accumulation consists of uptake and depuration, among the three pH conditions moderately acidified seawater enhanced most the accumulation of ⁵⁷Co. Accumulation of ¹³⁴Cs was not strongly influenced by a reduced pH condition, as represented by an analogous uptake constant rate and CF_ss in each treatment. Such results suggest that future seawater pH values that are projected to be lower in the next decades, may pose a risk for calcium-bearing organisms such as shellfish.

Toxicity assessment of arsenic and cobalt in the presence of aquatic humic substances of different molecular sizes

The release of contaminants in aquatic ecosystems can be influenced by humic acids. In this study, toxicity tests using environmentally relevant concentrations of arsenic and cobalt were conducted both in the presence and absence of aquatic humic substances (AHS) and the fractions of different molecular sizes in the range of (<5, 5-10;10-30; 30-100 and >100kDa) using the microcrustacean Ceriodaphnia dubia. AHS together with arsenic reduced the toxicity, and the toxicity decreased in fractions of larger molecular size AHS. Despite the presence of cobalt, the reduction in toxicity was not observed and that depended on the molecular size of AHS. There was a trend of enhanced toxicity for Co in fractions of larger molecular sizes, opposed to that found for arsenic. Thus, the humic substances alter toxicity of trace elements, and this effect varies depending on the size of the humic substances.

Prednisolone impairs embryonic and posthatching development and shell formation of the freshwater snail, Physa acuta

The aim of the present study was to investigate the lethal and sublethal effects of prednisolone exposure on the embryonic and posthatching stage of the freshwater snail, Physa acuta. The egg masses were exposed for 14 d to prednisolone concentrations ranging from 15.6 μg/L to 1000 μg/L. Treatment with prednisolone at 125 μg/L to 1000 μg/L resulted in significant decline in growth, survival, and heart rate, as well as notable abnormalities in embryonic development. Premature embryonic hatching was observed at lower concentrations of 31.25 μg/L and 62.5 μg/L, whereas delayed hatching was seen at concentrations from 125 μg/L to 1000 μg/L. To assess impacts of prednisolone exposure on the hatched juveniles, the drug exposure was extended for another 28 d. Impairment of shell development was noted in juveniles exposed to concentrations from 62.5 μg/L to 1000 μg/L at the end of 42 d, which resulted in thin and fragile shells. The thickness of shells in snails exposed to 1000 μg/L was significantly lower in comparison to those in the 15.6-μg/L and control treatments. In addition, lower calcium concentration in shells of the exposed juvenile snails at treatments of 62.5 μg/L to 1000 μg/L consequently reduced their growth. The present study confirms that continuous exposure to prednisolone can result in deleterious effects on calcium deposition, resulting in shell thinning in the freshwater snail P. acuta. Environ Toxicol Chem 2016;35:2339-2348. © 2016 SETAC.

Survival and Growth of Freshwater Pulmonate and Nonpulmonate Snails in 28-Day Exposures to Copper, Ammonia, and Pentachlorophenol

We performed toxicity tests with two species of pulmonate snails (Lymnaea stagnalis and Physa gyrina) and four taxa of nonpulmonate snails in the family Hydrobiidae (Pyrgulopsis robusta, Taylorconcha serpenticola, Fluminicola sp., and Fontigens aldrichi). Snails were maintained in static-renewal or recirculating culture systems with adults removed periodically to isolate cohorts of offspring for toxicity testing. This method successfully produced offspring for both species of pulmonate snails and for two hydrobiid species, P. robusta and Fluminicola sp. Toxicity tests were performed for 28 days with copper, ammonia, and pentachlorophenol in hard reconstituted water with endpoints of survival and growth. Tests were started with 1-week-old L. stagnalis, 2-week-old P. gyrina, 5- to 13-week-old P. robusta and Fluminicola sp., and older juveniles and adults of several hydrobiid species. For all three chemicals, chronic toxicity values for pulmonate snails were consistently greater than those for hydrobiid snails, and hydrobiids were among the most sensitive taxa in species sensitivity distributions for all three chemicals. These results suggest that the toxicant sensitivity of nonpulmonate snails in the family Hydrobiidae would not be adequately represented by results of toxicity testing with pulmonate snails.

Effects of chronic waterborne nickel exposure on growth, ion homeostasis, acid-base balance, and nickel uptake in the freshwater pulmonate snail, Lymnaea stagnalis

The freshwater pulmonate snail, Lymnaea stagnalis, is the most sensitive aquatic organism tested to date for Ni. We undertook a series of experiments to investigate the underlying mechanism(s) for this observed hypersensitivity. Consistent with previous experiments, juvenile snail growth in a 21-day exposure was reduced by 48% relative to the control when exposed to 1.3 μg l(-1) Ni (EC20 less than the lowest concentration tested). Ca(2+) homeostasis was significantly disrupted by Ni exposure as demonstrated by reductions in net Ca(2+) uptake, and reductions in Ca(2+) concentrations in the hemolymph and soft tissues. We also observed reduced soft tissue [Mg(2+)]. Snails underwent a significant alkalosis with hemolymph pH increasing from 8.1 to 8.3 and hemolymph TCO2 increasing from 19 to 22 mM in control versus Ni-exposed snails, respectively. Unlike in previous studies with Co and Pb, snail feeding rates were found to be unaffected by Ni at the end of the exposure. Snails accumulated Ni in the soft tissue in a concentration-dependent manner, and Ni uptake experiments with (63)Ni revealed a biphasic uptake profile - a saturable high affinity component at low exposure concentrations (36-189 nM) and a linear component at the high exposure concentrations (189-1,897 nM). The high affinity transport system had an apparent Km of 89 nM Ni(2+) and Vmax of 2.4 nmol g(-1)h(-1). This equates to a logK of 7.1, significantly higher than logK's (2.6-5.2) for any other aquatic organisms evaluated to date, which will have implications for Biotic Ligand Model development. Finally, pharmacological inhibitors that block Ca(2+) uptake pathways in snails did not inhibit Ni uptake, suggesting that the uptake of Ni does not occur via Ca(2+) uptake pathways. As with Cu and Pb, the exact mechanism for the significant disruption in Ca(2+) homeostasis and reduction in juvenile snail growth remains unknown.

Acute toxicity, critical body residues, Michaelis-Menten analysis of bioaccumulation, and ionoregulatory disturbance in response to waterborne nickel in four invertebrates: Chironomus riparius, Lymnaea stagnalis, Lumbriculus variegatus and Daphnia pulex

We investigated the bioaccumulation and acute toxicity (48 h or 96 h) of Ni in four freshwater invertebrate species in two waters with hardness of 40 (soft water) and 140 mg L(-1) as CaCO(3) (hard water). Sensitivity order (most to least) was Lymnaea stagnalis > Daphnia pulex > Lumbriculus variegatus > Chironomus riparius. In all cases water hardness was protective against acute Ni toxicity with LC(50) values 3-3.5× higher in the hard water vs. soft water. In addition, higher water hardness significantly reduced Ni bioaccumulation in these organisms suggesting that competition by Ca and Mg for uptake at the biotic ligand may contribute to higher metal resistance. CBR50 values (Critical Body Residues) were less dependent on water chemistry (i.e. more consistent) than LC(50) values within and across species by ~2 fold. These data support one of the main advantages of the Tissue Residue Approach (TRA) where tissue concentrations are generally less variable than exposure concentrations with respect to toxicity. Whole body Ni bioaccumulation followed Michaelis-Menten kinetics in all organisms, with greater hardness tending to decrease B(max) with no consistent effect on K(d). Across species, acute Ni LC(50) values tended to increase with both K(d) and B(max) values - i.e. more sensitive species exhibited higher binding affinity and lower binding capacity for Ni, but there was no correlation with body size. With respect to biotic ligand modeling, log K(NiBL) values derived from Ni bioaccumulation correlated well with log K(NiBL) values derived from toxicity testing. Both whole body Na and Mg levels were disturbed, suggesting that disruption of ionoregulatory homeostasis is a mechanism of acute Ni toxicity. In L. stagnalis, Na depletion was a more sensitive endpoint than mortality, however, the opposite was true for the other organisms. This is the first study to show the relationship between Na and Ni.

Mortality, bioaccumulation and physiological responses in juvenile freshwater mussels (Lampsilis siliquoidea) chronically exposed to copper

Several studies have indicated that the early life stages of freshwater mussels are among the most sensitive aquatic organisms to inorganic chemicals, including copper. However, little is known about the toxic mode of action and sub-lethal effects of copper exposure in this group of imperiled animals. In this study, the physiological effects of long-term copper exposure (survival, growth, copper bioaccumulation, whole-body ion content, oxygen consumption, filtration rate, ATPase activities, and biomarkers of oxidative stress) were evaluated in juvenile (6 month old) mussels (Lampsilis siliquoidea). The mussels' recovery capacity and their ability to withstand further acute copper challenge were also evaluated in secondary experiments following the 28 day exposure by assessing survival, copper bioaccumulation and whole-body ion content. Mussels chronically exposed to 2 and 12 μg Cu/L showed significantly higher mortality than those held under control conditions (mortality 20.9, 69.9 and 12.5%, respectively), indicating that juvenile L. siliquoidea is underprotected by the U.S. Environmental Protection Agency (USEPA) biotic ligand model (BLM)-derived chronic water quality criteria (WQC) (2.18 μg Cu/L) and the hardness-derived USEPA WQC (12.16 μg Cu/L). Soft tissue copper burden increased equally for both copper exposures, suggesting that chronic toxicity is not associated with copper bioaccumulation. Several physiological disturbances were also observed during chronic copper exposure. Most relevant was a decrease in whole-body sodium content paralleled by an inhibition of Na(+) K(+)-ATPase activity, indicating a metal-induced ionoregulatory disturbance. Filtration and oxygen consumption rates were also affected. Redox parameters (reactive oxygen production, antioxidant capacity against peroxyl radicals, glutathione-S-transferase (GST) activity, and glutathione (GSH) concentration) did not show clear responses, but membrane damage as lipid peroxidation (LPO) was observed in both copper exposures. Mussels previously held in control conditions or pre-exposed to 2 μg dissolved Cu/L were able to maintain their ionic homeostasis and did not experience mortality after the 4-d recovery period. In contrast, those previously exposed to 12 μg dissolved Cu/L exhibited 50% mortality indicating that they had already reached a 'point of no return'. Pre-exposure to copper did not influence mussel response to the copper challenge test. As observed for the chronic exposure, mortality of mussels held in the absence of copper and submitted to the challenge test was also associated with an ionoregulatory disturbance. These results indicate that ionoregulatory disruption in freshwater mussels chronically exposed to copper is the main mechanism of toxicity and that redox parameters do not appear to be useful as indicators of sub-lethal copper toxicity in these animals.

Investigations into the mechanism of lead toxicity to the freshwater pulmonate snail, Lymnaea stagnalis

The freshwater pulmonate snail, Lymnaea stagnalis, is the most sensitive aquatic organism tested to date for Pb with an estimated EC20 for juvenile snail growth of 3 μg l⁻¹. A previous study supported the hypothesis that this hypersensitivity to Pb was due to an extremely high Ca²⁺ uptake rate needed to support shell formation. The current study sought to build upon this working hypothesis and develop a mechanistic predictive model for inhibition of snail growth as a function of Pb exposure. Initial experiments confirmed previous predictions that juvenile snails have net Ca²⁺ uptake rates of 7000-8000 nmol g⁻¹ h⁻¹, approximately 100-fold higher than observed in a typical freshwater fish. However, an initial time course study revealed that the onset of growth inhibition occurs at least 4d prior to inhibition of net Ca²⁺ flux in Pb-exposed snails indicating the latter is not the primary mechanism of action. Qualitative observations during this experiment indicated snail feeding was inhibited in a dose-dependent manner. A subsequent experiment demonstrated that when food is withheld from snails for even 24 h, net Ca²⁺ uptake is significantly (∼50%) reduced. A second time course study demonstrated quantitatively that snail feeding is inhibited by Pb exposure by up to 98% at relatively high Pb concentrations (57 μg l⁻¹) but no inhibition was observed at ≤ 10 μg l⁻¹ Pb indicating feeding inhibition is not causing observed growth effects at concentrations approximating the EC20 of 3 μg l⁻¹ Pb. A final experiment testing whether Pb-induced growth effects are related to inhibition of carbonic anhydrase activity in the snail mantle also failed to demonstrate an effect. We conclude that while both feeding and net Ca²⁺ uptake in snails are affected by Pb exposure, they appear to be secondary effects. The primary mechanism of action explaining L. stagnalis hypersensitivity to Pb remains to be identified.

The toxicity and physiological effects of copper on the freshwater pulmonate snail, Lymnaea stagnalis

Several recent studies have demonstrated that the freshwater pulmonate snail Lymnaea stagnalis is extremely sensitive to metals (Co, Ni, Pb) in chronic exposures. The objective of the current study was to evaluate the acute and chronic sensitivity of L. stagnalis to Cu and investigate the underlying mechanism(s) of toxic action. A 96-h LC50 of 31μg L(-1) Cu was estimated indicating L. stagnalis was moderately acutely sensitive to Cu relative to other aquatic organisms. However, in a 30-day chronic exposure using juvenile snails an EC20 of 1.8μg L(-1) Cu was estimated for snail growth making L. stagnalis the most sensitive organism tested to date for Cu. Hardness-based and BLM-based water quality criteria for Cu at the water quality conditions used in this study were 7.8 and 1.5μg L(-1), respectively, indicating L. stagnalis is significantly under-protected by hardness-based WQC. Investigations into the mechanism(s) of toxic action for Cu were conducted on young adult snails necessitating higher Cu exposures. Exposure to Cu at 12μg L(-1) resulted in no detectable effects on hemolymph osmolality, net Ca(2+) uptake, titratable acid excretion, or ammonia excretion. Exposure to 48μg L(-1) Cu was shown to significantly reduce (91%) net Ca(2+) uptake which is strongly correlated with shell deposition and corresponding snail growth. Snails exposed to 48μg L(-1) Cu also exhibited reduced ammonia excretion, a marked hemolymph acidosis, and a compensatory increase in titratable acid excretion. The reduction in net Ca(2+) uptake was hypothesized to be a secondary effect of Cu-induced inhibition of carbonic anhydrase, but no reduction in carbonic anhydrase activity was detected. Overall, it remains unclear whether inhibition of Ca(2+) uptake is a direct result of Cu exposure or, along with the other observed physiological effects, is secondary to an unidentified primary mode of toxic action. Given the hypersensitivity of L. stagnalis to Cu, further study into the mechanisms of action and effects of varying water chemistry on Cu toxicity is clearly warranted.

Mechanisms of waterborne Cu toxicity to the pond snail Lymnaea stagnalis: physiology and Cu bioavailability

We examined the mechanisms of toxicity of waterborne Cu to the freshwater pond snail Lymnaea stagnalis. The snail is one of the most sensitive species to acute Cu exposure (96 h LC(50), LC(20): 24.9, 18.0 μgl(-1)); they are not protected by the water quality criteria of the US EPA. Tissue Na and Ca were also reduced by Cu in the acute exposure. In contrast, during 28 d chronic exposures to Cu in the presence of food, which resulted in higher DOC concentrations, there was no significant mortality but an inhibition of growth, which may reflect a re-allocation of resources to detoxification. Cu detoxification was evidenced in chronic exposure by increases in metallothionein-like protein concentrations and Cu binding to metal-rich granules, decreases in thiobarbituric acid-reactive substances, and changes in the subcellular distribution in the soft tissues. Our results demonstrated that apart from external Cu bioavailability, compartmentalization of metals within the cells can alter toxicity of Cu to the snails.

The chronic toxicity of molybdate to freshwater organisms. I. Generating reliable effects data

The European Union regulation on Registration, Evaluation, Authorization and Restriction of Chemical substances (REACH) (EC, 2006) requires the characterization of the chronic toxicity of many chemicals in the aquatic environment, including molybdate (MoO(4)(2-)). Our literature review on the ecotoxicity of molybdate revealed that a limited amount of reliable chronic no observed effect concentrations (NOECs) for the derivation of a predicted no-effect concentration (PNEC) existed. This paper presents the results of additional ecotoxicity experiments that were conducted in order to fulfill the requirements for the derivation of a PNEC by means of the scientifically most robust species sensitivity distribution (SSD) approach (also called the statistical extrapolation approach). Ten test species were chronically exposed to molybdate (added as sodium molybdate dihydrate, Na(2)MoO(4)·2H(2)O) according to internationally accepted standard testing guidelines or equivalent. The 10% effective concentrations (EC10, expressed as measured dissolved molybdenum) for the most sensitive endpoint per species were 62.8-105.6 (mg Mo)/L for Daphnia magna (21day-reproduction), 78.2 (mg Mo)/L for Ceriodaphnia dubia (7day-reproduction), 61.2-366.2 (mg Mo)/L for the green alga Pseudokirchneriella subcapitata (72h-growth rate), 193.6 (mg Mo)/L for the rotifer Brachionus calyciflorus (48h-population growth rate), 121.4 (mg Mo)/L for the midge Chironomus riparius (14day-growth), 211.3 (mg Mo)/L for the snail Lymnaea stagnalis (28day-growth rate), 115.9 (mg Mo)/L for the frog Xenopus laevis (4day-larval development), 241.5 (mg Mo)/L for the higher plant Lemna minor (7day-growth rate), 39.3 (mg Mo)/L for the fathead minnow Pimephales promelas (34day-dry weight/biomass), and 43.2 (mg Mo)/L for the rainbow trout Oncorhynchus mykiss (78day-biomass). These effect concentrations are in line with the few reliable data currently available in the open literature. The data presented in this study can serve as a basis for the derivation of a PNEC(aquatic) that can be used for national and international regulatory purposes and for setting water quality criteria. Using all reliable data that are currently available, a HC(5,50%) (median hazardous concentration affecting 5% of the species) of 38.2 (mg Mo)/L was derived with the statistical extrapolation approach.

Cross-phylum extrapolation of the Daphnia magna chronic biotic ligand model for zinc to the snail Lymnaea stagnalis and the rotifer Brachionus calyciflorus

We investigated if the chronic zinc biotic ligand model (BLM) developed earlier for the arthropod Daphnia magna could be extrapolated to predict chronic ecotoxicity of zinc as a function of water chemistry to two species from other phyla, i.e. the mollusc Lymnaea stagnalis and the rotifer Brachionus calyciflorus. We chronically exposed these two species to zinc in six natural surface waters. These water covered a wide range of pH (6.8-8.3), dissolved organic carbon (1.2-12.7mg/L) and Ca (8.8-118mg/L). Across all waters tested, the 28d-EC10s (200-1629μg Zn/L) and EC50s (382-2026μg Zn/L) for L. stagnalis spanned a 8.1-fold and 5.3-fold range, respectively. The 2d-EC10s (142-550μg Zn/L) and 2d-EC50s (195-1104μg Zn/L) for B. calyciflorus spanned a 3.9-fold and 5.7-fold range, respectively. The data indicated that higher pH and higher concentrations of Ca and DOC were generally associated with lower toxicity (higher ECx values). Furthermore, the chronic Zn BLM for D. magna, when calibrated only to reflect the intrinsic sensitivity of L. stagnalis and B. calyciflorus, was able to predict all ECx values with a less than 1.6-fold error, which demonstrates that the chronic D. magna Zn BLM can be extrapolated to other invertebrate phyla. This lends further support to the use of the chronic Zn BLM to account for bioavailability of zinc in aquatic risk assessment and the derivation of environmental quality standards.

Effects of copper on the egg development and hatching of a freshwater pulmonate snail Lymnaea luteola L

A freshwater invertebrate egg development and hatching toxicity test with an Indian freshwater pulmonate snail, Lymnaea luteola, comprising the following developmental endpoints was described: mortality, development, formation of eyes and foot structure, heart rate, duration of different larval stages, and hatching time. Developmental stages were morula, and at third, fifth, and eighth days; the trochophore, veliger, and hippo larvae, respectively. At the age of about 9th to 11th days after egg laying; more than 95% young snail hatched in control laboratory conditions. To evaluate effects on embryonic development, the pulmonate snail eggs of 24-h old were exposed to a series of nominal copper concentrations. The percentage survival of embryos treated in 10-32 microg l(-1) of Cu after 240 h of exposure drops sharply at veliger and hippo stages. All embryos died at 100-320 microg l(-1) of Cu within 168 h of exposure at trochophore and early veliger stages. The detected abnormalities were malformation of foot, eyes, thinness and incomplete formation of shell, growth retardation, and slow rotation of embryo within the egg capsule as compared to control embryos. Lethal and sublethal effects in terms of mortality and significant delay in hatching could be found in the 3.2, 5.6 and 10 microg l(-1) of Cu concentrations. This species is widely distributed in the Indian subcontinent freshwater reservoirs and more sensitive to Cu than other tested aquatic test organisms; therefore, could be used as a test model of Cu and possibly other pollutants for rapid risk assessment of environmental pollutants. The snail egg embryo bioassay is simple, rapid, highly sensitive, cost-effective, and easy to test under standardized laboratory conditions.

Environmental risk assessment of zinc in European freshwaters: a critical appraisal

A risk assessment report (RAR) on zinc and zinc compounds has recently been prepared in the framework of the European Union (EU) Council Regulation 793/93/EEC on Existing Chemicals. The EU Scientific Committee on Human and Environmental Risks (SCHER) has, however, expressed some fundamental, science-based concerns about the approach followed and the conclusions. The main objective of the present study was to assess the potential environmental risks associated with current use patterns of Zn in nine EU river basins in Germany, France and Belgium, thereby using more advanced methodologies which are largely in line with the recommendations made by SCHER. This included (i) avoiding working with measured Zn concentrations from monitoring stations that were potentially influenced by point sources and/or historical contamination, (ii) the full bioavailability normalization of all chronic ecotoxicity data to river basin specific physico-chemistry using biotic ligand models (BLM), prior to deriving predicted no effect concentrations (PNEC) with the species sensitivity distribution (SSD) approach, and (iii) the use of a probabilistic framework for risk characterization. Further, a total risk approach instead of an added risk approach was used, and the PNEC was equated to the HC5-50 without an additional assessment factor. Based on monitoring data we estimated predicted environmental concentrations (PEC) for the different EU river basins between 1.3 and 14.6 microg dissolved Zn/L. PNEC values varied between 22.1 and 46.1 microg dissolved Zn/L. This resulted in deterministic risk characterization ratios (RCR) that were below 1 in all river basins, suggesting that there is no deterministic regional risk associated with current use patterns of Zn in these river basins. With the probabilistic approach we identified rather limited risks, i.e., between <0.4 and 18.3%. When the EU RAR approach was applied to the same monitoring datasets, deterministic risks were found in different river basins. A detailed analysis showed that this different deterministic conclusion of risk is mainly due to the fact that the EU RAR (i) uses an additional assessment factor of 2 to derive the PNEC and (ii) uses a more conservative approach for implementing bioavailability (BioF approach). We argue that the larger conservatism in the EU RAR mainly originates from decisions made to deal in a pragmatic way with (i) uncertainty related to the across-species extrapolation of BLMs and (ii) the relatively high sensitivity of some multi-species toxicity studies.

Exposure routes of copper: short term effects on survival, weight, and uptake in Florida apple snails (Pomacea paludosa)

The uptake and effects (survival, weight) of copper (Cu) on Florida apple snails (Pomacea paludosa) via exposures to copper-enriched agricultural soil-water and water-only treatments were investigated. Soils were collected from citrus sites in south Florida and flooded with laboratory freshwater for 14d. Neonate apple snails (96-h-old) were then exposed to either Cu from a soil-overlying water (i.e., flooded agricultural soils) treatment or overlying water-only (i.e., equilibrated overlying water produced from 14d flooding of agriculture soils) treatment for 14d under standard laboratory conditions. Survival, weight (dry, wet), and whole body Cu uptake were measured. Copper exposure via soil-water exposures resulted in higher mortality and whole body Cu uptake than water-only exposures, indicating Cu uptake from soils. However, snail wet and dry weights were higher in soil-water treatments than in water-only treatments. Micronutrients from soils may be consumed by snails increasing weights. Survival, apple snail dry weight, and whole body Cu concentrations were significantly correlated with soil and water Cu concentrations in soil-water treatments. Survival was significantly correlated with the concentration of Cu(CO3)2(2-) in water-only treatments. This suggests that Cu(CO3)2(2-) is toxic to apple snails. Whole body Cu concentrations were higher in surviving snails than dead snails, suggesting that apple snails have the ability to detoxify accumulated Cu (e.g., through metallothionein induction, granules).