Effects of chronic copper exposure on development and survival in the southern leopard frog (Lithobates [Rana] sphenocephalus)

Influence of systemic copper toxicity on early development and metamorphosis in Xenopus laevis

The primary objective of the present study was to examine the influence of early systemic toxicity resulting from copper (Cu) exposure on metamorphic processes in Xenopus laevis. A 28-day exposure study with copper, initiated at developmental stage 10, was performed using test concentrations of 3.0, 9.0, 27.2, 82.5, and 250 μg Cu/L. The primary endpoints included mortality, developmental stage, embryo-larval malformation, behavioral effects, hindlimb length (HLL), growth (snout-vent length [SVL] and wet body weight), and histopathology. The 28-day LC50 value with 95% confidence intervals was 61.2 (51.4-72.9) μg Cu/L with 250 μg Cu/L resulting in complete lethality. Developmental arrest in the 82.5 and delay in the 27.2 μg Cu/L treatments was observed as early as study day 10 continuing throughout the remainder of exposure. SVL-normalized HLL, body weight, and SVL in the 27.2 and 82.5 μg Cu/L treatments were significantly decreased relative to control. At 82.5 μg Cu/L, and thyroid gland size was markedly reduced when compared with controls consistent with the stage of developmental and growth arrest. Concentration-dependent findings in the intestine, liver, gills, eyes, and pharyngeal mucosa were consistent with non-endocrine systemic toxicity. These were prevalent in the 9.0 and 27.2 μg Cu/L treatment groups but were minimally evident or absent in the 82.5 μg/L group, which was attributed to developmental arrest. In conclusion, developmental delay in larvae exposed to 27.2 and 82.5 μg Cu/L was the result of systemic toxicity occurring in early development prior hypothalomo-pituitary-thyroid axis (HPT)-driven metamorphosis and was not indicative of endocrine disruption.

Adverse Outcome Pathways for Chronic Copper Toxicity to Fish and Amphibians

In the present review, we synthesize information on the mechanisms of chronic copper (Cu) toxicity using an adverse outcome pathway framework and identify three primary pathways for chronic Cu toxicity: disruption of sodium homeostasis, effects on bioenergetics, and oxidative stress. Unlike acute Cu toxicity, disruption of sodium homeostasis is not a driving mechanism of chronic toxicity, but compensatory responses in this pathway contribute to effects on organism bioenergetics. Effects on bioenergetics clearly contribute to chronic Cu toxicity with impacts at multiple lower levels of biological organization. However, quantitatively translating these impacts into effects on apical endpoints such as growth, amphibian metamorphosis, and reproduction remains elusive and requires further study. Copper-induced oxidative stress occurs in most tissues of aquatic vertebrates and is clearly a significant driver of chronic Cu toxicity. Although antioxidant responses and capacities differ among tissues, there is no clear indication that specific tissues are more sensitive than others to oxidative stress. Oxidative stress leads to increased apoptosis and cellular damage in multiple tissues, including some that contribute to bioenergetic effects. This also includes oxidative damage to tissues involved in neuroendocrine axes and this damage likely alters the normal function of these tissues. Importantly, Cu-induced changes in hormone concentrations and gene expression in endocrine-mediated pathways such as reproductive steroidogenesis and amphibian metamorphosis are likely the result of oxidative stress-induced tissue damage and not endocrine disruption. Overall, we conclude that oxidative stress is likely the primary driver of chronic Cu toxicity in aquatic vertebrates, with bioenergetic effects and compensatory response to disruption of sodium homeostasis contributing to some degree to observed effects on apical endpoints. Environ Toxicol Chem 2022;41:2911-2927. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Bioremediation of copper in sediments from a constructed wetland ex situ with the novel bacterium Cupriavidus basilensis SRS

The H-02 constructed wetland was designed to remove metals (primarily copper and zinc) to treat building process water and storm water runoff from multiple sources associated with the Tritium Facility at the DOE-Savannah River Site, Aiken, SC. The concentration of Cu and Zn in the sediments has increased over the lifetime of the wetland and is a concern. A bioremediation option was investigated at the laboratory scale utilizing a newly isolated bacterium of the copper metabolizing genus Cupriavidus isolated from Tim's Branch Creek, a second-order stream that eventually serves as a tributary to the Savannah River, contaminated with uranium and other metals including copper, nickel, and mercury. Cupriavidus basilensis SRS is a rod-shaped, gram-negative bacterium which has been shown to have predatory tendencies. The isolate displayed resistance to the antibiotics ofloxacin, tetracycline, ciprofloxacin, select fungi, as well as Cu²⁺ and Zn²⁺. Subsequent ribosomal sequencing demonstrated a 100% confidence for placement in the genus Cupriavidus and a 99.014% match to the C. basilensis type strain. When H-02 wetland samples were inoculated with Cupriavidus basilensis SRS samples showed significant (p < 0.05) decrease in Cu²⁺ concentrations and variability in Zn²⁺ concentrations. Over the 72-h incubation there were no significant changes in the inoculate densities (10⁶-10⁸ cells/ML) indicating Cupriavidus basilensis SRS resiliency in this environment. This research expands our understanding of the Cupriavidus genus and demonstrates the potential for Cupriavidus basilensis SRS to bioremediate sites impacted with heavy metals, most notably copper.

Divergence in heritable life history traits suggests potential for local adaptation and trade-offs associated with a coal ash disposal site

Globally, human activities have resulted in rapid environmental changes that present unique challenges for wildlife. However, investigations of local adaptation in response to simultaneous exposure to multiple anthropogenic selection pressures are rare and often generate conflicting results. We used an in situ reciprocal transplant design within a quantitative genetic framework to examine how adaptive evolution and phenotypic plasticity contribute to the persistence of an amphibian population inhabiting an environment characterized by high levels of multiple toxic trace elements. We found evidence of phenotypic divergence that is largely consistent with local adaptation to an environment contaminated with multiple chemical stressors, tied to potential trade-offs in the absence of contaminants. Specifically, the population derived from the contaminated environment had a reduced risk of mortality and greater larval growth and in the contaminated environment, relative to offspring from the naïve population. Further, while survival in the uncontaminated environment was not compromised in offspring from the contaminant-exposed population, they did show delayed development and reduced growth rates over larval development, relative to the naïve population. We found no evidence of reduced additive genetic variation in the contaminant-exposed population, suggesting long-term selection in a novel environment has not reduced the evolutionary potential of that population. We also saw little evidence that past selection in the ASH environment had reduced trait plasticity in the resident population. Maternal effects were prominent in early development, but we did not detect any trends suggesting these effects were associated with the maternal transfer of toxic trace elements. Our results demonstrate the potential for adaptation to multiple contaminants in a wild amphibian population, which may have facilitated long-term persistence in a heavily impacted environment.

Sensitivity of Amphibians to Copper

Amphibian populations are declining globally. Major drivers of these global declines are known. However, the contribution of these major drivers to population declines varies by the presence or absence and the interactive effect of drivers, thus creating local challenges for conservation of populations. Studies have determined that environmental contaminants contribute to amphibian population declines. However, there is a disagreement over the use of amphibians as sentinel species in ecotoxicological testing rather than the traditional taxa used, fish and invertebrates. Reviews of ecotoxicological studies have demonstrated that amphibians are generally less sensitive than fish and invertebrates to different groups of contaminants. Nonetheless, because of the distinct nature and mechanism of toxicity of various contaminants, it is necessary to study contaminants individually to be able to come to any conclusion on the relative sensitivity of amphibians. Copper is one of the most studied environmental contaminants. We conducted a literature review of Cu toxicity to amphibians and the relative sensitivity of amphibians to other aquatic animals. The available data suggest that although amphibians may be tolerant of acute Cu exposure, they are relatively sensitive to chronic exposure (i.e., 100-fold greater sensitivity to chronic compared to acute exposure). In addition, ecologically relevant endpoints specific to amphibians (e.g., duration of metamorphosis and behavior) are shown to provide a better understanding of their sensitivity compared to traditional endpoints (e.g., survival and growth). Our current knowledge on amphibian sensitivity is far from complete. Considering the current status of this globally threatened class of animals, it is necessary to fill the knowledge gaps regarding their sensitivity to individual contaminants, beginning with Cu. Environ Toxicol Chem 2021;40:1810-1821. © 2021 SETAC.

Evaluating a developmental endocrine toxicity assay for Blanchard's cricket frog (Acris blanchardi) in outdoor enclosures

A developmental toxicity testing design was evaluated for larval and post-metamorphic Blanchard's cricket frogs (Acris blanchardi) raised in outdoor enclosures. Larvae were chronically exposed to 17β-estradiol (0.0-2.3 μg/L E2) from free swimming (Gosner stage 26) until metamorphosis. Juvenile frogs were allowed to mature within the enclosures for 60 days to assess effects of larval exposure on development, including body mass, snout-vent length (SVL), sex ratio, gonad size, and gonadal histopathology. Forty-eight percent of the initial 600 animals were recovered at the end of the study. Recovery was not influenced by E2 exposure, but larval losses were negatively impacted by unusually high spring rain events that flooded some larval tanks, and heat-related mortality of late stage larvae during summer. All surviving larvae completed metamorphosis within an average of 47 days. Overall, E2 exposure did not influence sex ratio, or the body mass, SVL, or gonad size of either males or females. Development of testes was not influenced by E2 exposure, but oviduct development in males was 4.5-fold greater in the highest treatment. Oviduct and ovary development in females exposed to the two highest E2 treatments were half that of control females. Although not treatment related and despite ad-lib feeding, variation in terminal body mass and SVL within enclosures was pronounced, with minimum - maximum differences ranging from 207 to 1442 mg for body mass and 1 mm to 15 mm for SVL. This design allowed us to assess the effects of larval exposure to a contaminant on post-metamorphic development of a native amphibian in a semirealistic field environment. With modifications to decrease flooding or overheating, this enclosure design and species is a good test system for assessing contaminant effects on development of an amphibian from early larval stages through reproductive maturity.

Sublethal Effects of Dermal Exposure to Poly- and Perfluoroalkyl Substances on Postmetamorphic Amphibians

Studies of the toxicity of poly- and perfluoroalkyl substances (PFAS) on amphibians, especially after metamorphosis, are limited. We examined effects of dermal PFAS exposure (30 d) on survival and growth of juvenile American toads (Anaxyrus americanus), eastern tiger salamanders (Ambystoma tigrinum), and northern leopard frogs (Rana pipiens). Chemicals included perfluorooctanoic acid, perfluorooctane sulfonate (PFOS), perfluorohexane sulfonate (PFHxS), and 6:2 fluorotelomer sulfonate (6:2 FTS) at 0, 80, 800, or 8000 ppb on a moss dry weight basis. Exposure to PFAS influenced final snout-vent length (SVL) and scaled mass index (SMI), a measure of relative body condition. Observed effects depended on species and chemical, but not concentration. Anurans exposed to PFOS, PFHxS (frogs only), and 6:2 FTS demonstrated reduced SVL versus controls, whereas salamanders exposed to 6:2 FTS showed increased SVL. Frogs exposed to PFHxS and 6:2 FTS and toads exposed to PFOS had increased SMI compared to controls; salamanders did not demonstrate effects. Concentrations of 6:2 FTS in substrate decreased substantially by 30 d, likely driven by microbial action. Perfluorooctane sulfonate had notable biota-sediment accumulation factors, but was still <1. Although a no-observable-effect concentration could not generally be determined, the lowest-observable-effect concentration was 50 to 120 ppb. Survival was not affected. The present study demonstrates that PFAS bioaccumulation from dermal exposures and sublethal effects are dependent on species, chemical, and focal trait. Environ Toxicol Chem 2021;40:717-726. © 2020 SETAC.

Agrochemicals disrupt multiple endocrine axes in amphibians

Concern over global amphibian declines and possible links to agrochemical use has led to research on the endocrine disrupting actions of agrochemicals, such as fertilizers, fungicides, insecticides, acaricides, herbicides, metals, and mixtures. Amphibians, like other species, have to partition resources for body maintenance, growth, and reproduction. Recent studies suggest that metabolic impairments induced by endocrine disrupting chemicals, and more particularly agrichemicals, may disrupt physiological constraints associated with these limited resources and could cause deleterious effects on growth and reproduction. Metabolic disruption has hardly been considered for amphibian species following agrichemical exposure. As for metamorphosis, the key thyroid hormone-dependent developmental phase for amphibians, it can either be advanced or delayed by agrichemicals with consequences for juvenile and adult health and survival. While numerous agrichemicals affect anuran sexual development, including sex reversal and intersex in several species, little is known about the mechanisms involved in dysregulation of the sex differentiation processes. Adult anurans display stereotypical male mating calls and female phonotaxis responses leading to successful amplexus and spawning. These are hormone-dependent behaviours at the foundation of reproductive success. Therefore, male vocalizations are highly ecologically-relevant and may be a non-invasive low-cost method for the assessment of endocrine disruption at the population level. While it is clear that agrochemicals disrupt multiple endocrine systems in frogs, very little has been uncovered regarding the molecular and cellular mechanisms at the basis of these actions. This is surprising, given the importance of the frog models to our deep understanding of developmental biology and thyroid hormone action to understand human health. Several agrochemicals were found to have multiple endocrine effects at once (e.g., targeting both the thyroid and gonadal axes); therefore, the assessment of agrochemicals that alter cross-talk between hormonal systems must be further addressed. Given the diversity of life-history traits in Anura, Caudata, and the Gymnophiona, it is essential that studies on endocrine disruption expand to include the lesser known taxa. Research under ecologically-relevant conditions will also be paramount. Closer collaboration between molecular and cellular endocrinologists and ecotoxicologists and ecologists is thus recommended.

Comparison of the characteristics of intestinal microbiota response in Bufo gargarizans tadpoles: Exposure to the different environmental chemicals (Cu, Cr, Cd and NO3-N)

Environmental pollutants could change the intestinal microbiota communities, while data concerning the dynamics of the intestinal microbiota in response to different environmental chemicals in amphibian are lacking. We compared the effects of Cu, Cd, Cr and NO₃-N on intestinal microbiota of B. gargarizans tadpoles by using high-throughput 16S rRNA sequencing technology. Our results revealed that responses of intestinal microbiota to three metals and NO₃-N showed different characteristics. At the phylum level, the most 100 OTUs were predominantly colonized by Proteobacteria, and meanwhile, expansion of Proteobacteria was observed in Cu 64 μg/L, Cd (100 μg/L and 200 μg/L) and NO₃-N100 mg/L treatment groups. In addition, the abundance of Bacteroidetes significantly increased in the gut administrate with Cu, Cd, Cr, NO₃-N 20 mg/L exposures, while declined abundance of Fusobacteria was observed in Cu64 μg/L Cd100 μg/L Cd200μg/L-exposed groups. At the genus level, several genera exhibited increased prevalence of abundance such as Shewanella, Azospira and Flavobacterium. The functional prediction revealed that exposures of three metals and NO₃-N increase the risks of metabolic disorders and diseases. Our research could be an important step toward an assessment of the ecological risks of different chemicals to aquatic organisms using intestinal microbiota.

Exposure to copper altered the intestinal microbiota in Chinese brown frog (Rana chensinensis)

The intestinal microbiota is a crucial physiological system that offers multiple services to the host and contributes to the health of host. However, substantially less is known concerning the interrelation between amphibian gut microbiota and Cu exposure. R. chensinensis larvae were exposed to different concentrations of Cu (0, 0.1, 0.25, 0.75 μM) until reached Gosner stage 38. Histological and morphological data were measured by four Cu exposure conditions. Then, the diversity, structure, and composition of intestinal microbiota were analyzed via 16S rRNA gene sequencing. These results indicated that total body length, intestinal wet weight, and total body wet weight were reduced in 0.75 μM CuSO₄ exposure group. Besides, obvious histopathologic alterations were observed in CuSO₄ exposure groups. Alpha diversity significantly differentiated in 0.75 μM CuSO₄ exposure group, and beta diversity showed 0.1 μM and 0.2 μM CuSO₄ exposure groups separation with the control group. At the phylum level of intestinal microbial community, the relative abundances of Fusobacteria were significantly decreased, while Bacteroidetes was no significant difference in all CuSO₄ exposure groups. Furthermore, at the genera level, Flavobacterium has a significant higher abundance in 0.75 μM CuSO₄ exposure group, and high abundance of Rahnella was found in 0.1 μM CuSO₄ exposure group. Also, Cu exposure affected the metabolism function of R. chensinensis tadpoles based on functional prediction analysis. This work provides new perspective to explore the effect of heavy metal on the intestinal health of amphibians.

Acute toxicity of copper to the larval stage of three species of ambystomatid salamanders

Copper (Cu) appears to be consistently more toxic to anuran species relative to other vertebrate taxa. There are limited Cu toxicity data for salamanders; of the few studies conducted on salamanders, most examined Cu effects on the embryonic, but not the larval, stage. We performed acute toxicity experiments, to quantify LC50s, on Harrison stage 46 larvae (free swimming hatchlings with egg yolk completely absorbed) of three ambystomatid salamander species. Each LC50 experiment used exposure concentrations of 10, 20, 30, 40, 50, and 60 µg/L with 10 replicates per concentration each containing one larva. We found very high toxicity for all species compared to previously published research on the embryo stage. Specifically, the 4-d LC50s for Ambystoma tigrinum and A. opacum were 35.3 and 18.73 µg/L, respectively. The same Cu concentrations caused similar toxicity to A. talpoideum (LC50 = 47.88 µg/L), but exposures required up to 48 d to elicit the same level of mortality. A time-to-event analysis indicated that time to mortality was significantly affected by Cu concentration. Additionally, for A. talpoideum, we observed that elevated levels of Cu decreased growth rate. Comparisons with previously reported Cu toxicity for embryos suggest that, as with fish, Cu may be more toxic to larval salamander stages than for embryos. Further, our data suggest that Cu is an important environmental contaminant that deserves increased scrutiny on the potential for population-level effects where contamination has occurred in wetlands and streams inhabited by salamanders.

Variation in metal tolerance associated with population exposure history in Southern toads (Anaxyrus terrestris)

Human activities have radically shaped the global landscape, affecting the structure and function of ecosystems. Habitat loss is one of the most visible changes to the landscape and a primary driver of species declines; however, anthropogenic environmental contamination also threatens population persistence, but is not as readily observed. Aquatic organisms are especially susceptible to chemical perturbations, which can negatively impact survival and fitness related traits. Some populations have evolved tolerance to chemical stressors, which could mitigate the consequences associated with contamination. Amphibians are experiencing global declines due to multiple stressors and are particularly at risk to aquatic chemical stressors due to their permeable skin and reliance on wetlands for reproduction and larval development. However, amphibians also have substantial plasticity in response to environmental variation. We designed our study to examine whether tolerance to heavy metals is greater in Southern toad (Anaxyrus terrestris) larvae from wetlands with a history of contamination. Considering many of the most common trace elements elicit acute toxicity by disrupting osmotic- and ionic-regulation, we hypothesized that alterations to these aspects of physiology resulting from multigenerational exposure to trace element mixtures would be the most likely routes by which tolerance would evolve. We used copper (Cu) as a proxy for heavy metal exposure because it is a widely distributed aquatic stressor known to cause osmotic stress that can also cause mortality at levels commonly encountered in the environment. We found considerable within and among population variation in Cu tolerance, as measured by time to death. Larvae from populations living in sites contaminated with mixtures of heavy metals associated with coal fly ash were no more tolerant to Cu than those from reference sites. However, larvae from a population inhabiting a constructed wetland complex with high Cu levels were significantly more tolerant; having half the risk of mortality as reference animals. This wetland complex was created < 20 years ago, thus if elevated Cu tolerance in this population is due to selection in the aquatic habitat, such adaptation may occur rapidly (i.e. ∼10 generation). Our results provide evidence that amphibians may be able to evolve tolerance in response to trace element contamination, though such tolerance may be specific to the combination of contaminants present.

Chronic effects of mercury on Bufo gargarizans larvae: Thyroid disruption, liver damage, oxidative stress and lipid metabolism disorder

Mercury is severely detrimental to organisms and is ubiquitous in both terrestrial and aquatic ecosystems. In the present study, we examined the effects of chronic mercury (Hg) exposure on metamorphosis, body size, thyroid microstructures, liver microstructural and ultrastructural features, and transcript levels of genes associated with lipid metabolism, oxidative stress and thyroid hormones signaling pathways of Chinese toad (Bufo gargarizans) tadpoles. Tadpoles were exposed to mercury concentrations at 0, 6, 12, 18, 24 and 30 µg/L from Gosner stage 26-42 of metamorphic climax. The present results showed that high dose mercury (24 and 30 µg/L) decelerated metamorphosis rate and inhibited body size of B. gargarizans larvae. Histological examinations have clearly exhibited that high mercury concentrations caused thyroid gland and liver damages. Moreover, degeneration and disintegration of hepatocytes, mitochondrial vacuolation, and endoplasmic reticulum breakdown were visible in the ultrastructure of liver after high dose mercury treatment. Furthermore, the larvae exposed to high dose mercury demonstrated a significant decrease in type II iodothyronine deiodinase (Dio2) and thyroid hormone receptor α and β (TRα and TRβ) mRNA levels. Transcript level of superoxide dismutase (SOD) and heat shock protein (HSP) were significantly up regulated in larvae exposed to high dose mercury, while transcript level of phospholipid hydroperoxide glutathione peroxidase (PHGPx) was significantly down regulated. Moreover, exposure to high dose mercury significantly down regulated mRNA expression of carnitine palmitoyltransferase (CPT), sterol carrier protein (SCP), acyl-CoA oxidase (ACOX) and peroxisome proliferator-activated receptor α (PPAPα), but significantly up regulated mRNA expression of fatty acid elongase (FAE), fatty acid synthetase (FAS) and Acetyl CoA Carboxylase (ACC). Therefore, we conclude that high dose mercury induced thyroid function disruption, liver oxidative stress and lipid metabolism disorder by damaging thyroid and liver cell structures and altering the expression levels of relevant genes.

Integrating copper toxicity and climate change to understand extinction risk to two species of pond-breeding anurans

Chemical contamination is often suggested as an important contributing factor to amphibian population declines, but direct links are rarely reported. Population modeling provides a quantitative method to integrate toxicity data with demographic data to understand the long-term effects of contaminants on population persistence. In this study we use laboratory-derived embryo and larval toxicity data for two anuran species to investigate the potential for toxicity to contribute to population declines. We use the southern toad (Anaxyrus terrestris) and the southern leopard frog (Lithobates sphenocephalus) as model species to investigate copper (Cu) toxicity. We use matrix models to project populations through time and quantify extinction risk (the probability of quasi-extinction in 35 yr). Life-history parameters for toads and frogs were obtained from previously published literature or unpublished data from a long-term (>35 yr) data set. In addition to Cu toxicity, we investigate the role of climate change on amphibian populations by including the probability of early pond drying that results in catastrophic reproductive failure (CRF, i.e., complete mortality of all larval individuals). Our models indicate that CRF is an important parameter for both species as both were unable to persist when CRF probability was >50% for toads or 40% for frogs. Copper toxicity alone did not result in significant effects on extinction risk unless toxicity was very high (>50% reduction in survival parameters). For toads, Cu toxicity and high probability of CRF both resulted in high extinction risk but no synergistic (or greater than additive) effects between the two stressors occurred. For leopard frogs, in the absence of CRF survival was high even under Cu toxicity, but with CRF Cu toxicity increased extinction risk. Our analyses highlight the importance of considering multiple stressors as well as species differences in response to those stressors. Our models were consistently most sensitive to juvenile and adult survival, further suggesting the importance of terrestrial stages to population persistence. Future models will incorporate multiple wetlands with different combinations of stressors to understand if our results for a single wetland result in a population sink within the landscape.

Environmental levels of Zn do not protect embryos from Cu toxicity in three species of amphibians

Contaminants often occur as mixtures in the environment, but investigations into toxicity usually employ a single chemical. Metal contaminant mixtures from anthropogenic activities such as mining and coal combustion energy are widespread, yet relatively little research has been performed on effects of these mixtures on amphibians. Considering that amphibians tend to be highly sensitive to copper (Cu) and that metal contaminants often occur as mixtures in the environment, it is important to understand the interactive effects that may result from multiple metals. Interactive effects of Cu and zinc (Zn) on amphibians have been reported as antagonistic and, conversely, synergistic. The goal of our study was to investigate the role of Zn in Cu toxicity to amphibians throughout the embryonic developmental period. We also considered maternal effects and population differences by collecting multiple egg masses from contaminated and reference areas for use in four experiments across three species. We performed acute toxicity experiments with Cu concentrations that cause toxicity (10-200 μg/L) in the absence of other contaminants combined with sublethal concentrations of Zn (100 and 1000 μg/L). Our results suggest very few effects of Zn on Cu toxicity at these concentrations of Zn. As has been previously reported, we found that maternal effects and population history had significant influence on Cu toxicity. The explanation for a lack of interaction between Cu and Zn in this experiment is unknown but may be due to the use of sublethal Zn concentrations when previous experiments have used Zn concentrations associated with acute toxicity. Understanding the inconsistency of amphibian Cu/Zn mixture toxicity studies is an important research direction in order to create generalities that can be used to understand risk of contaminant mixtures in the environment.

Copper and nickel effects on survival and growth of northern leopard frog (Lithobates pipiens) tadpoles in field-collected smelting effluent water

Trace metals can have subtle yet chronic impacts on organisms by inducing physiological stress that reduces their survival or impedes their ability to tolerate additional environmental stressors. The toxicity literature indicates, however, that aquatic organisms react differently to trace metals depending on the environments in which they reside. The objective of the present study was to understand the response of northern leopard frog (Lithobates pipiens) larvae to ionic copper (Cu), nickel (Ni), and their combination within an effluent water collected downstream of a tailings wetland area. Tadpoles were assigned randomly to 1 of 8 Cu concentrations (8-200 μg/L), 7 Ni concentrations (160-1200 μg/L), or 8 Cu and Ni combined concentrations (8:160-200:1200 μg/L) and showed significant differences in survival and life history traits among treatments. In the Cu and Cu and Ni combined treatments, tadpole survival decreased with increased Cu exposure starting at Cu = 160 μg/L and in the Ni treatment, tadpole survival decreased with increased Ni exposure starting at Ni = 650 μg/L. All Cu-exposed treatments induced a growth increase as the concentration increased, whereas the tadpoles showed a significant decrease in growth rate in Ni treatments. These contrasting outcomes suggest a plastic response to trace metals whereby tadpoles allocate energy reserves toward either escaping or coping with stress. Finally, the authors' argue that future studies will benefit from examining the impacts of multiple stressors in aquatic ecosystems to provide better environmental mitigation.

Effects of copper on growth, metamorphosis and endocrine disruption of Bufo gargarizans larvae

Chinese toad (Bufo gargarizans) tadpoles were exposed to copper (1, 6.4, 32 and 64μgL(-1) copper) from the beginning of larval period through completion of metamorphosis. We examined the effects of chronic copper exposure on mortality, growth, time to metamorphosis, tail resorption time, body size at the metamorphic climax (Gs 42) and completion of metamorphosis (Gs 46) and thyroid gland histology. In addition, type 2 and 3 iodothyronine deiodinase (Dio2 and Dio3), thyroid hormone receptors (TRα and TRβ) mRNA levels were also measured to assess disruption of TH synthesis. Our result showed that 6.4-64μgL(-1) copper concentration increased the mortality and inhibited the growth of B. gargarizans tadpoles. In addition, significant reduction in size at Gs 42 and a time delay to Gs 42 were observed at 6.4-64μgL(-1) copper treatments. Moreover, histological examinations have clearly revealed that 64μgL(-1) copper caused follicular cell hyperplasia in thyroid gland. According to real-time PCR results, exposure to 32 and 64μgL(-1) copper significantly up-regulated mRNA expression of Dio3, but down-regulated mRNA expression of TRα and TRβ mRNA level. We concluded that copper delayed amphibian metamorphosis through changing mRNA expression of Dio3, TRα and TRβ, which suggests that copper might have the endocrine-disrupting effect.

Lethal and sublethal measures of chronic copper toxicity in the eastern narrowmouth toad, Gastrophryne carolinensis

Many metals are acutely toxic to aquatic organisms at high concentrations and for some metals, such as copper (Cu), even low-level chronic contamination may be cause for conservation concern. Amphibian susceptibility to Cu has been examined in only a few species, and susceptibility is highly variable. The lethal and sublethal effects were examined of chronic aqueous Cu exposure on embryonic and larval eastern narrowmouth toads, Gastrophryne carolinensis. Copper levels as low as 10 μg Cu/L reduced embryonic and larval survival. Embryonic survivorship varied within- and between-source populations, with embryos derived from uncontaminated-wetland parents having greater survival at lower Cu levels than embryos from parents from a metal-contaminated constructed wetland. At 30 μg/L, embryos from the contaminated site had greater survival. Overall survival from oviposition to metamorphosis was 68.9% at 0 μg/L and 5.4% at 10 μg/L. Similarly, embryos exposed to ≥50 μg/L demonstrated developmental delays in transition from embryo to free-swimming larva. These results demonstrate a negative population-specific response to environmentally relevant levels of Cu.

Effects of Cutrine-Plus® algaecide and predators on wood frog (Lithobates sylvaticus) tadpole survival and growth

Copper contamination is increasing in many aquatic ecosystems. One mode by which copper can be introduced into aquatic ecosystems is as an algaecide, such as Cutrine-Plus®. Using a mesocosm experiment, we examined the effects of Cutrine-Plus® on wood frog (Lithobates sylvaticus) tadpoles. In addition, we examined how the presence of a nonnative predator the Western mosquitofish (Gambusia affinis) may interact with exposure to Cutrine-Plus®. Exposure to our low and high Cutrine-Plus® treatments had a strong negative effect on the wood frog tadpoles, and survivorship was greatly decreased in the low treatment, and no tadpoles survived in the high treatment. Additionally, the tadpoles that survived the low treatment were significantly smaller than those in the control treatment. Mosquitofish had no effect on the survivorship or growth of wood frog tadpoles, and mosquitofish presence did not have a significant interaction with the Cutrine-Plus® treatments. Cutrine-Plus® clearly had a negative effect on wood frog tadpoles at the concentrations used in our experiment, which were at and below the label-recommended dosages, suggesting that the use of Cutrine-Plus® in natural ponds may have negative consequences for wood frog populations and possibly other amphibians.

Effects of copper exposure on hatching success and early larval survival in marbled salamanders, Ambystoma opacum

The creation of wetlands, such as urban and industrial ponds, has increased in recent decades, and these wetlands often become enriched in pollutants over time. One metal contaminant trapped in created wetlands is copper (Cu(2+)). Copper concentrations in sediments and overlying water may affect amphibian species that breed in created wetlands. The authors analyzed the Cu concentration in dried sediments from a contaminated wetland and the levels of aqueous Cu released after flooding the sediments with different volumes of water, mimicking low, medium, and high pond-filling events. Eggs and larvae of Ambystoma opacum Gravenhorst, a salamander that lays eggs on the sediments in dry pond beds that hatch on pond-filling, were exposed to a range of Cu concentrations that bracketed potential aqueous Cu levels in created wetlands. Embryo survival varied among clutches, but increased Cu levels did not affect embryo survival. At Cu concentrations of 500 µg/L or greater, however, embryos hatched earlier, and the aquatic larvae died shortly after hatching. Because Cu concentrations in sediments increase over time in created wetlands, even relatively tolerant species such as A. opacum may be affected by Cu levels in the posthatching environment.

Chironomidae bloodworms larvae as aquatic amphibian food

Different species of chironomids larvae (Diptera: Chironomidae) so-called bloodworms are widely distributed in the sediments of all types of freshwater habitats and considered as an important food source for amphibians. In our study, three species of Chironomidae (Baeotendipes noctivagus, Benthalia dissidens, and Chironomus riparius) were identified in 23 samples of larvae from Belgium, Poland, Russia, and Ukraine provided by a distributor in Belgium. We evaluated the suitability of these samples as amphibian food based on four different aspects: the likelihood of amphibian pathogens spreading, risk of heavy metal accumulation in amphibians, nutritive value, and risk of spreading of zoonotic bacteria (Salmonella, Campylobacter, and ESBL producing Enterobacteriaceae). We found neither zoonotic bacteria nor the amphibian pathogens Ranavirus and Batrachochytrium dendrobatidis in these samples. Our data showed that among the five heavy metals tested (Hg, Cu, Cd, Pb, and Zn), the excess level of Pb in two samples and low content of Zn in four samples implicated potential risk of Pb accumulation and Zn inadequacy. Proximate nutritional analysis revealed that, chironomidae larvae are consistently high in protein but more variable in lipid content. Accordingly, variations in the lipid: protein ratio can affect the amount and pathway of energy supply to the amphibians. Our study indicated although environmentally-collected chironomids larvae may not be vectors of specific pathogens, they can be associated with nutritional imbalances and may also result in Pb bioaccumulation and Zn inadequacy in amphibians. Chironomidae larvae may thus not be recommended as single diet item for amphibians.

Copper-driven avoidance and mortality in temperate and tropical tadpoles

Amphibians have experienced an accentuated population decline in the whole world due to many factors, one of them being anthropogenic contamination. The present study aimed to assess the potential effect of copper, as a worldwide and reference contaminant, on the immediate decline of exposed population due to avoidance and mortality responses in tadpoles of three species of amphibians across climatic zones: a South American species, Leptodactylus latrans, a North American species, Lithobates catesbeianus, and a European species, Pelophylax perezi. A non-forced exposure system with a copper gradient along seven compartments through which organisms could freely move was used to assess the ability of tadpoles to detect and avoid copper contamination. All species were able to avoid copper at a concentration as low as 100 μg L(-1). At the lowest (sublethal) concentrations (up to 200 μg L(-1)) avoidance played an exclusive role for the population decline, whereas at the highest concentrations (> 450 μg L(-1)) mortality was the response determining population decline. The median concentrations causing exposed population immediate decline were 93, 106 and 180 μg L(-1) for Le. latrans, Li. catesbeianus and P. perezi, respectively. Contaminants might, therefore, act as environmental disruptors both by generating low-quality habitats and by triggering avoidance of tadpoles, which could be an important response contributing to dispersion patterns, susceptibility to future stressors and decline of amphibian populations (together with mortality).

Effects of chronic aluminum and copper exposure on growth and development of wood frog (Rana sylvatica) larvae

Wood frogs (Rana sylvatica) were exposed to aluminum (Al; 10, 100, 500, 1000, or 2000 μgL(-1)) or copper (Cu; 1, 10, 50, 100, 200 μgL(-1)) at a pH of 4.70 from the beginning of the larval period through the completion of metamorphosis (range=43-102 days). Observations on mortality, malformation, time to reach specific developmental stages, body mass at these stages, and metamorphic success were made throughout the larval developmental period. Only one case of malformation was observed and mortality was ≤ 10% at all concentrations except the highest Cu concentration where the rate was 33%. All larvae that survived the experiment successfully completed metamorphosis, but significant effects on growth and development occurred for both metals and these were most prominent for Cu. At the highest Al concentration (2000 μgL(-1)), body mass of larvae was significantly lower (reduced by 17% compared to the control) at 20 days post hatching (DPH) and the time to reach the hind-limb (HL), front-limb (FL), and tail resorption (TR) stages was significantly increased (9-10 days longer than the control). Body mass of larvae exposed to the three highest concentrations of Cu (50, 100, 200 μgL(-1)) was reduced by 30-34% at 20 DPH. Exposure to these concentrations also resulted in increased time to reach the HL, FL, and TR stages with larvae in the highest concentration taking 21-29 days longer to reach these stages. Larvae exposed to 10 μgL(-1) Cu also took longer to reach the FL and TR stages of development, and exposure to all Cu concentrations increased tail resorption time by more than two days compared to the control. Although the only observed effects of Al were for a concentration that is probably not ecologically relevant, results demonstrate that environmentally-realistic levels of Cu may have significant biological effects that could influence individual fitness and population-level processes.

Within- and among-population level differences in response to chronic copper exposure in southern toads, Anaxyrus terrestris

Environmental contaminants are implicated in the global decline of amphibian populations. Copper (Cu) is a widespread contaminant that can be toxic at concentrations just above the normal physiological range. In the present study we examined the effects of chronic Cu aqueous exposure on embryos and larvae of southern toads, Anaxyrus (Bufo) terrestris. Measurable levels of Cu were found in larvae, with tissue concentrations up to 27.5 μg Cu/g dry mass. Aqueous concentrations of Cu as low as 10 μg/L significantly reduced survival to the free-swimming stage and no larvae reached metamorphosis at concentrations above 15 μg/L. Clutches from populations with prior Cu exposure had the lowest survivorship. Among several populations there was significant variation in survivorship at different levels of Cu. More data are needed to understand the underlying causes of within- and among-population resilience to anthropogenic stressors.