Age-dependent changes in sensitivity to a pesticide in tadpoles of the common toad (Bufo bufo)

Chytridiomycosis causes high amphibian mortality prior to the completion of metamorphosis

Amphibian populations are undergoing extensive declines globally. The fungal disease chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), is a primary contributor to these declines. The amphibian metamorphic stages (Gosner stages 42-46) are particularly vulnerable to a range of stressors, including Bd. Despite this, studies that explicitly examine host response to chytridiomycosis throughout the metamorphic stages are lacking. We aimed to determine how Bd exposure during the larval stages impacts metamorphic development and infection progression in the endangered Fleay's barred frog (Mixophyes fleayi). We exposed M. fleayi to Bd during pro-metamorphosis (Gosner stages 35-38) and monitored infection dynamics throughout metamorphosis. We took weekly morphological measurements (weight, total body length, snout-vent-length and Gosner stage) and quantified Bd load using qPCR. While we observed minimal impact of Bd infection on animal growth and development, Bd load varied throughout ontogeny, with an infection load plateau during the tadpole stages (Gosner stages 35-41) and temporary infection clearance at Gosner stage 42. Bd load increased exponentially between Gosner stages 42 and 45, with most exposed animals becoming moribund at Gosner stage 45, prior to the completion of metamorphosis. There was variability in infection outcome of exposed individuals, with a subgroup of animals (n = 5/29) apparently clearing their infection while the majority (n = 21/29) became moribund with high infection burdens. This study demonstrates the role that metamorphic restructuring plays in shaping Bd infection dynamics and raises the concern that substantial Bd-associated mortality could be overlooked in the field due to the often cryptic nature of these latter metamorphic stages. We recommend future studies that directly examine the host immune response to Bd infection throughout metamorphosis, incorporating histological and molecular methods to elucidate the mechanisms responsible for the observed trends.

Prednisone and prednisolone effects on development, blood, biochemical and histopathological markers of Aquarana catesbeianus tadpoles

Synthetic glucocorticoids are often found in surface waters and can cause harmful effects to aquatic organisms such as amphibians. In this work we evaluated the effects of the drugs prednisone (PD) and prednisolone (PL) on developmental, molecular, blood, biochemical and histological markers. Aquarana catesbeianus tadpoles were exposed for 16 days to environmentally relevant concentrations of 0, 0.1, 1 and 10 µg/L of both drugs. PD increased the transcript levels of the enzyme deiodinase III (Dio3), the hormones cortisol and T4 and delayed development. Changes in the thyroid gland occurred after tadpoles were exposed to both drugs, with a reduction in the diameter and number of follicles and an increase/or decrease in area. Also, both drugs caused a decrease in lymphocytes (L) and an increase in neutrophils (N), thrombocytes, the N:L ratio and lobed and notched erythrocytes. Increased activity of the enzymes superoxide dismutase, glutathione S-transferase and glucose 6-phosphate dehydrogenase was observed after exposure to PD. Furthermore, both drugs caused an increase in the activity of the enzymes catalase and glutathione peroxidase. However, only PD caused oxidative stress in exposed tadpoles, evidenced by increased levels of malondialdehyde and carbonyl proteins. Both drugs caused an increase in inflammatory infiltrates, blood cells and melanomacrophages in the liver. Our results indicate that PD was more toxic than PL, affecting development and causing oxidative stress.

Marsh frog response to urea fertilizer during the embryonic, larval, and metamorphosis stages: a new perspective into urea toxicity on amphibians

Amphibian populations are declining worldwide. These declines are caused by a variety of factors, one of which is the use of fertilizers in agriculture. This is especially true for tadpoles, which may develop in fertilizer-polluted agricultural water bodies. Nevertheless, there is little data on the toxicological consequences of fertilizers on amphibians. The goal of this study was to determine the acute and chronic toxicity of urea fertilizer on marsh frogs' (Pelophylax sp.) embryonic, larval, and metamorphic stages. For this purpose, in a static-renewal test, individuals were exposed to twelve nominal concentrations (0 to 15000 mg/L) of urea for 122 days to determine hatching success, survival, growth, development, and metamorphic traits, as well as histological consequences. Based on the results, at concentrations greater than 500 mg/L, no hatching occurred. Survivorship was unaffected for the first 72 hours, but it reached 0% on day 26 at concentrations greater than 150 mg/L. Survival and development rates decreased significantly in 100 and 150 mg/L treatments after a longer duration (day 86). Growth was reduced as well, but it was only significant at 150 mg/L. Metamorphosis time and percentage were significantly impacted, but not metamorphosis size. Increased urea fertilizer concentrations had significant histopathological consequences for the skin, gills, liver, kidneys, and striated muscles. Our results suggest that urea fertilizer, at concentrations commonly found in agroecosystems, may pose a serious threat to temperate anuran species inhabiting these conditions.

Impact of commercial formulations of herbicides alone and in mixtures on the antioxidant system and body condition parameters in tadpoles of Rhinella icterica (Spix 1824)

Pesticide usage has increased over the last decades, leading to concerns regarding its effects on non-target organisms, especially amphibians. Tadpoles of Rhinella icterica were collected in a pesticide-free place, acclimated in the laboratory (21 days) and exposed (7 days) to three herbicides (20µg/L atrazine (A); 250µg/L glyphosate (G); 20µg/L quinclorac (Q)) and their mixtures. Only 2% mortality was observed over the 28 days of the study. Despite this, significant variations were observed for markers of oxidative balance and body condition when comparing all experimental groups. K and Kn factor showed the lowest values in the group A+G+Q, as well as the activity levels of GST and SOD. In contrast to this, the CAT activity was higher in the same group (A+G+Q). The mixture of the three herbicides proved to be more harmful, which points to the need for more restrictive laws for the use of mixed herbicides.

Occurrence, spatio-temporal distribution, and human health risk assessment of pesticides in fish along the Ayuquila-Armería River, Mexico

Pesticide usage has contributed to increasing food production; it has also caused them to be found in ecosystems inducing adverse effects on biota. Fish are the most abundant and diverse vertebrates in the world and are of great importance both economically and ecologically. Some fishes are indicators of the environmental quality of aquatic ecosystems and provide insight as to how pollutants might influence public health. The tilapias species can be considered biomonitors because they present little displacement representing the contamination level of a site. This study aimed at three goals: (1) to determine the concentration of 20 pesticides in tilapia muscle in the Ayuquila-Armería basin, (2) to describe the spatiotemporal variation of analytes and (3) to evaluate the risk of consuming contaminated fish. The presence of 11 pesticides was determined. Ametrine, glyphosate and malathion concentrations showed significant differences by season. The risk assessment showed that the consumption of tilapia muscle from the Ayuquila-Armería basin does not represent a risk for the population. Diazinon concentrations were relatively low compared to the other pesticides concentrations, but its toxic characteristics were the ones that most negatively influenced the risk assessment. The results obtained are relevant from the social and economic points of view.

Do immune system changes at metamorphosis predict vulnerability to chytridiomycosis? An update

Amphibians are among the vertebrate groups suffering great losses of biodiversity due to a variety of causes including diseases, such as chytridiomycosis (caused by the fungal pathogens Batrachochytrium dendrobatidis and B. salamandrivorans). The amphibian metamorphic period has been identified as being particularly vulnerable to chytridiomycosis, with dramatic physiological and immunological reorganisation likely contributing to this vulnerability. Here, we overview the processes behind these changes at metamorphosis and then perform a systematic literature review to capture the breadth of empirical research performed over the last two decades on the metamorphic immune response. We found that few studies focused specifically on the immune response during the peri-metamorphic stages of amphibian development and fewer still on the implications of their findings with respect to chytridiomycosis. We recommend future studies consider components of the immune system that are currently under-represented in the literature on amphibian metamorphosis, particularly pathogen recognition pathways. Although logistically challenging, we suggest varying the timing of exposure to Bd across metamorphosis to examine the relative importance of pathogen evasion, suppression or dysregulation of the immune system. We also suggest elucidating the underlying mechanisms of the increased susceptibility to chytridiomycosis at metamorphosis and the associated implications for population persistence. For species that overlap a distribution where Bd/Bsal are now endemic, we recommend a greater focus on management strategies that consider the important peri-metamorphic period.

"Heat waves" experienced during larval life have species-specific consequences on life-history traits and sexual development in anuran amphibians

Extreme temperatures during heat waves can induce mass-mortality events, but can also exert sublethal negative effects by compromising life-history traits and derailing sexual development. Ectothermic animals may, however, also benefit from increased temperatures via enhanced physiological performance and the suppression of cold-adapted pathogens. Therefore, it is crucial to address how the intensity and timing of naturally occurring or human-induced heat waves affect life-history traits and sexual development in amphibians, to predict future effects of climate change and to minimize risks arising from the application of elevated temperature in disease mitigation. We raised agile frog (Rana dalmatina) and common toad (Bufo bufo) tadpoles at 19 °C and exposed them to a simulated heat wave of 28 or 30 °C for six days during one of three ontogenetic periods (early, mid or late larval development). In agile frogs, exposure to 30 °C during early larval development increased mortality. Regardless of timing, all heat-treatments delayed metamorphosis, and exposure to 30 °C decreased body mass at metamorphosis. Furthermore, exposure to 30 °C during any period and to 28 °C late in development caused female-to-male sex reversal, skewing sex ratios strongly towards males. In common toads, high temperature only slightly decreased survival and did not influence phenotypic sex ratio, while it reduced metamorph mass and length of larval development. Juvenile body mass measured 2 months after metamorphosis was not adversely affected by temperature treatments in either species. Our results indicate that heat waves may have devastating effects on amphibian populations, and the severity of these negative consequences, and sensitivity can vary greatly between species and with the timing and intensity of heat. Finally, thermal treatments against cold-adapted pathogens have to be executed with caution, taking into account the thermo-sensitivity of the species and the life stage of animals to be treated.

Multigeneration toxicity of Geunsami® (a glyphosate-based herbicide) to Allonychiurus kimi (Lee) (Collembola) from sub-individual to population levels

Glyphosate-based herbicide (GBH) is the most widely used herbicide worldwide and has long been considered to have significantly low toxicity to non-target soil invertebrates based on short-term toxicity tests (<56 d). However, long-term GBH toxicity assessment is necessary as GBH is repeatedly applied in the same field annually because of the advent of glyphosate-resistant crops. In this study, a multigeneration test was conducted where Allonychiurus kimi (Collembola) was exposed to GBH for three generations (referred to as F0, F1, and F2) to evaluate the long-term toxic effect. The endpoints used were adult survival and juvenile production for the individual level toxicity assessment. Phospholipid profile and population age structure were the endpoints used for sub-individual and population levels, respectively. GBH was observed to have no negative effects on adult survivals of all generations, but juvenile production was found to decrease in a concentration-dependent manner, with EC50s being estimated as 572.5, 274.8, and 59.8 mg a.i. kg-1 in the F0, F1, and F2 generations, respectively. The age structure of A. kimi population produced in the test of all generations was altered by GBH exposure, mainly because of the decrease in the number of young juveniles. Further, differences between the phospholipid profiles of the control and GBH treatments became apparent over generations, with PA 16:0, PA 12:0, and PS 42:0 lipids not being detected at the highest concentration of 741 mg kg-1 in F2. Considering all our findings from sub-individual to population levels, repeated and long-term use of GBH could have significantly higher negative impacts on non-target soil organisms than expected.

Juvenile African Clawed Frogs (Xenopus laevis) Express Growth, Metamorphosis, Mortality, Gene Expression, and Metabolic Changes When Exposed to Thiamethoxam and Clothianidin

Neonicotinoids (NEO) represent the main class of insecticides currently in use, with thiamethoxam (THX) and clothianidin (CLO) primarily applied agriculturally. With few comprehensive studies having been performed with non-target amphibians, the aim was to investigate potential biomarker responses along an adverse outcome pathway of NEO exposure, whereby data were collected on multiple biological hierarchies. Juvenile African clawed frogs, Xenopus laevis, were exposed to commercial formulations of THX and CLO at high (100 ppm) and low (20 ppm) concentrations of the active ingredient. Mortality, growth, development, liver metabolic enzyme activity, and gene expression endpoints were quantified. Tadpoles (n > 1000) from NF 47 through tail resorption stage (NF 66) were exposed to NEO or to NEO-free media treatments. Liver cell reductase activity and cytotoxicity were quantified by flow cytometry. Compared to control reference gene expressions, levels of expression for NEO receptor subunits, cell structure, function, and decontamination processes were measured by RT-qPCR by using liver and brain. Mortality in THX high was 21.5% compared to the control (9.1%); the metabolic conversion of THX to CLO may explain these results. The NF 57 control tadpoles were heavier, longer, and more developed than the others. The progression of development from NF 57-66 was reduced by THX low, and weight gain was impaired. Liver reductases were highest in the control (84.1%), with low NEO exhibiting the greatest reductions; the greatest cytotoxicity was seen with THX high. More transcriptional activity was noted in brains than in livers. Results affirm the utility of a study approach that considers multiple complexities in ecotoxicological studies with non-target amphibians, underscoring the need for simultaneously considering NEO concentration-response relationships with both whole-organism and biomarker endpoints.

Effects of mancozeb on heat Shock protein 70 (HSP70) and its relationship with the thermal physiology of Physalaemus henselii (Peters, 1872) tadpoles (Anura: Leptodactylidae)

Negative impacts on amphibians have been reported due to contamination by agrochemicals. However, until now, no study has tested the effect of the fungicide mancozeb (MZ) on thermal tolerance and its relationship with the expression of heat shock proteins (HSPs). MZ is the best-selling broad-spectrum fungicide in the world, which negatively affects non-target organisms. Here, we tested for the first time the effects of MZ on critical thermal maximum (CTmax) and its relationship to the expression of heat shock protein 70 (HSP70) in tadpoles of Physalameus henselii, a colder-adapted species in southernmost of the Neotropical region. A sublethal concentration of 2 mg/L was used. We found that the CTmax of the MZ-treated group was lower than that of the control group. In addition, there was an increase in HSP70 expression in tadpoles exposed to MZ and in tadpoles that underwent heat treatment. However, tadpoles subjected to MZ and heat treatment showed no induced HSP70 protein expression. Our results demonstrated that sublethal doses of the fungicide MZ negatively affected the thermal physiology and heat shock protein expression in tadpoles of P. henselii by inducing an increase in HSP70 concentration and by reducing the critical CTmax supported by tadpoles. It is important to understand the relationship between environmental contamination and physiological thermal limits in our current scenario of high rates of habitat conversion associated with unrestricted use of agrochemicals, as well as the challenging environmental changes induced by global warming.

Could Glyphosate and Glyphosate-Based Herbicides Be Associated With Increased Thyroid Diseases Worldwide?

The increased incidence of thyroid diseases raises a series of questions about what the main predisposing factors are nowadays. If dietary restriction of iodine was once a major global health concern, today, the processes of industrialization of food and high exposure to a wide variety of environmental chemicals may be affecting, directly or indirectly, thyroid function. The homeostasis of hypothalamus-pituitary-thyroid (HPT) axis is finely regulated through the negative feedback mechanism exerted by thyroid hormones. Allostatic mechanisms are triggered to adjust the physiology of HPT axis in chronic conditions. Glyphosate and glyphosate-based herbicides are pesticides with controversial endocrine disrupting activities and only few studies have approached their effects on HPT axis and thyroid function. However, glyphosate has an electrophilic and nucleophilic zwitterion chemical structure that may affect the mechanisms involved in iodide oxidation and organification, as well as the oxidative phosphorylation in the ATP synthesis. Thus, in this review, we aimed to: (1) discuss the critical points in the regulation of HPT axis and thyroid hormones levels balance, which may be susceptible to the toxic action of glyphosate and glyphosate-based herbicides, correlating the molecular mechanisms involved in glyphosate toxicity described in the literature that may, directly or indirectly, be associated to the higher incidence of thyroid diseases; and (2) present the literature regarding glyphosate toxicity in HPT axis.

How to disinfect anuran eggs? Sensitivity of anuran embryos to chemicals widely used for the disinfection of larval and post-metamorphic amphibians

Emerging infectious diseases are major drivers of global and local amphibian biodiversity loss. Therefore, developing effective disinfection methods to manage the impact of diseases in wild and captive "ark" populations are an important goal in amphibian conservation. While chemical disinfectants have been used safely and effectively in larval and adult amphibians infected with pathogenic microbes, their applicability to amphibian egg masses has remained untested. To bridge this gap, we exposed embryos of the common toad (Bufo bufo) and agile frog (Rana dalmatina) experimentally to three widely used disinfectants: voriconazole, chloramphenicol and chlorogen-sesquihydrate. For 3 days we exposed portions of egg masses to these disinfectants at 1×, 2×, 5× and 10× the concentration recommended for the disinfection of tadpoles and adults. Subsequently, we recorded embryonic and larval survival, as well as larval body mass and the incidence of abnormalities 12 days after hatching. Application of voriconazole had species- and concentration-dependent negative impacts on survival and body mass, and caused marked malformations in the viscerocranial structure of B. bufo tadpoles. Exposure to chlorogen-sesquihydrate also resulted in significant mortality in B. bufo embryos and negatively affected body mass of R. dalmatina larvae. Chloramphenicol had little negative effects on embryos or larvae in either species. Based on these results, the application of voriconazole and chlorogen-sesquihydrate cannot be recommended for the disinfection of amphibian eggs, whereas treatment with chloramphenicol appears to be a safe method for eliminating potential pathogens from anuran egg masses and their immediate aquatic environment.

Can Constructed Wetlands be Wildlife Refuges? A Review of Their Potential Biodiversity Conservation Value

The degradation of wetland ecosystems is currently recognized as one of the main threats to global biodiversity. As a means of compensation, constructed wetlands (CWs), which are built to treat agricultural runoff and municipal wastewater, have become important for maintaining biodiversity. Here, we review studies on the relationships between CWs and their associated biodiversity published over the past three decades. In doing so, we provide an overview of how wildlife utilizes CWs, and the effects of biodiversity on pollutant transformation and removal. Beyond their primary aim (to purify various kinds of wastewater), CWs provide sub-optimal habitat for many species and, in turn, their purification function can be strongly influenced by the biodiversity that they support. However, there are some difficulties when using CWs to conserve biodiversity because some key characteristics of these engineered ecosystems vary from natural wetlands, including some fundamental ecological processes. Without proper management intervention, these features of CWs can promote biological invasion, as well as form an ‘ecological trap’ for native species. Management options, such as basin-wide integrative management and building in more natural wetland components, can partially offset these adverse impacts. Overall, the awareness of managers and the public regarding the potential value of CWs in biodiversity conservation remains superficial. More in-depth research, especially on how to balance different stakeholder values between wastewater managers and conservationists, is now required.

Endocrine disruptors in breeding ponds and reproductive health of toads in agricultural, urban and natural landscapes

Many chemical pollutants have endocrine disrupting effects which can cause lifelong reproductive abnormalities in animals. Amphibians are the most threatened group of vertebrates, but there is little information on the nature and quantity of pollutants occurring in typical amphibian breeding habitats and on the reproductive capacities of amphibian populations inhabiting polluted areas. In this study we investigated the occurrence and concentrations of endocrine disrupting chemicals in the water and sediment of under-studied amphibian breeding habitats in natural, agricultural and urbanized landscapes. Also, we captured reproductively active common toads (Bufo bufo) from these habitats and let them spawn in a 'common garden' to assess among-population differences in reproductive capacity. Across 12 ponds, we detected 41 out of the 133 contaminants we screened for, with unusually high concentrations of glyphosate and carbamazepine. Levels of polycyclic aromatic hydrocarbons, nonylphenol and bisphenol-A increased with urban land use, whereas levels of organochlorine and triazine pesticides and sex hormones increased with agricultural land use. Toads from all habitats had high fecundity, fertilization rate and offspring viability, but the F1 generation originating from agricultural and urban ponds had reduced development rates and lower body mass both as larvae and as juveniles. Females with small clutch mass produced thicker jelly coat around their eggs if they originated from agricultural and urban ponds compared with natural ponds. These results suggest that the observed pollution levels did not compromise reproductive potential in toads, but individual fitness and population viability may be reduced in anthropogenically influenced habitats, perhaps due to transgenerational effects and/or costs of tolerance to chemical contaminants.

Chemical application strategies to protect water quality

Management of turfgrass on golf courses and athletic fields often involves application of plant protection products to maintain or enhance turfgrass health and performance. However, the transport of fertilizer and pesticides with runoff to adjacent surface waters can enhance algal blooms, promote eutrophication and may have negative impacts on sensitive aquatic organisms and ecosystems. Thus, we evaluated the effectiveness of chemical application setbacks to reduce the off-site transport of chemicals with storm runoff. Experiments with water soluble tracer compounds confirmed an increase in application setback distance resulted in a significant increase in the volume of runoff measured before first off-site chemical detection, as well as a significant reduction in the total percentage of applied chemical transported with the storm runoff. For example, implementation of a 6.1 m application setback reduced the total percentage of an applied water soluble tracer by 43%, from 18.5% of applied to 10.5% of applied. Evaluation of chemographs revealed the efficacy of application setbacks could be observed with storms resulting in lesser (e.g. 100 L) and greater (e.g. > 300 L) quantities of runoff. Application setbacks offer turfgrass managers a mitigation approach that requires no additional resources or time inputs and may serve as an alternative practice when buffers are less appropriate for land management objectives or site conditions. Characterizing potential contamination of surface waters and developing strategies to safeguard water quality will help protect the environment and improve water resource security. This information is useful to grounds superintendents for designing chemical application strategies to maximize environmental stewardship. The data will also be useful to scientists and regulators working with chemical transport and risk models.

Age-dependent antioxidant responses to the bioconcentration of microcystin-LR in the mysid crustacean, Neomysis awatschensis

Microcystins (MCs) are naturally occurring algal toxins in the aquatic environment and pose a serious threat to the ecosystem. In general, aquatic populations are structured by organisms of different ages, with varying degrees of biochemical and physiological responses. In this study, juvenile and adult marine mysids (Neomysis awatschensis) were exposed to MC-Leucine Arginine (MC-LR) (0.1, 1, and 10 μg L^-1) for 7 days, and the bioconcentration dynamics and responses of antioxidant defense system were measured during the exposure and additional depuration periods (7 days). MC-LR bioconcentrated in a dose-dependent manner, from a threshold concentration of 1 μg L^-1 in both stages, and the levels reduced gradually during the depuration phase. Bioconcentration patterns of MC-LR were highly age-specific, as juvenile mysids showed peaks during the exposure period, whereas adults exhibited a peak on the first day of depuration. After exposure to 10 μg L^-1 concentration, elevated levels of malondialdehyde (MDA) and glutathione (GSH) were observed during the late (days 5 and 7) exposure and early (days 1 and 3) depuration periods in juvenile mysids, while adult mysids showed a peak on day 7 of the exposure period. Age-specific responses were also observed in the enzymatic activities of glutathione S-transferase (GST), catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione reductase (GR). Juvenile mysids showed a significant elevation in all enzymatic activities during the exposure and/or depuration phase upon exposure to 10 μg L^-1 MC-LR, but only CAT and SOD enzymes showed significant changes during the exposure and/or depuration periods in adults. Overall, our results indicate the bioconcentration potential of MC-LR and its threshold in the marine mysid, in addition to age-specific MC-LR dynamics and subsequent biochemical responses.

Chronic exposure to a glyphosate-based herbicide makes toad larvae more toxic

Chemical pollutants can exert various sublethal effects on wildlife, leading to complex fitness consequences. Many animals use defensive chemicals as protection from predators and diseases, yet the effects of chemical contaminants on this important fitness component are poorly known. Understanding such effects is especially relevant for amphibians, the globally most threatened group of vertebrates, because they are particularly vulnerable to chemical pollution. We conducted two experiments to investigate how exposure to glyphosate-based herbicides, the most widespread agrochemicals worldwide, affects the production of bufadienolides, the main compounds of chemical defence in common toads (Bufo bufo). In both experiments, herbicide exposure increased the amount of bufadienolides in toad tadpoles. In the laboratory, individuals exposed to 4 mg a.e./L glyphosate throughout their larval development had higher bufadienolide content at metamorphosis than non-exposed tadpoles, whereas exposure for 9 days to the same concentration or to 2 mg a.e./L throughout larval development or for 9 days had no detectable effect. In outdoor mesocosms, tadpoles from 16 populations exhibited elevated bufadienolide content after three-weeks exposure to both concentrations of the herbicide. These results show that pesticide exposure can have unexpected effects on non-target organisms, with potential consequences for the conservation management of toxin-producing species and their predators.