Increased Conductivity Affects Corticosterone Levels and Prey Consumption in Larval Amphibians

Amphibian Dispersal Traits Not Impacted by Triclopyr Exposure during the Juvenile Stage

Exposure to agrochemicals can have lethal and sublethal effects on amphibians. Most toxicology studies only examine exposure during the aquatic larval stage. Survival of the juvenile stage is the most important for population persistence and it is critical to understand the potential impacts of exposure during this life stage. We investigated how short-term exposure to triclopyr, an herbicide commonly used in forestry management, might impact several juvenile traits. To determine if juveniles perceived exposure as an environmental stressor, we measured their release of corticosterone. We also examined dispersal traits by measuring foraging and hopping behavior. We found no evidence that exposure negatively impacted these traits or was a stressor. Our results provide a preliminary assessment of the potential impact of triclopyr on juvenile amphibians, but we recommend additional research on the effects of agrochemicals on juvenile amphibians.

Contributions of water-borne corticosterone as one non-invasive biomarker in assessing nitrate pollution stress in tadpoles of Rana temporaria

Among a multitude of stressors to which wildlife is exposed, environmental pollution is a pervasive one that poses a serious threat. The permeable skin of amphibians is likely to increase direct contact of the body with pollutants, making them a group worth studying to access environmental quality. Consequently, finding reliable and complementary biomarkers that will present detectable and predictable changes in response to pollutants is essential to identify pollution sublethal effects on amphibians and to investigate whether these are in part responsible for population declines. The glucocorticoid hormone corticosterone (CORT), involved in many metabolic functions, is often used to measure the physiological stress response to environmental stressors in amphibians. In this study, we evaluated whether water-borne CORT can serve as a non-invasive biomarker for nitrate pollution stress in the European common frog (Rana temporaria) by comparing the effect of nitrate exposure on hormone release rates and on other physiological downstream biomarkers, i.e., ultimate physiological effects of the stressor. Specifically, we investigated the effect of different nitrate concentrations (0, 10, 50, and 100 mg/L) on water-borne CORT release rates, age, size, and body condition. Exposure to nitrate pollution significantly increased age at metamorphosis and water-borne CORT release rates, and led to reduced mass and body condition, but only at higher nitrate concentrations (i.e., 50 and 100 mg/L). Considering this similar sensitivity to other acknowledged biomarkers, water-borne CORT was a reliable biomarker of physiological stress in R. temporaria exposed to nitrate pollution stress in a controlled single-stressor laboratory approach. Thus, water-borne CORT is a promising method to be included in more holistic approaches. We recommend that such approaches keep testing multiple biomarker combinations, as species are exposed to several stressors likely to interact and produce varied outcomes in different biomarkers in their natural habitats.

Impaired negative feedback and death following acute stress in glucocorticoid receptor knockout Xenopus tropicalis tadpoles

Blood glucocorticoid levels are regulated by the hypothalamo-pituitary-adrenal/interrenal axis (HPA axis in mammals, HPI axis in amphibians), and negative feedback by glucocorticoid signaling is a key player in that regulation. Glucocorticoid and mineralocorticoid receptors (GR and MR) mediate negative feedback in mammals, but little is known about nuclear receptor-mediated feedback in amphibians. Because amphibians have only one corticosteroidogenic cell type responsible for glucocorticoid and mineralocorticoid production, we hypothesized that GR knockout (GRKO) tadpoles have elevated levels of glucocorticoids and mineralocorticoids as well as axis components regulating their production. We also examined the response to stress and potential for increased aldosterone signaling in GRKO tadpoles. We found that GRKO tadpoles have severe hyperactivity of the HPI axis, namely high mRNA expression levels of pomc, cyp17a1, cyp21a2, cyp11b2, and star, and high tissue content of corticosterone, aldosterone, 17-hydroxyprogesterone, 21-deoxycortisol, and progesterone. Such aberrant HPI activity was accompanied by reduced survival after acute temperature shock and shaking stress. Like mammalian models of HPA hyperactivity, GRKO tadpoles have high MR mRNA expression levels in brain, kidney, heart, and skin and high levels of the inflammatory cytokine tnf-α and the profibrotic factor tgf-β in kidneys. This study showed GR is critical for negative feedback to the amphibian HPI axis and for survival from acute stressors. This study also showed GRKO tadpoles exhibit altered expression/overproduction of regulators of salt-water homeostasis and associated biomarkers of kidney disease.

From the Field to the Lab: Physiological and Behavioural Consequences of Environmental Salinity in a Coastal Frog

Environmental salinization is recognized as a global threat affecting biodiversity, particularly in coastal ecosystems (affected by sea level rise and increased frequency and severity of storms), and the consequent osmoregulatory challenges can negatively affect wildlife. In order to assess whether coastal species can respond to changes in environmental salinity, it remains essential to investigate the consequences of exposure to salinity in an environmentally-relevant context. In this study, we assessed the consequences of exposure to environmental salinity in coastal frogs (Pelophylax sp., N = 156) both in the field and experimentally, using a comprehensive combination of markers of physiology, behaviour and ecology. Exposure to salinity in the field negatively affected physiological parameters (osmolality, monocytes and eosinophils counts), as well as body condition and locomotor performance, and influenced size- and sex-specific habitat selection. Further, we demonstrated in a controlled experiment that short-term exposure to salinity strongly affected physiological parameters (salt influxes, water effluxes, immunity-related stress markers) and locomotor performance. Most of these effects were transient (water and salt fluxes, locomotor performance) once optimal conditions resumed (i.e., freshwater). Taken together, our results highlight the need to investigate whether exposure to environmental salinity can ultimately affect individual fitness and population persistence across taxa.

Distance to coastline modulates morphology and population structure in a coastal amphibian

Salinization due to sea-level rise and marine submersions is expected to strongly impact coastal ecosystems. Exposure to salinity can negatively impact biodiversity especially in coastal wetlands. To understand comprehensively the consequences of environmental salinization on coastal biodiversity, it is essential to document how coastal species currently respond to exposure to salinity. In this study, we investigated how variations of environmental salinity relative to the distance to the ocean influence population structure (age ratio and sex ratio), and individual hydro-mineral balance (osmolality) and morphology (size, mass, condition) in the western spadefoot toad (Pelobates cultripes) in two populations from the French Atlantic coast. We show that distance to coastline strongly influences exposure to salt on a small spatial scale. Some variables (e.g., abundances, osmolality, morphology) responded similarly in both sites and may be related to salt deposition due to landward sea-spray. Interestingly, other variables (sex ratio and age ratio) displayed site-specific responses and appeared to be linked to the salinity of breeding sites. Distance to the shoreline appears to be a critical population structuring factor in this coastal salt-tolerant species. Future studies should investigate how distance to shoreline—and thus environmental salinity—can ultimately affect individual performances and fitness.

Multiscale determinants of Pacific chorus frog occurrence in a developed landscape

Pacific chorus frog (Pseudacris regilla) populations have persisted despite urban and rural development throughout the species’ range; yet it is possible that P. regilla, like other anurans with which it historically co-occurred, will become extirpated from cities and suburbs if urbanization intensifies as predicted. An improved understanding of the conditions that enable this species to persist in developed landscapes is needed to identify and conserve suitable habitats. We investigated species-habitat relationships for P. regilla in a mixed urban-rural landscape in southwestern British Columbia, Canada, to identify potential criteria for habitat suitability. We conducted repeat auditory surveys of chorusing males at 52 potential breeding wetlands and modeled occupancy at 26 of these sites using local and landscape variables representing competing hypotheses and spatial scales of influence. The models that best explained P. regilla occupancy included a combination of terrestrial habitat and connectivity factors and the presence of non-native predators. We found that the proportion of impervious cover within 250 m of a wetland had the strongest negative impact on occupancy. Our findings suggest that availability of terrestrial habitat adjacent to breeding sites is the primary driver of species presence in the developed landscape. Conservation efforts should seek to limit impervious cover to less than 20% within a 250-m buffer around breeding wetlands. Further, restored and created wetlands in urban and rural areas may be more likely to support P. regilla if they are designed with a seasonal hydroperiod that excludes non-native aquatic predators and are placed in an area of high pond density.

Brain plasticity in response to short-term exposure to corticosterone in larval amphibians

Exposure to stressors and elevation of glucocorticoid hormones such as corticosterone (CORT) has widespread effects on vertebrate brain development. Previous studies have shown that exposure to environmental stressors alters larval amphibian brain morphology and behavior, yet the effects of CORT on amphibian brain morphology are still unknown. We exposed prometamorphic Northern Leopard Frog (Lithobates pipiens (Schreber, 1782)) tadpoles for 7 days to a concentration of exogenous CORT (45.56 μg/L) that produced physiologically relevant increases in plasma CORT. This brief exposure to CORT, relatively late in development, resulted in a significantly larger diencephalon width (relative to body mass) when compared with controls. Although we were unable to detect changes in behavior or body morphology, our results indicate that brain shape is modulated by exposure to CORT. More studies are needed to better understand what accounts for the CORT-induced change in brain shape as well as the functional consequences of these changes.

Evaluating corticosterone as a biomarker for amphibians exposed to increased salinity and ambient corticosterone

Physiological biomarkers are commonly used to assess the health of taxa exposed to natural and anthropogenic stressors. Glucocorticoid (GC) hormones are often used as indicators of physiological stress in wildlife because they affect growth, reproduction and survival. Increased salinity from human activities negatively influences amphibians and their corticosterone (CORT; the main amphibian GC) physiology; therefore, CORT could be a useful biomarker. We evaluated whether waterborne CORT could serve as a biomarker of salt stress for three free-living amphibian species that vary in their sensitivity to salinity: boreal chorus frogs (Pseudacris maculata), northern leopard frogs (Rana pipiens) and barred tiger salamanders (Ambystoma mavortium). Across a gradient of contamination from energy-related saline wastewaters, we tested the effects of salinity on baseline and stress-induced waterborne CORT of larvae. Stress-induced, but not baseline, CORT of leopard frogs increased with increasing salinity. Salinity was not associated with baseline or stress-induced CORT of chorus frogs or tiger salamanders. Associations between CORT and salinity were also not related to species-specific sensitivities to salinity. However, we detected background environmental CORT (ambient CORT) in all wetlands and spatial variation was high within and among wetlands. Higher ambient CORT was associated with lower waterborne CORT of larvae in wetlands. Therefore, ambient CORT likely confounded associations between waterborne CORT and salinity in our analysis and possibly influenced physiology of larvae. We hypothesize that larvae may passively take up CORT from their environment and downregulate endogenous CORT. Although effects of some hormones (e.g. oestrogen) and endocrine disruptors on aquatic organisms are well described, studies investigating the occurrence and effects of ambient CORT are limited. We provide suggestions to improve collection methods, reduce variability and avoid confounding effects of ambient CORT. By making changes to methodology, waterborne CORT could still be a promising, non-invasive conservation tool to evaluate effects of salinity on amphibians.

Effects of Salinity on Hatchling Diamond-Backed Terrapin (Malaclemys terrapin) Growth, Behavior, and Stress Physiology

Diamond-backed Terrapins inhabit coastal salt marshes along the eastern and Gulf coasts of North America. Terrapins are adapted to intermediate salinities yet frequently face saltwater-inundated marsh habitat exceeding 25 ppt (or grams/kilogram). We investigated the effect of salinity on the growth of hatchling terrapins and on their compensatory responses to salinity stress. We randomly assigned 30 terrapin hatchlings each to one of five salinity treatments (1, 5, 10, 20, or 35 ppt). Over 75 d, we regularly monitored behavior, appetite, and changes in growth; and calculated ratios of heterophils to lymphocytes (H:L ratio) to assess responses to prolonged salinity stress. Consistent with prior studies, chronic exposure to high salinity significantly reduced hatchling growth. Hatchlings in 20-ppt and 35-ppt salinities exhibited appetite suppression and saltwater avoidance and were more likely to show freshwater-seeking behaviors. H:L ratios were higher among hatchlings in 20-and 35-ppt salinities, consistent with a corticosterone-driven stress response to sustained high-salinity exposure, which may play a role in limiting growth. Our findings suggest hatchling growth and distribution among local habitats will vary spatially depending on habitat salinity and freshwater accessibility. The growth-limiting effects of chronically high salinity or limited access to freshwater could therefore increase hatchling mortality and be an important driver of spatial variation in terrapin demography and abundance. However, when freshwater sources are available, compensatory behaviors might reduce growth-limiting effects. Terrapin recruitment is likely to be impacted as rising sea levels, increased human water use, land development, and other anthropogenic changes alter freshwater inputs to coastal marshes.

Stressed tadpoles mount more efficient glucocorticoid negative feedback in anthropogenic habitats due to phenotypic plasticity

Coping with anthropogenic environmental change is among the greatest challenges faced by wildlife, and endocrine flexibility is a potentially crucial coping mechanism. Animals may adapt to anthropogenic environments by dampening their glucocorticoid stress response, but empirical tests of this hypothesis have provided mixed evidence. An alternative hypothesis is that a non-attenuated stress response and efficient negative feedback are favored in anthropogenic habitats. To test this idea, we non-invasively sampled corticosterone release rates of common toad (Bufo bufo) tadpoles in agricultural, urban, and natural habitats, and quantified their stress response and negative feedback by a standardized stress-and-recovery protocol. We repeated the same sampling with tadpoles raised from eggs from the same ponds in a common-garden experiment to infer if the differences observed between populations in different habitats were due to individual phenotypic plasticity rather than microevolution or transgenerational effects. We found that, compared to tadpoles in natural ponds, urban tadpoles had higher baseline and stressed corticosterone release rates, and tadpoles in agricultural ponds had similar corticosterone release rates but greater stress-induced change, indicating stronger stress responses in both types of anthropogenic habitats. As predicted, tadpoles in both agricultural and urban ponds showed more efficient negative feedback than did tadpoles in natural ponds. Water pollution levels, as indicated by the concentrations of carbamazepine and corticoid-disrupting compounds in pond water, contributed to elevating the stress response regardless of land use. Infection by neither Batrachochytrium dendrobatidis nor Ranavirus was detected in free-living tadpoles. No habitat-related glucocorticoid differences persisted in the common-garden experiment. These results suggest that toad tadpoles in anthropogenic habitats increased their glucocorticoid flexibility via phenotypic plasticity. The coupling of stronger stress response and stronger negative feedback in these habitats supports the importance of rapidly "turning on and off" the stress response as a mechanism for coping with anthropogenic environmental change.

Warmer temperatures interact with salinity to weaken physiological facilitation to stress in freshwater fishes

Management of stressors requires an understanding of how multiple stressors interact, how different species respond to those interactions and the underlying mechanisms driving observed patterns in species' responses. Salinization and rising temperatures are two pertinent stressors predicted to intensify in freshwater ecosystems, posing concern for how susceptible organisms achieve and maintain homeostasis (i.e. allostasis). Here, glucocorticoid hormones (e.g. cortisol), responsible for mobilizing energy (e.g. glucose) to relevant physiological processes for the duration of stressors, are liable to vary in response to the duration and severity of salinization and temperature rises. With field and laboratory studies, we evaluated how both salinity and temperature influence basal and stress-reactive cortisol and glucose levels in age 1+ mottled sculpin (Cottus bairdii), mountain sucker (Catostomus platyrhynchus) and Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus). We found that temperature generally had the greatest effect on cortisol and glucose concentrations and the effect of salinity was often temperature dependent. We also found that when individuals were chronically exposed to higher salinities, baseline concentrations of cortisol and glucose usually declined as salinity increased. Reductions in baseline concentrations facilitated stronger stress reactivity for cortisol and glucose when exposed to additional stressors, which weakened as temperatures increased. Controlled temperatures near the species' thermal maxima became the overriding factor regulating fish physiology, resulting in inhibitory responses. With projected increases in freshwater salinization and temperatures, efforts to reduce the negative effects of increasing temperatures (i.e. increased refuge habitats and riparian cover) could moderate the inhibitory effects of temperature-dependent effects of salinization for freshwater fishes.

Relative Toxicity and Sublethal Effects of NaCl and Energy-Related Saline Wastewaters on Prairie Amphibians

Increasing salinity in freshwater environments is a growing problem due both to the negative influences of salts on ecosystems and their accumulation and persistence in environments. Two major sources of increased salinity from sodium chloride salts (NaCl) are saline wastewaters co-produced during energy production (herein, wastewaters) and road salts. Effects of road salts have received more attention, but legacy contamination from wastewaters is widespread in some regions and spills still occur. Amphibians are sensitive to contaminants, including NaCl, because of their porous skin and osmoregulatory adaptations to freshwater. However, similarities and differences between effects of wastewaters and road salts have not been investigated. Therefore, we investigated the relative influence of wastewaters and NaCl at equivalent concentrations of chloride on three larval amphibian species that occur in areas with increased salinity. We determined acute toxicity and growth effects on Boreal Chorus Frogs (Pseudacris maculata), Northern Leopard Frogs (Rana pipiens), and Barred Tiger Salamanders (Ambystoma mavortium). We posited that wastewaters would have additive effects on amphibians compared to NaCl because wastewaters often have additional toxic heavy metals and other contaminants. For NaCl, toxicity was higher for frogs than the salamander. Toxicity of wastewaters was also similar between chorus and leopard frogs. Only chorus frog survival was lower when exposed to wastewater compared to NaCl. Mass and length of leopard and chorus frog larvae decreased with increasing salinity after only 96 hours of exposure but did not for tiger salamanders. Size of leopard frogs was lower when exposed to NaCl compared to wastewater. However, growth effects were similar between wastewater and NaCl for chorus frogs. Taken together, our results suggest that previous studies on effects of road salt could inform future studies and management of wastewater-contaminated ecosystems, and vice versa. Nevertheless, effects of road salts and wastewaters may be context-, species-, and trait-specific and require further investigations. The negative influence of salts on imperiled amphibians underscores the need to restore landscapes with increased salinity and reduce future salinization of freshwater ecosystems.

Changes in physiology and microbial diversity in larval ornate chorus frogs are associated with habitat quality

Environmental change associated with anthropogenic disturbance can lower habitat quality, especially for sensitive species such as many amphibians. Variation in environmental quality may affect an organism's physiological health and, ultimately, survival and fitness. Using multiple health measures can aid in identifying populations at increased risk of declines. Our objective was to measure environmental variables at multiple spatial scales and their effect on three indicators of health in ornate chorus frog (Pseudacris ornata) tadpoles to identify potential correlates of population declines. To accomplish this, we measured a glucocorticoid hormone (corticosterone; CORT) profile associated with the stress response, as well as the skin mucosal immune function (combined function of skin secretions and skin bacterial community) and bacterial communities of tadpoles from multiple ponds. We found that water quality characteristics associated with environmental variation, including higher water temperature, conductivity and total dissolved solids, as well as percent developed land nearby, were associated with elevated CORT release rates. However, mucosal immune function, although highly variable, was not significantly associated with water quality or environmental factors. Finally, we examined skin bacterial diversity as it aids in immunity and is affected by environmental variation. We found that skin bacterial diversity differed between ponds and was affected by land cover type, canopy cover and pond proximity. Our results indicate that both local water quality and land cover characteristics are important determinants of population health for ornate chorus frogs. Moreover, using these proactive measures of health over time may aid in early identification of at-risk populations that could prevent further declines and aid in management decisions.

Effect of Individual and Combined Treatments of Pesticide, Fertilizer, and Salt on Growth and Corticosterone Levels of Larval Southern Leopard Frogs (Lithobates sphenocephala)

Human activities have introduced a variety of chemicals, including pesticides, fertilizers, and salt, into the environment, which may have deleterious effects on the organisms inhabiting these areas. Amphibians are especially susceptible to absorption of chemical pollutants. To determine the possible combined effects of these chemicals on amphibian development and stress levels, Southern leopard frog (Lithobates sphenocephala) larvae were exposed to one of eight individual or combined treatments of atrazine, ammonium nitrate fertilizer, and sodium chloride salt. Stress levels, indicated by release of the stress hormone corticosterone, were measured premetamorphosis at week 8 of development. Water hormone samples were processed to analyze corticosterone levels. Changes in tadpole growth were determined by surface area measurements taken from biweekly photographs. The combined chemical treatment of atrazine, salt, and fertilizer had a significant interactive effect by increasing stress levels before metamorphosis (p = 0.003). After a month of larval development, tadpoles exposed to ammonium nitrate had larger surface area (p = 0.035). Tadpoles exposed to atrazine had a lower growth rate throughout larval development (p = 0.025) and the lowest number of individuals reaching metamorphosis at 33%. However, the frogs in the atrazine treatment that did successfully metamorphose did so in fewer days (p = 0.002). Because amphibians are exposed to multiple chemicals simultaneously in the environment, assessing the effects of a combination of contaminants is necessary to improve application strategies and ecosystem health.

Degradation and dispersion limit environmental DNA detection of rare amphibians in wetlands: Increasing efficacy of sampling designs

The detection of rare macroorganisms using environmental DNA (eDNA) is a powerful new method for conservation and management; the efficacy of this method is affected by physiological, ecological, and hydrological processes. Understanding the processes limiting eDNA detection and accounting for those factors with optimized sampling designs is critical for realizing the potential of this tool. Amphibians are a focus of conservation programs globally and are often difficult to detect, presenting a challenge for effective action. To increase the ability of eDNA techniques to inform conservation and management programs, we investigated the eDNA detection of amphibians compared with field surveys for six species across a gradient of environmental factors expected to affect eDNA detection in three different systems: perennial wetlands, intermittent wetlands, and acidic intermittent wetlands. We applied a baseline sampling design in each wetland and used an occupancy modeling approach to evaluate evidence for processes limiting detection for each species given the presence of the target species. Evidence weights indicated that limiting processes varied across systems and included those associated with increased degradation (pH<5, temperature>25°C) and limited dispersion (wetland area>1200m², sample volume<200mL). Optimized sampling protocols based on model results included an increased number of sampling locations in large and highly degradative (acidic) wetlands and increased filter pore size in high-particulate systems. These improved designs compensated for the previously limiting factors and yielded average detection rates of 0.62-0.86 per water sample. Degradation and dispersion processes appear to strongly influence the detection of amphibians in wetlands. Optimized, adaptive sampling designs can greatly increase the efficacy of eDNA monitoring approaches.

Salinisation of rivers: an urgent ecological issue

Secondary salinisation of rivers and streams is a global and growing threat that might be amplified by climate change. It can have many different causes, like irrigation, mining activity or the use of salts as de-icing agents for roads. Freshwater organisms only tolerate certain ranges of water salinity. Therefore secondary salinisation has an impact at the individual, population, community and ecosystem levels, which ultimately leads to a reduction in aquatic biodiversity and compromises the goods and services that rivers and streams provide. Management of secondary salinization should be directed towards integrated catchment strategies (e.g. benefiting from the dilution capacity of the rivers) and identifying threshold salt concentrations to preserve the ecosystem integrity. Future research on the interaction of salinity with other stressors and the impact of salinization on trophic interactions and ecosystem properties is needed and the implications of this issue for human society need to be seriously considered.

Potential for local adaptation in response to an anthropogenic agent of selection: effects of road deicing salts on amphibian embryonic survival and development

The application of millions of tons of road deicing salts every winter in North America presents significant survival challenges to amphibians inhabiting roadside habitats. While much is known of the effects of NaCl on anuran tadpoles, less is known of effects on amphibian eggs, or any caudate life stage. In addition, little is known of the effects of MgCl₂, which is now the 2nd most commonly used road deicer. Most studies have considered amphibians to be helpless victims of deicing salts, and ignore the possibility of the evolution of local adaptation to this stressor. We attempt to address these knowledge gaps and explore this evolutionary potential by examining the effects of NaCl and MgCl₂ on the survival and development of eggs from different female rough-skinned newts (Taricha granulosa) from the same population. We demonstrate that both salts, at environmentally relevant concentrations, severely affect the embryonic survival and development of this amphibian, but that the effects of the salt are dependent on the identity of the mother. This female × treatment interaction results in substantial variation in tolerance to road deicing salts among newt families, providing the raw material necessary for natural selection and the evolution of local adaptation in this amphibian.