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

Differential sensitivity of aquatic life stages of Pelophylax perezi to an acidic metal-contaminated effluent

Acid mine drainage (AMD) involves complex mixtures of metals and hydrogen ions that can be highly toxic to the biota. Assessing the effects of AMD to aquatic stages of amphibians is key, as this group constitutes the vertebrate class with the highest proportion of species considered as threatened. Thus, the present work aimed at assessing the sensitivity of two aquatic life stages of the green frog Pelophylax perezi to an AMD originated from a cupric-pyrite mine. Embryos (Gosner stages 10-11) and tadpoles (Gosner stages 20-21) of P. perezi were exposed, for 96 h, to six AMD dilutions (1.39% to 7.5%). Endpoints involving responses at different levels of biological organization were monitored: mortality, malformations, hatching rates, body length and growth rate, enzymatic activity related with detoxification metabolism (glutathione S-transferase), and histopathologies (anatomical structures of the digestive, respiratory, and excretory systems). Embryos presented high mortality and malformation rates at AMD levels equal or above 5.36%, as well as premature hatching at 1.95% of AMD or higher. A significant reduction in body length and growth rate occurred in embryos and tadpoles exposed to 1.95% or higher levels of AMD, respectively. At the histological level, several abnormalities were observed for AMD-exposed tadpoles in a variety of tissues. One of the most noticeable histological changes occurred in the intestine that exhibited papillary epithelial hyperplasia along with a yellowish content and was more pronounced in tadpoles exposed to higher AMD levels. FEmbryos were more sensitive to lethal levels of AMD than tadpoles, suggesting embryos as a useful model life stage when performing amphibian risk assessment of mine drainage. Furthermore, AMD was highly toxic for P. perezi aquatic life stages since levels as low as 1.95% induced lethal effects. These results emphasize the importance of implementing efficient remediation methodologies for AMD, given its high toxicity.

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.

Acute and chronic sensitivity to copper of a promising ecotoxicological model species, the annual killifish Nothobranchius furzeri

Nothobranchius furzeri is a promising model for ecotoxicological research due to the species' short life cycle and the production of drought-resistant eggs. Although the species is an emerging vertebrate fish model for several fundamental as well as applied research domains, its potential for ecotoxicological research has not yet been tested. The aim of this study was to characterise the acute and chronic sensitivity of this species to copper as compared to other model organisms. Effects of both acute and chronic copper exposure were tested on survival, life history and physiological traits. We report a 24h-LC₅₀ of 53.93µg Cu/L, which is situated within the sensitivity range of other model species such as Brook Trout, Fathead Minnow and Rainbow Trout. Moreover, in the full life cycle exposure, we show that an exposure concentration of 10.27µg/L did not cause acute adverse effects (96h), but did cause mortality after prolonged exposure (3-4 weeks). Also chronic, sublethal effects were observed, such as a reduction in growth rate, delayed maturation and postponed reproduction. Based on our results, we define the NOEC at 6.68µg Cu/L, making N. furzeri more sensitive to copper as compared to Brook Trout and Fathead Minnow. We found stimulatory effects on peak fecundity at subinhibitory levels of copper concentrations (hormesis). Finally, we found indications for detoxifying and copper-excreting mechanisms, demonstrating the ability of the fish to cope with this essential metal, even when exposed to stressful amounts. The successful application of current ecotoxicological protocols on N. furzeri and its sensitivity range comparable to that of other model organisms forms the basis to exploit this species in further ecotoxicological practices.