Determinants of mercury contamination in viperine snakes, Natrix maura, in Western Europe

Mercury bioaccumulation and Hepatozoon spp. infections in two syntopic watersnakes in South Carolina

Mercury (Hg) is a ubiquitous environmental contaminant known to bioaccumulate in biota and biomagnify in food webs. Parasites occur in nearly every ecosystem and often interact in complex ways with other stressors that their hosts experience. Hepatozoon spp. are intraerythrocytic parasites common in snakes. The Florida green watersnake (Nerodia floridana) and the banded watersnake (Nerodia fasciata) occur syntopically in certain aquatic habitats in the Southeastern United States. The purpose of this study was to investigate relationships among total mercury (THg) concentrations, body size, species, habitat type and prevalence and parasitemia of Hepatozoon spp. infections in snakes. In the present study, we sampled N. floridana and N. fasciata from former nuclear cooling reservoirs and isolated wetlands of the Savannah River Site in South Carolina. We used snake tail clips to quantify THg and collected blood samples for hemoparasite counts. Our results indicate a significant, positive relationship between THg and snake body size in N. floridana and N. fasciata in both habitats. Average THg was significantly higher for N. fasciata compared to N. floridana in bays (0.22 ± 0.02 and 0.08 ± 0.006 mg/kg, respectively; p < 0.01), but not in reservoirs (0.17 ± 0.02 and 0.17 ± 0.03 mg/kg, respectively; p = 0.29). Sex did not appear to be related to THg concentration or Hepatozoon spp. infections in either species. We found no association between Hg and Hepatozoon spp. prevalence or parasitemia; however, our results suggest that species and habitat type play a role in susceptibility to Hepatozoon spp. infection.

Using Crocodylians for monitoring mercury in the tropics

Mercury contamination is a widespread phenomenon that impacts ecosystems worldwide. Artisanal Small Scale Gold Mining (ASGM) activities are responsible for more than a third of atmospheric Hg emission. Due to Hg toxicity and its broad and elevated prevalence in the environment resulting from ASGM activities in the tropics, its biomonitoring is essential to better understand the availability of its methylmercury (MeHg) form in the environment. The Minamata Convention was ratified with the objective to "protect human health and the environment from anthropogenic emissions and releases of mercury compounds". Biomagnification of MeHg occurs through the trophic food web, where it biomagnifies and bioaccumulates in top predators. To monitor environmental MeHg contamination, studies have evaluated the use of living organisms; however, reptiles are among the least documented vertebrates regarding MeHg exposure. In this review we evaluate the use of crocodylians for Hg biomonitoring in tropical ecosystems. We found that out of the 28 crocodiles species, only 10 have been evaluated regarding Hg contamination. The remaining challenges when using this taxon for Hg biomonitoring are inconsistencies in the applied methodology (e.g., wet versus dry weight, tissues used, quantification method). However, due to their life history traits, crocodylians are particularly relevant for monitoring MeHg contamination in regions where ASGM activities occur. In conclusion and given their ecological and socio-economic importance, crocodylians are at great risk of MeHg contamination and are excellent bioindicators for tropical ecosystems.

Heavy Metal Concentration in Neotropical Aquatic Snakes (Helicops pastazae) and Its Potential as a Bioindicator of Water Pollution

The purpose of this study was to test the potential role of the aquatic snake Helicops pastazae as an indicator of water pollution caused by heavy metals. In particular, we tested whether the total heavy metal concentration is related to (1) the position (upstream vs downstream) of the sampling point and its distance from the point where wastewater is discharged; (2) the taxonomic group studied: piscivorous snakes vs characid fish that occupy the same habitats; and (3) the organ or tissue examined: snake liver versus muscle. We used atomic absorption spectrophotometry with electrothermal atomization to quantify cadmium (Cd), chromium (Cr) and lead (Pb) and found significant differences between some of the sampling points, with particularly high metal concentrations detected upstream at point 1. However, we found no clear spatial pattern nor any significant differences in the concentration of any of the metals in fish and snake muscle, suggesting that both species accumulate similar amounts of the sampled elements. With regard to interactions, snake liver had the highest concentrations of Cd, while muscle had the highest concentrations of Pb and Cr, which may indicate tissue affinity differences for certain metals. Altogether, our results indicate that H. pastazae accumulates contaminants differentially, depending on the tissue and location, which highlights their potential as bioindicators of water contamination. Further research is necessary to understand their role as bioindicators based on extensive sampling and environmental contaminant data.

Mercury and Radiocesium Accumulation and Associations With Sublethal Endpoints in the Florida Green Watersnake (Nerodia floridana)

Mercury (Hg) and radiocesium (¹³⁷ Cs) are well-known environmental contaminants with the potential to impact the health of humans and wildlife. Snakes have several characteristics conducive to studying environmental contamination but have rarely been included in the monitoring of polluted sites. We investigated the bioaccumulation of Hg and ¹³⁷ Cs and associations with sublethal effects (standard metabolic rate [SMR] and hemoparasite infections) in Florida green watersnakes (Nerodia floridana). We captured 78 snakes from three former nuclear cooling reservoirs on the US Department of Energy's Savannah River Site in South Carolina (USA). For captured snakes, we (1) determined whole-body ¹³⁷ Cs, (2) quantified total Hg (THg) using snake tail clips, (3) conducted hemoparasite counts, and (4) measured the SMR. We used multiple regression models to determine associations among snake body size, capture location, sex, tail THg, whole-body ¹³⁷ Cs, Hepatozoon spp. prevalence and parasitemia, and SMR. Average whole-body ¹³⁷ Cs (0.23 ± 0.08 Becquerels [Bq]/g; range: 0.00-1.02 Bq/g) was correlated with snake body size and differed significantly by capture site (Pond B: 0.67 ± 0.05 Bq/g; Par Pond: 0.10 ± 0.02 Bq/g; Pond 2: 0.03 ± 0.02 Bq/g). Tail THg (0.33 ± 0.03 mg/kg dry wt; range: 0.16-2.10 mg/kg) was significantly correlated with snake body size but did not differ by capture site. We found no clear relationship between SMR and contaminant burdens. However, models indicated that the prevalence of Hepatozoon spp. in snakes was inversely related to increasing whole-body ¹³⁷ Cs burdens. Our results indicate the bioaccumulation of Hg and ¹³⁷ Cs in N. floridana and further demonstrate the utility of aquatic snakes as bioindicators. Our results also suggest a decrease in Hepatozoon spp. prevalence related to increased burdens of ¹³⁷ Cs. Although the results are intriguing, further research is needed to understand the dynamics between ¹³⁷ Cs and Hepatozoon spp. infections in semiaquatic snakes. Environ Toxicol Chem 2022;41:758-770. © 2022 SETAC.

Variation of Total Mercury Concentrations in Different Tissues of Three Neotropical Caimans: Implications for Minimally Invasive Biomonitoring

Mercury (Hg) is a global environmental contaminant that affects ecosystems. It is known to biomagnify through food webs and to bioaccumulate especially in the tissues of top predators. Large-scale comparisons between taxa and geographic areas are needed to reveal critical trends related to Hg contamination and its deleterious effects on wildlife. Yet, the large variety of tissues (keratinized tissues, internal organs, blood) as well as the variability in the units used to express Hg concentrations (either in wet- or dry-tissue weight) limits straightforward comparisons between studies. In the present study, we assessed the moisture content that could influence the total Hg (THg) concentrations measured in several tissues (claws, scutes, total blood, and red blood cells) of three caiman species. We evaluated the moisture content from the different tissues to provide information on THg concentrations in various matrices. Our results show a difference of THg concentrations between the tissues and intra- and interspecific variations of moisture content, with the highest THg values found in keratinized tissues (scute keratinized layers and claws). For the three species, we found positive relationships between body size and THg concentration in keratinized tissues. In the blood, the relationship between body size and THg concentration was species-dependent. Our results emphasize the need for a standardized evaluation of THg concentration and trace elements quantification based on dry weight analytical procedures. In addition, the use of both blood and keratinized tissues offers the possibility to quantify different time scales of THg exposure by non-lethal sampling.

Multi-decadal trends in mercury and methylmercury concentrations in the brown watersnake (Nerodia taxispilota)

Mercury (Hg) is an environmental contaminant that poses a threat to aquatic systems globally. Temporal evaluations of Hg contamination have increased in recent years, with studies focusing on how anthropogenic activities impact Hg bioavailability in a variety of aquatic systems. While it is common for these studies and ecological risk assessments to evaluate Hg bioaccumulation and effects in wildlife, there is a paucity of information regarding Hg dynamics in reptiles. The goal of this study was to investigate temporal patterns in total mercury (THg) and methylmercury (MeHg) concentrations across a 36-year period, as well as evaluate relationships among and between destructive (kidney, liver, muscle) and non-destructive (blood, tail) tissue types in a common watersnake species. To accomplish this, we measured THg and MeHg concentrations in multiple tissues from brown watersnakes (Nerodia taxispilota) collected from Steel Creek on the Savannah River Site (SRS; Aiken, SC, USA) from two time periods (1983-1986 and 2019). We found significant and positive relationships between tail tips and destructive tissues. In both time periods, THg concentrations varied significantly by tissue type, and destructive tissues exhibited higher but predictable THg values relative to tail tissue. Methylmercury concentrations did not differ among tissues from the 1980s but was significantly higher in muscle compared to other tissues from snakes collected in 2019. Percent MeHg of THg in N. taxispilota tissues mirrored patterns reported in other reptiles, although the range of % MeHg in liver and kidney differed between time periods. Both THg and MeHg concentrations in N. taxispilota declined significantly from the 1980s to 2019, with average values 1.6 to 4-fold lower in contemporary samples. Overall, our data add further evidence to the utility of watersnakes to monitor Hg pollution in aquatic environments and suggest attenuation of this contaminant in watersnakes in our study system.

Brown watersnakes (Nerodia taxispilota) as bioindicators of mercury contamination in a riverine system

Mercury (Hg) is a contaminant that enters the environment through natural or anthropogenic means. Ecological risk assessments have examined Hg bioaccumulation and effects in many taxa, but little is known about Hg dynamics in reptiles, or their potential use as bioindicator species for monitoring Hg in aquatic systems. Numerous snake species, like North American watersnakes (Nerodia spp.), are piscivorous and are exposed to Hg through their diet. The purpose of this study was to identify factors associated with Hg accumulation in a common watersnake species and compare Hg concentrations of the snakes to those in fish occupying the same habitats. To this end, we sampled brown watersnakes (Nerodia taxispilota) from the Savannah River, a major river system in the southeastern U.S., and compared N. taxispilota Hg accumulation trends to those of bass (Micropterus salmoides), catfish (Ictalurus and Ameiurus spp.), and panfish (Lepomis and Pomoxis spp.) collected from the same reach. Total Hg (THg) in N. taxispilota tail tips ranged from 0.020 to 0.431 mg/kg (wet weight; mean: 0.104 ± 0.008). Snake tail THg was significantly correlated with blood THg, which ranged from 0.003 to 1.140 mg/kg (0.154 ± 0.019). Snake size and site of capture were significantly associated with tail THg. Snake tail THg increased at sites along and downstream of the area of historic Hg pollution, consistent with fish THg. Snake muscle THg was predicted based on tail THg and ranged from 0.095 to 1.160 (0.352 ± 0.022). To gauge Hg biomagnification in N. taxispilota, we compared predicted snake muscle THg concentrations to THg in fish of consumable size. Average biomagnification factors for THg in N. taxispilota were 3.1 (panfish) and 5.4 (catfish), demonstrating N. taxispilota likely biomagnify Hg through their diet. These results reveal N. taxispilota to be an effective bioindicator species for monitoring Hg in aquatic environments.

Contamination of habu (Protobothrops flavoviridis) in Okinawa, Japan by persistent organochlorine chemicals

Persistent organochlorine chemicals (OCs), including chlordane compounds (CHLs), DDTs, PCBs, and chlorinated dioxins and related compounds (DRCs), were examined in the adipose tissue and liver from 33 specimens of habu (Protobothrops flavoviridis), a species of venomous pit viper endemic to the Japanese Southwest Islands. The median concentrations of CHLs, DDTs, and PCBs in adipose tissue of 22 habus collected from an urban area were 4400 ng g^-1 lipid weight (lw), 610 ng g^-1 lw, and 1600 ng g^-1 lw, respectively. Their DDT and PCB concentrations were higher in comparison with the specimens from a rural area. Liver of 10 specimens from the urban area were subjected to DRCs analysis, and PCDDs, PCDFs, and DL-PCBs were detected with median values of 1300, 350, and 150,000 pg g^-1 lw, respectively. Among PCDD/F congeners, octa-CDD was detected at the highest concentrations in seven liver samples, but considerable concentrations of penta- and hexa-CDD/Fs were found in two samples. Relatively higher concentrations of PCB, DDTs, and PCDD/Fs were found in habus collected within 1 km of the boundary of military facilities, suggesting that OCs from some unknown sources of these OCs inside and/or around some of the facilities accumulated in habus.

Influence of sex, size and trophic level on blood Hg concentrations in Black caiman, Melanosuchus niger (Spix, 1825) in French Guiana

Mercury (Hg) is a contaminant that is impacting ecosystems worldwide. Its toxicity is threatening wildlife and human populations, leading to the necessity of identifying the most affected ecosystems. Therefore, it is essential to identify pertinent bioindicator organisms to monitor Hg contamination. In this study, we determined the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios in the red blood cells (RBCs), and the total Hg concentration in total blood of 72 Melanosuchus niger in French Guiana. The goals of our study were to assess the level of Hg contamination in total blood of Black caimans and to further investigate the influence of individual traits (i.e., sex, size/age, diet) on Hg concentrations. Mercury concentration in total blood of Black caimans ranged from 0.572 to 3.408 μg g^-1 dw (mean ± SD is 1.284 ± 0.672 μg g^-1 dw) and was positively correlated to individual body size and trophic position (δ¹⁵N). We did not find any sexual or seasonal effects on Hg concentrations in the blood. The use of blood of M. niger is relevant to determine Hg concentrations within the population and suggests that this species can be used as a bioindicator for environmental contamination. In addition, our results emphasize trophic position as a major source of Hg variation and further suggest that it is essential to take trophic position (δ¹⁵N) into account for future studies.

The Broad-Scale Analysis of Metals, Trace Elements, Organochlorine Pesticides and Polycyclic Aromatic Hydrocarbons in Wetlands Along an Urban Gradient, and the Use of a High Trophic Snake as a Bioindicator

Wetlands and their biodiversity are constantly threatened by contaminant pollution from urbanisation. Despite evidence suggesting that snakes are good bioindicators of environmental health, the bioaccumulation of contaminants in reptiles is poorly researched in Australia. We conducted the first broad-scale analysis of 17 metals and trace elements, 21 organochlorine pesticides, and 14 polycyclic aromatic hydrocarbons in the sediments (4 samples per site, December 2018) from four wetlands along an urban gradient in Perth, Western Australia, and from the livers (5 livers per site, February-April 2019) of western tiger snakes Notechis scutatus occidentalis captured at those sites. All 17 metals and trace elements were detected in the sediments of wetlands as well as 16 in the livers of tiger snakes. Arsenic, Cu, Hg, Pb, Se, and Zn were at concentrations exceeding government trigger values in at least one sediment sample. Two organochlorine pesticides and six of seven polycyclic aromatic hydrocarbons were detected in the sediments of a single wetland, all exceeding government trigger values, but were not detected in tiger snakes. Metals and trace elements were generally in higher concentration in sediments and snake livers from more heavily urbanised wetlands. The least urbanised site had some higher concentrations of metals and trace elements, possibly due to agriculture contaminated groundwater. Concentrations of nine metals and trace elements in snake livers were statistically different between sites. Arsenic, Cd, Co, Hg, Mo, Sb, and Se near paralleled the pattern of contamination measured in the wetland sediments; this supports the use of high trophic wetland snakes, such as tiger snakes, as bioindicators of wetland contamination. Contamination sources and impacts on these wetland ecosystems and tiger snakes are discussed herein.

Methylmercury exposure in wildlife: A review of the ecological and physiological processes affecting contaminant concentrations and their interpretation

Exposure to methylmercury (MeHg) can result in detrimental health effects in wildlife. With advances in ecological indicators and analytical techniques for measurement of MeHg in a variety of tissues, numerous processes have been identified that can influence MeHg concentrations in wildlife. This review presents a synthesis of theoretical principals and applied information for measuring MeHg exposure and interpreting MeHg concentrations in wildlife. Mercury concentrations in wildlife are the net result of ecological processes influencing dietary exposure combined with physiological processes that regulate assimilation, transformation, and elimination. Therefore, consideration of both physiological and ecological processes should be integrated when formulating biomonitoring strategies. Ecological indicators, particularly stable isotopes of carbon, nitrogen, and sulfur, compound-specific stable isotopes, and fatty acids, can be effective tools to evaluate dietary MeHg exposure. Animal species differ in their physiological capacity for MeHg elimination, and animal tissues can be inert or physiologically active, act as sites of storage, transformation, or excretion of MeHg, and vary in the timing of MeHg exposure they represent. Biological influences such as age, sex, maternal transfer, and growth or fasting are also relevant for interpretation of tissue MeHg concentrations. Wildlife tissues that represent current or near-term bioaccumulation and in which MeHg is the predominant mercury species (such as blood and eggs) are most effective for biomonitoring ecosystems and understanding landscape drivers of MeHg exposure. Further research is suggested to critically evaluate the use of keratinized external tissues to measure MeHg bioaccumulation, particularly for less-well studied wildlife such as reptiles and terrestrial mammals. Suggested methods are provided to effectively use wildlife for quantifying patterns and drivers of MeHg bioaccumulation over time and space, as well as for assessing the potential risk and toxicological effects of MeHg on wildlife.

Short- and medium-chain chlorinated paraffins in aquatic organisms from an e-waste site: Biomagnification and maternal transfer

Chlorinated paraffins (CPs) are globally pervasive contaminants that are toxic to humans and wildlife. Inconsistent biomagnification behaviors in different food chains have been reported, and very few studies have been conducted to investigate the maternal transfer of CPs in ovoviviparous species. This study investigated the biomagnification of short- and medium-chain chlorinated paraffins (S/MCCPs) in two aquatic food chains, as well as maternal transfer of S/MCCPs in watersnakes collected from an e-waste polluted pond in southern China. The concentrations of SCCPs and MCCPs varied from 1.2 to 250 μg/g lipid weight (lw) and from 2.3 to 200 μg/g lw in the collected organisms. The SCCP homologue profiles in prey (fish and prawn) differed from those in predators (watersnake and waterbird egg), while MCCP homologue group patterns were homogeneous. All maternal transfer concentration ratios (egg to muscle) of S/MCCPs in the watersnakes were lower than 1 and negatively correlated with the octanol-water partition coefficients (log K_OW), different from the maternal transfer of halogenated aromatic pollutants in the watersnake. Biomagnification factors (BMFs) of S/MCCPs for fish-watersnake muscle food chain were larger than 1, while BMFs for the fish-waterbird egg food chain were less than 1. However, when watersnake egg was used to calculate BMF, no biomagnification was found. BMFs in the two food chains showed significant positive linear correlations with chlorine atoms, but no significant correlation with carbon atom numbers, which suggested that a congener-group-specific elimination and excretion process for S/MCCPs exist.

Snakes as Novel Biomarkers of Mercury Contamination: A Review

Mercury (Hg) is an environmental contaminant that has been reported in many wildlife species worldwide. The organic form of Hg bioaccumulates in higher trophic levels, and thus, long-lived predators are at risk for higher Hg exposure. Although ecological risk assessments for contaminants such as Hg include pertinent receptor species, snakes are rarely considered, despite their high trophic status and potential to accumulate high levels of Hg. Our current knowledge of these reptiles suggests that snakes may be useful novel biomarkers to monitor contaminated environments. The few available studies show that snakes can bioaccumulate significant amounts of Hg. However, little is known about the role of snakes in Hg transport in the environment or the individual-level effects of Hg exposure in this group of reptiles. This is a major concern, as snakes often serve as important prey for a variety of taxa within ecosystems (including humans). In this review, we compiled and analyzed the results of over 30 studies to discuss the impact of Hg on snakes, specifically sources of exposure, bioaccumulation, health consequences, and specific scientific knowledge gaps regarding these moderate to high trophic predators.

Do different diets affect oxidative stress biomarkers and metal bioaccumulation in two snake species?

In this study we examined possible differences in heavy metal accumulation and oxidative stress parameters in the liver and muscle of two semi-aquatic snakes: grass snake (Natrix natrix) and dice snake (N. tessellata), that inhabit the same environment but differ in prey diversity. The obtained results revealed some interspecies, inter-tissue, prey-snake and prey-prey differences in heavy metal concentrations. Grass snakes pray contained significantly higher concentrations of Al, Cr and Fe as compared to food of dice snakes. Both investigated snakes accumulated generally lower concentrations of metals than their prey, indicating that they are not at risk of contaminant biomagnification. A significant interspecies difference in accumulation was observed only for Cu and Mn concentrations. On the other hand, analysis of oxidative stress biomarkers showed clear differences between the investigated snake species and the two investigated tissues. The liver of grass snake had increased superoxide dismutase, glutathione reductase and glutathione-S-transferase activities in comparison to dice snake. In muscle, a reverse trend was observed for the activities of these three enzymes, as well as for glutathione peroxidase activity. The higher number of significant correlations observed between oxidative stress biomarkers and heavy metal concentrations in grass snake points to upregulation of the antioxidative system (AOS), which resulted in a lower TBARS concentration. Results show that while the investigated snake species did not differ significantly in the accumulated metals, their defense mechanisms were different. This reveals the complexity of the AOS and points to the cooperation of different AOS components in individuals from natural populations.

Environmental causes and reproductive correlates of mercury contamination in European pond turtles (Emys orbicularis)

Assessing Hg contamination in aquatic ecosystems is difficult because wetlands are part of large and complex networks, and potential sources of Hg contamination are highly diverse. To investigate environmental determinants of Hg contamination, we studied one of the largest continental French wetlands structured as a dense network of artificial ponds. Such context allows to investigate the influence of pond characteristics on Hg contamination in an area relatively disconnected from direct sources of pollution. We relied on a bioindicator organism, the European pond turtle (Emys orbicularis) to assess Hg contamination in a relatively large number of sites (N > 255 turtles from 15 ponds sampled in 2016 and 2017). Non-invasive sampling in the claws of turtles show that Hg concentrations were not related to their sex or size, but we found an effect of age (1.62 ± 0.20 in juveniles and 2.21 ± 0.06 µg g^-1 dw in adults), suggesting that turtles do bioaccumulate Hg through their life. Turtle Hg was different between ponds, and we found that pond age and pond usage (draining events linked to pond maintenance) were the main environmental determinants of Hg concentrations in turtles. Finally, and more importantly, our dataset allowed us to highlight potential negative effect of Hg concentrations on the proportion of reproductive females, suggesting an influence of Hg on reproductive mechanisms in this species. This result indicates that Hg contamination, even in absence of direct and strong sources of pollution, may have a critical impact on reproduction and thus the persistence of a long-lived vertebrate.

Town and Country Reptiles: A Review of Reptilian Responses to Urbanization

The majority of the world population is now inhabiting urban areas, and with staggering population growth, urbanization is also increasing. While the work studying the effects of changing landscapes and specific urban pressures on wildlife is beginning to amass, the majority of this work focuses on avian or mammalian species. However, the effects of urbanization likely vary substantially across taxonomic groups due to differences in habitat requirements and life history. The current article aims first to broaden the review of urban effects across reptilian species; second, to summarize the responses of reptilian fauna to specific urban features; and third, to assess the directionality of individual and population level responses to urbanization in reptile species. Based on our findings, urban research in reptilian taxa is lacking in the following areas: (1) investigating interactive or additive urban factors, (2) measuring multiple morphological, behavioral, and physiological endpoints within an animal, (3) linking individual to population-level responses, and (4) testing genetic/genomic differences across an urban environment as evidence for selective pressures.

Trace Element Concentrations in European Pond Turtles (Emys orbicularis) from Brenne Natural Park, France

We assessed trace elements concentration in European pond turtle (Emys orbicularis) from Brenne Natural Park (France). We sampled road-killed turtles (N = 46) to measure the concentrations of 4 non-essential (Ag, Cd, Hg, and Pb) and 10 essential (As, Co, Cr, Cu, Fe, Mn, Ni, Se, V, and Zn) elements in muscle, skin, liver and claws. Body size or sex did not influence the concentrations of most elements; except for Hg (liver, skin and claws) and Zn (muscle) which increased with body size. We found relatively high concentrations of Hg and Zn, possibly linked to fish farming. This result deserves future investigations to evaluate possible ecotoxicological effects on E. orbicularis.