Inter‐ and Intraspecific Variation in Mercury Bioaccumulation by Snakes Inhabiting a Contaminated River Floodplain

Abstract

Although mercury (Hg) is a well-studied contaminant, knowledge about Hg accumulation in snakes is limited. The authors evaluated Hg bioaccumulation within and among four snake species (northern watersnakes, Nerodia sipedon; queen snakes, Regina septemvittata; common garter snakes, Thamnophis sirtalis; and rat snakes, Elaphe obsoleta [Pantherophis alleghaniensis]) from a contaminated site on the South River (Waynesboro, VA, USA) and two nearby reference sites. Total Hg (THg) concentrations in northern watersnake tail tissue at the contaminated site ranged from 2.25 to 13.84 mg/kg dry weight (mean: 4.85 ± 0.29), or 11 to 19 times higher than reference sites. Blood THg concentrations (0.03-7.04 mg/kg wet wt; mean: 2.24 ± 0.42) were strongly correlated with tail concentrations and were the highest yet reported in a snake species. Within watersnakes, nitrogen stable isotope values indicated ontogenetic trophic shifts that correlated with THg bioaccumulation, suggesting that diet plays a substantial role in Hg exposure. Female watersnakes had higher mean THg concentrations (5.67 ± 0.46 mg/kg) than males (4.93 ± 0.49 mg/kg), but no significant differences between sexes were observed after correcting for body size. Interspecific comparisons identified differences in THg concentrations among snake species, with more aquatic species (watersnakes and queen snakes) accumulating higher mean concentrations (5.60 ± 0.40 and 4.59 ± 0.38 mg/kg in tail tissue, respectively) than the more terrestrial species, garter snakes and rat snakes (1.28 ± 0.32 and 0.26 ± 0.09 mg/kg, respectively). The results of the present study warrant further investigation of potential adverse effects and will aid in prioritizing conservation efforts.

High levels of maternally transferred mercury do not affect reproductive output or embryonic survival of northern watersnakes (Nerodia sipedon)

“…We found that female N. sipedon transferred up to 1.24 mg of THg to their offspring, but our desire to avoid lethal sampling of adults prohibited us from calculating the percentage of body burden that this amount represents. Field sampling of adult N. sipedon at our site has revealed that similarly sized males and females do not differ significantly in THg concentrations of tail tissue [14]. Thus, although females transfer a relatively large absolute amount of Hg to their offspring, this appears to be a relatively small proportion of their overall body burden.…”

“…A recent study suggests that the primary reason for higher litter THg concentrations in N. sipedon is that they have higher exposure to Hg as indicated by high maternal blood THg concentrations when compared to other species [14]. Studies of other vertebrate species at the South River report mean %MeHg values up to 91% as well as positive relationships between THg concentrations and %MeHg in eggs [4,27,28].…”

“…Gravid female N. sipedon (n ¼ 31) were captured by hand at sites along the Middle and South Rivers between June 14 and July 31, 2011 (see Drewett et al [14] for map). Nine females were captured from reference locations; the remaining 22 females were collected along the contamination gradient (river miles 1-22 downstream of the Hg source) at the South River.…”

“…In addition, aquatic snake species may be particularly at risk of Hg bioaccumulation. A recent study found that northern watersnakes (Nerodia sipedon) from an Hg-contaminated river had among the highest mean Hg concentrations among vertebrates inhabiting the site [14]. Within N. sipedon, large females had the highest overall Hg concentrations [14].…”

“…A recent study found that northern watersnakes (Nerodia sipedon) from an Hg-contaminated river had among the highest mean Hg concentrations among vertebrates inhabiting the site [14]. Within N. sipedon, large females had the highest overall Hg concentrations [14]. Presumably, females that accumulate high Hg concentrations subsequently transfer high concentrations to their offspring.…”