Effects of Maternally-Transferred Methylmercury on Stress Physiology in Northern Water Snake (Nerodia sipedon) Neonates

Abstract

Biomagnification of methylmercury in aquatic systems can cause elevated tissue mercury (Hg) and physiological stress in top predators. Mercury is known to affect stress hormone levels in mammals, birds and fish. In this study, the effects of maternally-transferred methylmercury on the stress physiology of Northern Water Snake (Nerodia sipedon) neonates were tested. Gravid females were dosed via force-fed capsules during late gestation with 0, 0.01, or 10 µg methylmercury per gram of body mass. Plasma corticosterone levels and leukocyte differentials were analyzed in baseline and confinement-stressed neonates from all dose levels. Neither Hg nor confinement stress had a significant effect on leukocyte differentials nor was Hg related to corticosterone levels. However, stress group neonates showed lower heterophil/lymphocyte ratios and this study was the first to show that neonate N. sipedon can upregulate CORT in response to stress. These results indicate that N. sipedon may be somewhat tolerant to Hg contamination.

Placentation in watersnakes I: Placental histology and development in North American Nerodia (Colubridae: Natricinae)

“…have demonstrated placental transfer of amino acids, sodium, potassium, water, and iodine (Conaway & Fleming, ; Stewart & Castillo, ; Van Dyke & Beaupre, ). In addition, recent analytical studies on this species have demonstrated maternal‐fetal transfer of environmental pollutants (mercury, PCBs, and pesticide chemicals) during pregnancy (Bishop & Rouse, ; Chin et al, ,2013b; Cusaac et al, ). The specific placenta that transfers such substances has not been established experimentally.…”

“…However, what little has been reported about the morphological basis for such transport (Conaway & Fleming, ) is not consistent with information on other viviparous snakes (Blackburn, ). Placentation in N. sipedon is of particular interest given ongoing use of this species to monitor maternal‐fetal transfer of environmental contaminants during pregnancy (Bishop & Rouse, ; Chin, Willson, Cristol, Drewett, & Hopkins, ; Cusaac, Kremer, Wright, Henry, Otter, & Bailey, ; Neuman‐Lee, Gaines, Baumgartner, Voorhees, Novak, & Mullin, ). Likewise, information on placentation in Nerodia contributes to a long‐term effort to reconstruct placental evolution in North American thamnophine snakes, a group that has become a model for understanding squamate placentation (Blackburn & Stewart, ; Stewart & Thompson, ).…”

The use of glucocorticoid hormones or leucocyte profiles to measure stress in vertebrates: What’s the difference?

“…McMahon, Boughton, Martin, & Rohr, 2017), aquatic reptiles (e.g. Camacho et al, 2013;Cusaac et al, 2016) and aquatic birds (e.g. Nisbet, Tseng, Fiorello, & Apanius, 2015).…”

Placentation in watersnakes II: Placental ultrastructure in Nerodia erythrogaster (Colubridae: Natricinae)

“…Recent studies on various Nerodia have demonstrated that environmental contaminants can cross the placenta from maternal to fetal tissues during pregnancy (Bishop & Rouse, ; Chin et al, ; Cusaac et al, ). Electron microscopy reveals the chorioallantoic placenta to be a likely route for maternal‐fetal transfer, given its extreme vascularity and the very thin barriers that lie between its uterine and fetal blood vessels…”

“…Past research on various Nerodia has shown that whereas most nutrients for development come from the ovulated yolk, placental membranes are responsible for gas exchange as well as maternal‐fetal transfer of organic and inorganic nutrients (Conaway & Fleming, ; Stewart & Castillo, ; Van Dyke & Beaupre, ). Other work has indirectly implicated the placental membranes in maternal‐fetal transfer of environmental contaminants (Bishop & Rouse, ; Chin, Willson, Cristol, Drewett, & Hopkins, ; Cusaac et al, ). Our study tests the hypothesis that fine structure of the placental membranes reflects specific functions, while offering detailed information about placental composition, development, and evolution.…”