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

Abstract

Abstract-Maternal transfer is an important exposure pathway for contaminants because it can directly influence offspring development. Few studies have examined maternal transfer of contaminants, such as mercury (Hg), in snakes, despite their abundance and high trophic position in many ecosystems where Hg is prevalent. The objectives of the present study were to determine if Hg is maternally transferred in northern watersnakes (Nerodia sipedon) and to evaluate the effects of maternal Hg on reproduction. The authors captured gravid female watersnakes (n ¼ 31) along the South River in Waynesboro, Virginia, USA, where an extensive Hg-contamination gradient exists. The authors measured maternal tissue and litter Hg concentrations and, following birth, assessed (1) reproductive parameters (i.e., litter size and mass, neonate mass); (2) rates of infertility, death during development, stillbirths, malformations, and runts; and (3) the overall viability of offspring. Mercury concentrations in females were strongly and positively correlated with concentrations in litters, suggesting that N. sipedon maternally transfer Hg in proportion to their tissue residues. Maternal transfer resulted in high concentrations (up to 10.10 mg/kg dry wt total Hg) of Hg in offspring. The authors found little evidence of adverse effects of Hg on these measures of reproductive output and embryonic survival, suggesting that N. sipedon may be more tolerant of Hg than other vertebrate species. Given that this is the first study to examine the effects of maternally transferred contaminants in snakes and that the authors did not measure all reproductive endpoints, further research is needed to better understand factors that influence maternal transfer and associated sublethal effects on offspring. Environ. Toxicol. Chem. 2013;32:619-626. # 2012 SETAC

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

“…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, ).…”

Like mother, like offspring: maternal and offspring wound healing correlate in snakes

“…This is contrary to previous studies that have shown that Hg affects vertebrate immune function (Spalding et al, 2000;Finkelstein et al, 2007;Hawley et al, 2009). However, two recent studies found few adverse sublethal effects of maternally transferred Hg in neonatal N. sipedon (Chin et al, 2013a;Chin et al, 2013b), suggesting that our study species may be more tolerant of Hg than many other species. Future studies that examine species known to experience deleterious effects in response to Hg exposure [e.g.…”

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.…”