Quantifying maternal transfer of trace elements and stable isotopes in the endangered pelagic thresher shark (Alopias pelagicus)

To quantify maternal provisioning of nutrients in the pelagic thresher shark (Alopias pelagicus) and the potential for negative impacts, the concentrations of trace elements (essential: Co, Cr, Cu, Mn, Ni, Se, and Zn; nonessential: As, Ba, Cd, Hg, and Pb) and fractionation of stable isotopes (C-13 and N-15) were analyzed in the muscle and liver of 10 pregnant females and 18 associated embryos. Essential trace elements were observed to be offloaded at higher concentrations to embryos, with the exception of Zn and Ni in liver, while nonessential trace elements were unevenly distributed between maternal-embryo tissues. Observed Hg concentrations were at levels considered toxic in A. pelagicus, but the Se: Hg molar ratios in all embryonic tissues were all greater than one. A negative correlation was observed between transfer ratios and concentrations of all elements in maternal tissue, indicating the existence of a regulatory mechanism in maternal ovaries of A. pelagicus. Compared with maternal specimens, associated embryos had higher delta C-13 and delta N-15 values in muscle and liver tissue. Negative correlations were observed between delta C-13, delta N-15, and Delta delta C-13 values and precaudal length in embryonic muscle tissue potentially reflecting either a dietary-habitat shift in pregnant females during the latter period of gestation or a physiological change modifying fractionation. Higher concentrations of essential elements are linked to potential benefits for embryos during early development, levels of Hg suggested a degree of anthropogenic impact with unknown consequences while the directionality of isotopic fractionation could suggest a potential reproductive migration as a protective mechanism for birthing.

Automated summary

This study analyzed the concentration and distribution of trace elements and stable isotopes in pregnant female pelagic thresher sharks and their embryos. The results showed that essential trace elements were mostly transferred to the embryos, while nonessential trace elements were unevenly distributed between maternal and embryo tissues. Mercury concentrations reached toxic levels, but the molar ratios of selenium to mercury in embryonic tissues were all greater than one. A negative correlation was observed between concentrations of elements in maternal tissue and transfer ratios, indicating the existence of a regulatory mechanism in maternal ovaries. Compared to maternal specimens, embryos had higher isotopic values.