Lipid-Extracted Muscle and Liver Tissues: Can They Reveal Mercury Exposure of Pelagic Sharks?

Abstract

Pelagic sharks are apex predators in oceanic ecosystems and tend to accumulate high amount of mercury (Hg). The conventional method for assessing Hg exposure in sharks involves analyzing tissue samples without any chemical treatment. However, a substantial number of chemically treated tissue samples are still being preserved in laboratories or museums. It is critical to maximize the utilization of existing samples to reduce the need for additional sampling of pelagic sharks, especially endangered species. Lipid extraction is a widely employed pretreatment process for carbon isotope analysis in shark trophic ecology, while its impact on Hg quantification remains uncertain. Here, we evaluated the feasibility of using lipid-extracted muscle and liver tissues for investigation of Hg exposure in four endangered pelagic sharks inhabiting the eastern Pacific, including bigeye thresher (Alopias superciliosus), pelagic thresher (A. pelagicus), blue shark (Prionace glauca) and silky shark (Carcharhinus falciformis). Results showed that total Hg concentrations (THg) differed between untreated (THga) and lipid-extracted (THgb) samples for each tissue type of each species. In addition, dichloromethane-methanol extractions significantly altered the amount of Hg. This may result from the removal of lipoprotein compounds that vary between tissues and species. The THga can be calculated by THgb using the following formulas, THga = 1.14 × THgb + 0.30 and THga = 0.33 × THgb + 0.18, for muscle and liver tissues, respectively. These findings emphasize the applications of lipid-extracted tissues in THg analysis. This study may have important implications for improving evaluation of Hg exposure in endangered pelagic sharks.

Keywords: Pelagic shark, Mercury exposure, Lipid extraction, Muscle, liver