Mercury and Selenium in Stranded Indo-Pacific Humpback Dolphins and Implications for Their Trophic Transfer in Food Chains

Abstract

As top predators in the Pearl River Estuary (PRE) of China, Indo-Pacific humpback dolphins (Sousa chinensis) are bioindicators for examining regional trends of environmental contaminants in the PRE. We examined samples from stranded S. chinensis in the PRE, collected since 2004, to study the distribution and fate of total mercury (THg), methylmercury (MeHg) and selenium (Se) in the major tissues, in individuals at different ages and their prey fishes from the PRE. This study also investigated the potential protective effects of Se against the toxicities of accumulated THg. Dolphin livers contained the highest concentrations of THg (32.34±58.98 µg g−1 dw) and Se (15.16±3.66 µg g−1 dw), which were significantly different from those found in kidneys and muscles, whereas the highest residue of MeHg (1.02±1.11 µg g−1 dw) was found in dolphin muscles. Concentrations of both THg and MeHg in the liver, kidney and muscle of dolphins showed a significantly positive correlation with age. The biomagnification factors (BMFs) of inorganic mercury (Hginorg) in dolphin livers (350×) and MeHg in muscles (18.7×) through the prey fishes were the highest among all three dolphin tissues, whereas the BMFs of Se were much lower in all dolphin tissues. The lower proportion of MeHg in THg and higher Se/THg ratios in tissues were demonstrated. Our studies suggested that S. chinensis might have the potential to detoxify Hg via the demethylation of MeHg and the formation of tiemannite (HgSe) in the liver and kidney. The lower threshold of hepatic THg concentrations for the equimolar accumulation of Se and Hg in S. chinensis suggests that this species has a greater sensitivity to THg concentrations than is found in striped dolphins and Dall’s porpoises.

Mercury, cadmium, zinc and copper concentrations and stable isotope ratios of carbon and nitrogen in tiger sharks (Galeocerdo cuvier) culled off Ishigaki Island, Japan

“…2): all Hg concentrations in liver from sharks exceeding 2.6 m PCL for males and 2.8 m PCL for females were higher than those in muscle. The marked increase in hepatic Hg concentrations after maturation may be explained by the continuous intake of Hg via their diet and the reduction in growth at maturation, as well as the formation of Hg-Se complexes (Storelli and Marcotrigiano, 2002;Endo et al, 2002Endo et al, , 2006Endo et al, , 2013Lailson-Brito et al, 2012;Karimi et al, 2013;Gui et al, 2014). In agreement with our definition of mature males (exceeding 2.6 m PCL) and females Wintner and Dudley (2000) report that the 50% maturities for males and females to be 2.50 and 2.74 m PCL, respectively.…”

“…Marine predators, particularly long-lived species such as sharks, tuna, swordfish, and cetaceans, accumulate high levels of mercury (Hg) via the food web, and the hepatic Hg concentration in marine cetaceans is greater than the Hg concentration in the muscle (Honda et al, 1983;Andre et al, 1991;Holsbeek et al, 1998;Zhou et al, 2001) as the hepatic Hg concentration increases greatly after maturation due to the formation of Hg-Se complexes (Endo et al, 2002(Endo et al, , 2006Lailson-Brito et al, 2012;Karimi et al, 2013;Gui et al, 2014). In contrast to cetaceans, there are only limited data available on Hg concentration in the shark liver because most Hg surveys on shark species were undertaken for the edible part of the muscle.…”

“…Increases in the Cd burden induce the synthesis of metallothionein (MT) in the liver and Cd subsequently binds to MT along with Zn and Cu (Das et al, 2000). Increases in the Hg burden leads to an increase in Hg-Se complex formation in liver and this Hg-Se is characteristically accumulated in this organ (Storelli and Marcotrigiano, 2002;Endo et al, 2002Endo et al, , 2006Lailson-Brito et al, 2012;Karimi et al, 2013;Gui et al, 2014). A large-scale survey performed under well-designated plan is necessary to elucidate the body length (lifestage)-dependent changes in Zn and Cu concentrations in the liver of predators.…”

In vivo formation of natural HgSe nanoparticles in the liver and brain of pilot whales

“…The relatively low percentage of the methylated fraction of total Hg found in the tissues of juvenile whales suggests ( Supplementary Tables S1 and S2 ), that the detoxification process is fully developed from the early age of the animals, which is in contradiction with previous suggestions that young animals are not able to efficiently demethylate MeHg 29 . Additionally, several authors 30 31 suggested that initiation of a demethylation process requires the total Hg concentration to exceed a certain threshold value. In striped dolphin, the threshold value was suggested to be 0.49 μmol of total Hg kg −1 of wet weight of liver 32 .…”

Mercury and selenium status of bottlenose dolphins ( Tursiops truncatus ): A study in stranded animals on the Canary Islands

“…This effect has been shown in studies in a variety of species, including marine mammals (Cuvin-Aralar and Furness, 1991;Frodello et al, 2000;Gui et al, 2014;Sakamoto et al, 2013) exposed to these elements for a long time, even before the industrial period (Holsbeek et al, 1998). Thus, several mechanisms for resistance to the adverse effects of Hg have been proposed.…”