Selenium:Mercury Molar Ratios in Muscle and Organs of Immature Sharks in Texas Bays




Pitman, Natalie

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Mercury (Hg) is a nonessential trace element that bioaccumulates in marine organisms and biomagnifies in marine food webs, so top predators, including sharks have the greatest Hg concentrations. Most studies investigate Hg concentrations in adult sharks, with little research on immature (young-of-the-year and juvenile) sharks, even though high tissue Hg concentrations could result in adverse health effects that impact growth and development. Selenium (Se), an essential trace element, has an antagonistic relationship with Hg and may have a protective effect against Hg toxicity if the Se:Hg molar ratio is >1:1. Muscle and liver Se:Hg molar ratios have been investigated in adult sharks, however, there are no studies examining Se:Hg molar ratios in tissues of immature sharks due, in part, to the difficulty acquiring samples. Texas Bays are essential habitat that provide nursery areas for shark species that utilize the bays for protection and growth, but industrial pollution may threaten these habitats. This study investigated the Hg and Se concentrations and calculated the Se:Hg molar ratios in dorsal muscle, heart, brain, kidney, and liver of immature bull sharks (Carcharhinus leucas), blacktip sharks (Carcharhinus limbatus), and bonnethead sharks (Sphyrna tiburo), in four bays (Sabine Lake, Aransas Bay, Corpus Christi Bay, and Lower Laguna Madre) along the Texas coast. Sharks were collected during the Texas Parks and Wildlife Department 2020-2022 fall and spring gill net seasons (n = 20-30 per species and bay). Mercury concentrations were determined using a direct mercury analyzer (DMA-80) and Se concentrations measured using microwave acid digestion and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis. Overall, Hg concentrations were greatest in muscle and heart and lowest in brain and liver in all species and bays. In comparison, Se concentrations were greatest in kidney and lowest in muscle for all species and bays. There were inconsistent trends within and among bays when examining the relationship between Hg concentration and fork length, Se concentration and fork length, and Se:Hg molar ratios and fork length for each tissue, most likely due to the variability in Hg and Se availability and factors affecting uptake. For all bays combined, 38.3% (23 out of 60) bull sharks, 6.0% (3 out of 50) blacktip sharks, and 1.7% (1 out of 60) bonnethead sharks had a muscle Hg concentration that exceeded the Hg adverse biological threshold level (0.5 μg/g Hg wet weight). The Se:Hg molar ratios were >1:1 in all tissues, indicating that Se may have a protective effect against Hg toxicity in immature sharks. High tissue Se:Hg molar ratios in immature sharks (e.g., 155:1 in heart, 654:1 in kidney, and 367:1 in the brain of bonnethead sharks) could reduce the adverse effects of Hg on growth and development, however, the Se:Hg molar ratio at which Se is protective against Hg toxicity (e.g., 1:1, 5:1, or greater) is not known, highlighting the need for further research in taxa that have higher Hg concentrations due to their reproductive strategy, long-life span, and high trophic position.



mercury, selenium, molar ratio, sharks, Texas bays


Pitman, N. (2023). Selenium:Mercury molar ratios in muscle and organs of immature sharks in Texas bays (Unpublished thesis). Texas State University, San Marcos, Texas.


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