The relationship between mercury and selenium in the eggshell and egg content of the Whiskered tern (Chlidonias hybridus).

Mercury and its compounds do not have beneficial biological roles in living organisms. In contrast, selenium, within permissible limits, is an essential micronutrient and antioxidant. There is an antagonistic relationship between mercury and selenium. Therefore, the molar ratio of selenium to mercury is used as an appropriate index. Measuring the mercury in the environment without considering the protective effects of selenium does not accurately reflect the risks posed by mercury. This study aimed to measure the mercury, selenium, and the molar ratio Se:Hg in the shell and content of the eggs of the Whiskered Tern (Chlidonias hybridus) in northern Iran. After the onset of egg laying, one egg was collected from each nest (38 eggs in total). The results showed that the concentration of mercury in the eggshell was higher, while the concentration of selenium and the molar ratio Se:Hg were higher in the egg content. A molar ratio Se:Hg in both the eggshell and the egg content indicates a positive effect of selenium in reducing the risks associated with mercury toxicity. The parameters of egg shape index, egg weight, and clutch size did not have a significant effect on the concentrations of mercury, selenium, and the molar ratio Se:Hg in the eggshell and egg content. There was a significant negative correlation between the concentration of mercury in the eggshell and the molar ratio Se:Hg in both the eggshell and egg content. The lack of a positive correlation between the concentrations of mercury or selenium in the eggshell and egg content suggests that the transferability of heavy metals from the egg to the embryo in the egg content cannot be confirmed.