Experimental Evidence for Mobilization of Mercury From the Eggshell by the Developing Embryo in an Avian Model Organism

Of the many pollutants wildlife is exposed to, mercury ranks among the most toxic. In birds, mercury does not only accumulate within generations, but also is transferred across generations via maternal offloading in eggs, with higher mercury concentrations found in shells of unhatched eggs. The latter may be explained either by mercury reducing hatching success, or by reduced embryonic development limiting the mobilization of mercury from the shell by the embryo. Here, we distinguished between these pathways by experimentally exposing mated female Japanese quail (Coturnix japonica) to a single sublethal dose of methylmercury, and randomly selecting their eggs to be incubated or not. A control group of unexposed females allowed us to test whether maternal mercury transfer affects embryo survival within the exposure range observed. We found markedly lower total mercury concentrations in the eggshells of incubated versus unincubated eggs. In addition, among incubated eggs, successful embryonic development was associated with lower total mercury concentrations. The eggs from mercury-exposed and control-treated females, however, did not differ in hatching success. As such, our findings provide evidence consistent with embryonic mobilization of mercury from the eggshell during development. This implies that elevated mercury in the shells of unhatched eggs should not, by default, be interpreted as evidence of mercury-related embryo mortality, and illustrates how assessing causality is crucial for understanding how pollution may put wildlife at risk.