Toxicity of methylmercury injected into eggs of thick-billed murres and arctic terns.

Mercury (Hg) has been increasing in some marine birds in the Canadian Arctic over the past several decades. To evaluate the potential reproductive impact of Hg exposure, eggs of two species of arctic-breeding seabirds, the thick-billed murre and arctic tern, were dosed with graded concentrations of methylmercury (MeHg) and artificially incubated in the laboratory to determine species differences in sensitivity. Based on the dose-response curves, the median lethal concentrations (LC(50)) for thick-billed murre and arctic tern embryos were 0.48 and 0.95 μg g(-1) Hg on a wet-weight (ww) basis, respectively. Compared with published LC(50) values for other avian species, the murres and terns had a medium sensitivity to MeHg exposure. LC(50) values were also calculated for the actual Hg concentration measured in the embryos, that is, the maternally-deposited Hg plus the injected MeHg dose. This increased the LC(50) values to 0.56 μg g(-1) Hg ww in the thick-billed murre and to 1.10 μg g(-1) Hg ww in the arctic tern. Although muscarinic acetylcholine and N-methyl-D-aspartic acid glutamate receptor levels have been correlated with increasing Hg concentrations in brains of adult birds, no significant associations were found in brain tissue of the murre or tern embryos. The incidence of gross external anatomical deformities was 4.3 % in the murre embryos and 3.6 % in the tern embryos. However, given that the eggs were taken from wild populations, it is unlikely that the deformities observed in this study were due to MeHg exposure alone.