Selenium and mercury have a synergistic negative effect on fish reproduction.

Selenium (Se) can reduce the negative impacts of mercury (Hg) toxicity on growth and survival, but little is known about how these two elements interact in reproduction. In the following study we explored the effects of organic Hg and Se on the growth, survival and reproduction of female zebrafish (Danio rerio). Fish were fed one of four diets from 73 until 226 dpf in a 2 × 2 factorial design, using selenomethionine (SeMet) and methylmercury (MeHg) as the Se and Hg sources, respectively. Each diet contained Se at either requirement (0.7 mg Se/kg DM) or elevated levels (10 mg Se/kgDM), and Hg at either low (0.05 mg Hg/kg DM) or elevated (12 mg Hg/kg DM) levels. Between 151 and 206 dpf the female fish were pairwise crossed against untreated male fish and the mating success, fecundity, embryo survival, and subsequent overall reproductive success were measured. Elevated dietary Se reduced Hg levels in both the adult fish and their eggs. Elevated dietary Hg and Se increased egg Se levels to a greater extent than when dietary Se was elevated alone. At elevated maternal intake levels, egg concentrations of Se and Hg reflected the maternal dietary levels and not the body burdens of the adult fish. Elevated dietary Hg reduced the growth and survival of female fish, but these effects were largely prevented with elevated dietary Se. Elevated dietary Se alone did not affect fish growth or survival. Compared to other treatments, elevated dietary Hg alone increased both mating and overall reproductive success with <100 days of exposure, but decreased these parameters with >100 days exposure. Elevated dietary Se decreased fecundity, embryo survival, and overall reproductive success. The combination of elevated Se and Hg had a synergistic negative effect on all aspects of fish reproduction compared to those groups fed elevated levels of either Se or Hg. Overall the data demonstrate that while increased dietary Se may reduce adverse effects of Hg on the growth and survival in adult fish, it can negatively affect fish reproductive potential, and the effect on reproduction is enhanced in the presence of elevated Hg.