Toxicity of mercury, copper, nickel, lead, and cobalt to embryos and larvae of zebrafish, Brachydanio rerio.

The toxicity of mercury (HgCl2), copper (CuCl2: 5 H20), nickel (NiSO4: 6 H2O), lead (Pb(CH3COO)2: 3 H2O) and cobalt (CoCl2: 6 H2O) was studied under standardized conditions in embryos and larvae of the zebrafish, Brachydanio rerio. Exposures were started at the blastula stage (2-4 h after spawning) and the effects on hatching and survival were monitored daily for 16 days. Copper and nickel were more specific inhibitors of hatching than cobalt, lead, and mercury. Nominal "no effect" concentrations determined from the dose-response relationships (ZEPs, Zero Equivalent Points) for effect on hatching time were 0.05 microgram Cu/L, 10 micrograms Hg/L, 20 micrograms Pb/L, 40 micrograms Ni/L and 3,840 micrograms Co/L, and those for effect on survival time were 0.25 microgram Cu/L, 1.2 micrograms Hg/L, 30 micrograms Pb/L, 80 micrograms Ni/L, and 60 micrograms Co/L. The "no effect" concentrations for Ni, Hg and Pb are consistent with previously reported MATC values for sensitive species of fish. The "no effect" concentrations for copper are 1-2 orders of magnitude lower than previously reported values. The major reason for the latter discrepancy was considered to be the absence of organics that can complex copper ions in the reconstituted water that we used, which had a hardness of 100 mg/L (as CaCO3) and a pH of 7.5-7.7. Unexposed controls were started with embryos from different parental zebrafishes and the parental-caused variability in early embryo mortality, median hatching time and median survival time were estimated.