Divalent metal toxicity to the Japanese medaka embryo in single embryo culture

Embryos of the Japanese Medaka, Oryzias latipes, were exposed to a range of divalent metals including cadmium (Cd), copper (Cu), manganese (Mn), mercury (Hg), nickel (Ni) and zinc (Zn). Embryos were cultured individually to better track the course of developmental events and to remove neighbor effects. The hierarchy of toxicity measured as mortality was Hg > Cu > Cd > Zn > Ni > Mn. Other factors such as endurance time, time to hatch, and the incidences of gross developmental abnormalities were also evaluated. Mercury in particular produced both sublethal and lethal toxic effects at concentrations relevant to the situation in streams draining the US Department of Energy`s Oak Ridge Reservation. Medaka embryos have proven in both laboratory studies such as these and routine testing of ambient water sources on the reservation to be sensitive test organisms for a variety of environmental contaminants including the divalent metals.