Levels of MEP in medaka eggs and the effects of MEP on hatchability were investigated. When medaka eggs were exposed to MEP-containing water, the eggs contained more MEP the higher the MEP concentration of the water, the more advanced the stage of development, and the longer the exposure. The MEP contents accumulated in the eggs were reduced to 2-19% after they were transferred to dechlorinated tap water for 96h. There was a significant correlation between the MEP concentration in the water and the MEP content in the eggs.
When medaka eggs in an early stage of development were exposed to 4ppm MEP for 96h or 64ppm MEP for 6h and were then placed in dechlorinated tap water until hatching, their hatching rates became significantly lower than those of the control, and abnormal fry occurred frequently.
This experiment clarified that increases of MEP in medaka eggs were greatly affected by the MEP concentration in the water, and that the hatching rate was reduced and abnormal fry increased, even though the eggs contained the MEP only temporarily.