Effects of Ethinylestradiol on Medaka (Oryzias latipes) as Measured by Sperm Motility and Fertilization Success

We investigated the effects of 30–480 ng/L 17α-ethinylestradiol (EE 2 ) on the sperm motility and fertility of Japanese medaka ( Oryzias latipes ). Sperm motility was examined by computer-assisted image analysis. In male medaka, the velocity of sperm was found to have increased after 3 weeks of exposure at 60–480 ng/L. This result suggests that higher sperm velocities depleted sperm energy reserves more rapidly and shortened the time for which sperm were viable to fertilize eggs. In a separate experiment that studied whether EE 2 exposure of males affects the fertilization rate or hatchability, sexually mature male medaka were exposed for 3 weeks and subsequently evaluated for their reproductive ability after pairing with unexposed females for 7 days. Exposure of males to EE 2 exerted a potent inhibitory effect on a reproduction parameter (fertilization rate × hatchability), and the highest inhibition was observed at 60 ng/L. The results offer toxicological data for the assessment of EE 2 exposure in medaka and suggest that short-term exposure to EE 2 might reduce sperm function and fertility in adult male medaka.