The role of development and duration of exposure to the embryotoxicity of diazinon.

Medaka, Oryzias latipes, were used as a laboratory surrogate for species of concern to define the effects of diazinon exposure on teleost embryogenesis. Medaka embryos were placed in a static, non-renewal system and exposures initiated on days 1, 3, or 5 of development. Following initiation of exposure, replicates (n=5) remained in diazinon for a total of 4 days or from the day of initiation to day 9 of development. This exposure scenario was designed to elucidate sensitive periods in development for diazinon-induced toxicity but also shows the effect of added exposure duration on the degree of toxicity. Embryos were observed daily and endpoints recorded included: edema formation, total hatch, mean day of hatch, percentage of larvae with swim bladder inflation, and total length of larvae on day 14, when observations were terminated. Diazinon exposure resulted in decreases in hatch success, swim bladder inflation and the total length of larvae. In addition, dose-response increases in the incidence of edemas of the pericardial sac and vitelline veins were recorded. As expected, severity of embryotoxicity was positively correlated with duration of exposure. While no developmental period was the most sensitive for all toxic effects, for certain endpoints the severity of effects was dependent on exposure timing. Total hatch was greatly affected in embryos exposed from day 1 until day 5 whereas edema was more prevalent in embryos exposed later in development. Finally, among endpoints recorded, total length of larvae was the most sensitive indicator of exposure with all exposure groups showing significant (P<0.05) decreases in length at 5 ppm.