Behavioral and sex ratio modification of Japanese medaka (Oryzias latipes) in response to environmentally relevant mixtures of three pesticides.

We exposed Japanese medaka (Oryzias latipes) to environmentally relevant concentrations of azinphos-methyl, chlorothalonil, endosulfan, and mixtures of all three to determine if combinations of these pesticides result in additive, less-than-additive, or more-than-additive effects. Medaka were exposed from fertilization until 7 days posthatching, and end points included survival, time to hatch, size at 7 days posthatching, activity level (as measured by distance swam) and foraging ability at 3 weeks posthatching, and adult size, liver size, and sex ratio at 5 months posthatching. Although exposure to individual pesticides or pesticide mixtures did not affect survival, hatching time, or foraging ability, fry exposed to azinphos-methyl were significantly smaller at 1 week of age, and those exposed to chlorothalonil and a combination of the chemicals showed reduced activity. Adult sex ratios were biased toward females in all groups exposed to pesticides, with those exposed to azinphos-methyl, chlorothalonil, and the pesticide mixture departing significantly from an even sex ratio. There was no evidence of additive or synergistic effects of pesticide mixtures.