Alterations in social behavior of Japanese medaka (Oryzias latipes) in response to sublethal chlorpyrifos exposure

The behavioral and biochemical responses of Japanese medaka (Oryzias latipes) to acute and subacute (sublethal) levels of chlorpyrifos were studied. In the acute exposure test, medaka were exposed to 0.018, 0.055, 0.166, or 0.500mgL^-1 chlorpyrifos for 4d. As a result, fish showed hypoactivity compared to the control (at 0.018, 0.055, and 0.166mgL^-1, swimming speeds were 55.6%, 39.0%, and 27.3% those of the control), Brain acetylcholinesterase activity and swimming speed were significantly correlated. In the subacute toxicity test, medaka were exposed to 0.012mgL^-1 chlorpyrifos (10% of LC₅₀) for 8d. On day 4, there were no significant differences in behavioral and biochemical endpoints in exposed fish as compared to the control. On day 8, exposed fish became hyperactive, and the swimming speed of the social group increased to 2times that of the control, whereas acetylcholinesterase activity was decreased to 68% that of the control. In addition, fish exhibited significant alterations in social behavior (schooling duration increased to 2.6times and solitary duration decreased to 28% that of the control). Our findings clearly demonstrate a subacute effect of chlorpyrifos on the social behavior of medaka, which may pose a risk at population level because of the disturbance of social behavior. In addition, the recorded behavioral alterations may provide a useful tool for assessing the toxicity of organophosphorous pesticides to aquatic organisms.