Life‐cycle toxicity of 4‐nonylphenol to medaka (Oryzias latipes)

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Abstract
We studied the chronic effects of 4‐nonylphenol (4‐NP) on reproductive status of medaka (Oryzias latipes) over two generations of continuous exposure. The exposure study of the parental (F0) medaka was begun on embryos within 24 h postfertilization and continued with monitoring through embryological development, hatching, posthatch survival, growth, sexual differentiation, and reproduction under flow‐through exposures to mean measured 4‐NP concentrations of 4.2, 8.2, 17.7, 51.5, and 183 μg/L for up to 104 d. Eggs spawned from the F0 fish at 102 and 103 d posthatch were also examined for hatchability, survival after hatching, growth, and sexual differentiation until 60 d posthatch. The 183‐μg/L treatment significantly reduced the embryo survival and swim‐up success of the F0 fish. The cumulative mortality after swim‐up of the F0 fish exposed to 17.7 and 51.5 μg/L were significantly higher than the control mortality. No concentration‐related effect of 4‐NP was observed on the growth of surviving F0 fish at 60 d posthatch. However, the sex ratio estimated from the appearance of their secondary sex characteristics was skewed toward female in the 51.5μg/L treatment. Additionally, gonadal histology showed that 20% of the fish in the 17.7‐μg/L treatment and 40% in the 51.5‐μg/L treatment had testis–ova, indicating that 4‐NP affects the gonadal development and survival of medaka at similar concentrations in juveniles. The sex ratio of the F0 fish in the 51.5‐μg/L treatment was completely skewed toward female; subsequently, the effects on fecundity and fertility in this generation were monitored at mean measured concentrations of 4.2, 8.2, and 17.7 μg/L from 71 to 103 d posthatch. Fecundity was unaffected by any of the treatments examined. The mean fertility in the 17.7‐μg/L treatment was reduced to 76% of that in the controls, although no statistically significant differences were determined. Overall, these results indicate that the lowest‐observed‐effect concentration (LOEC) and no‐observed‐effect concentration (NOEC) of 4‐NP through the life cycle of the F0 medaka were 17.7 and 8.2 μg/L, respectively. In the F1 medaka, no significant effects were observed on hatching success, posthatch mortality, or growth, but sexual differentiation at 60 d posthatch was affected. Induction of testis–ova in the gonads of the F1 fish was observed in both the 8.2‐ and the 17.7‐μg/L concentrations. The results indicate that 4‐NP can have significant effects on reproductive potential of medaka at concentrations as low as 17.7 μg/L.