Developmental toxicity of fipronil and its two metabolites towards zebrafish (Danio rerio) embryos

Fipronil (FIL) use is currently regulated in several countries due to its specific toxicity to bees. This study investigated the possible developmental toxicities as well as the acute toxicities of FIL, fipronil sulfide (FIL-SI), and fipronil sulfone (FIL-SO) to zebrafish (Danio rerio) embryos. At concentrations up to 5000 μg L⁻¹, FIL- and FIL–SI–treated embryos exhibited significant mortality at 96 h postfertilization. Body length was significantly shortened with increasing concentrations in FIL- and FIL–SI–treated embryos. However, FIL–SO–treated embryos exhibited low mortality with high hatching rates. Body length was also significantly shortened in FIL–SO–treated embryos. Regarding the number of intersegmental vessels (ISVs), all chemical-treated embryos showed high ISV numbers with increasing concentrations of each chemical. FIL and FIL-SI induced abnormal heart formation with heart dysfunction in embryos, whereas FIL-SO did not induce any difference in heart development compared with the control. Abnormal heart formation may be related to the upregulation of nppa responsible for the expression of natriuretic peptides in embryos. Embryonic acetylcholinesterase activity was decreased gradually according to the increase in FIL and FIL-SI concentrations, whereas FIL-SO did not cause any change in enzyme activity. Il-1β responsible for the occurrence of injury or infection was highly upregulated in FIL–SI– and FIL–SO–treated embryos. Therefore, reduction to FIL-SI may be associated with FIL toxicity, whereas oxidation to FIL-SO may be a detoxification route in the environment.