Abstract
Neonicotinoids, including imidacloprid, are pesticides that resemble nicotine and undergo slight chemical alterations through metabolic changes in the environment. However, the effects of these metabolites on organisms remain unknown. In this study, we assessed the developmental processes of medaka embryos exposed to neonicotinoid metabolites. The target compounds were imidacloprid metabolites: 2-chloro-5-pyridine carbaldehyde (CPC) and 6-chloronicotinic acid (6-CNA). Medaka embryos within 6 h of fertilization were exposed to the compounds, and their developmental processes were observed under a stereomicroscope. Medaka embryos exposed to 5 mg/L CPC showed no abnormalities compared to the controls. Contrastingly, medaka embryos exposed to 10, 15, and 20 mg/L CPC showed abnormalities such as thrombus formation, asymmetry, disorganized development of the eyeballs, and low blood flow. This trend was more pronounced at higher CPC concentrations. On the other hand, embryos exposed to 80 and 160 mg/L 6-CNA showed no abnormalities until day 7 of exposure. However, on day 8 of exposure, sudden embryo death was observed. Both compounds may have bound to acetylcholine receptors as agonists; however, their effects were different. CPC caused abnormal development and 6-CNA caused inhibition of hatching gland development and/or synthesis of the hatching enzyme.