Application of an amphibian (Silurana tropicalis) metamorphosis assay to the testing of the chronic toxicity of three rice paddy herbicides: Simetryn, mefenacet, and thiobencarb

• Using an amphibian metamorphosis assay, we tested three rice paddy herbicides.
• Simetryn retarded tadpole growth and development at 1/100 of the 96-h LC50 value.
• Simetryn also induced axial malformations at 1/10 of the 96-h LC 50 value.
• Mefenacet and thiobencarb induced no abnormalities at 1/10 of the 96-h LC50 values.
• Simetryn may exert chronic toxicity in wild anuran tadpoles in paddy water.

We examined the chronic toxicity of three rice paddy herbicides (simetryn, mefenacet, and thiobencarb) using an amphibian (Silurana tropicalis) metamorphosis assay (a 28-day semistatic test under an individual-separated exposure system). Each herbicide was tested at two concentrations (1/100 and 1/10 of the 96-h LC50 value reported previously) with morphometric, gravimetric, and thyroid-histological endpoints. Simetryn caused significant retardation in growth and development at both test concentrations (0.04 and 0.40 mg/L), as indicated by significantly shorter total body lengths and hind limb lengths, smaller wet body masses, and delayed developmental stages compared to those observed in the control tadpoles. However, no clear histopathology was observed in the thyroid glands of the tadpoles exposed to simetryn. These results suggest that simetryn can act as a chemical stressor retarding tadpole growth and development without disrupting thyroid functions, even at 1/100 of the 96-h LC50 value. In addition, scoliosis near the tail base was observed in the tadpoles exposed to 0.40 mg/L of simetryn at a significantly high incidence (7/30=23.3%). Therefore, simetryn can also act as a teratogen inducing axial malformations at 1/10 of the 96-h LC50 value. During the 28 days of exposure, neither mefenacet (0.03 and 0.30 mg/L) nor thiobencarb (0.008 and 0.080 mg/L) induced any abnormalities, although the test concentrations measured immediately before the solution renewals decreased to nearly 50 percent of the nominal concentrations since day 14. Because the concentrations tested for simetryn are likely to occur in paddy water, wild anuran tadpoles in paddy water may therefore be adversely impacted by simetryn.