Examination of an amphibian metamorphosis assay under an individual-separated exposure system using Silurana tropicalis tadpoles

Acute and chronic toxicity tests of systemic insecticides, four neonicotinoids and fipronil, using the tadpoles of the western clawed frog Silurana tropicalis

Extensive use of neonicotinoids and fipronil, which are popular systemic insecticides used in Japanese rice paddies, has raised concerns about their impacts on nontarget aquatic organisms such as amphibians. This study employed premetamorphic tadpoles of Silurana tropicalis and addressed the toxicity of four neonicotinoids (acetamiprid, clothianidin, dinotefuran, and imidacloprid) and fipronil. Acute toxicity tests were conducted under a 96-h semistatic exposure regime and median lethal concentration (LC50) values were calculated at 24-h intervals. All LC50 values of the four neonicotinoids exceeded 100 mg/L, suggesting their low acute toxicity to amphibians. Fipronil yielded much lower LC50 values (3.00-1.34 mg/L) and was highly toxic compared to the four neonicotinoids. Additionally, exposure to fipronil at >1 mg/L induced axial malformations, suggesting its teratogenicity. However, the LC50 values of fipronil were three orders of magnitude higher than the realistic concentrations in paddy water. Chronic toxicity tests were conducted with morphometric, gravimetric, and thyroid-histological endpoints. Premetamorphic tadpoles were exposed to each insecticide at two test concentrations: 0.1 and 1.0 mg/L for the four neonicotinoids; and 1/100 and 1/10 of the 96-h LC50 value for fipronil. Exposure to each insecticide continued until all tadpoles in the control reached late prometamorphic stages or the initial stage of metamorphic climax. At test termination, all insecticides showed no significant differences in any of the endpoints between the respective controls and chemical exposure groups. Overall, our results suggest that these insecticides alone do not directly affect amphibians through their larval stages at concentrations that likely occur in paddy water.

Thyroid disruption properties of three indoor dust chemicals tested in Silurana tropicalis tadpoles

Indoor dust contains a multitude of industrial chemicals, and ingestion of dust is considered an important exposure route to organic contaminants. Some of these contaminants have been shown to interfere with the thyroid system, which may result in significant consequences on public health. The amphibian metamorphosis is a thyroid hormone-dependent process, which can be used as an in vivo model for studies on thyroid hormone-disrupting potency. Three contaminants of indoor dust were tested on metamorphosing Silurana (Xenopus) tropicalis tadpoles. The tested chemicals were Tris (1,3-dichloroisopropyl) phosphate (TDCiPP), tetrabromobisphenol-A (TBBPA) and propylparaben (PrP). Measurements reflecting general growth, development progress and thyroid epithelial cell height were performed on the exposed tadpoles as well as chemical analyses of the exposure water. It was shown that TDCiPP acts as a thyroid hormone-disrupting chemical in metamorphosing tadpoles by causing increased epithelial cell height in thyroid glands after exposure to a nominal concentration of 0.010 mg/L and in higher concentrations. TBBPA caused reductions in general growth of tadpoles at the nominal concentration 0.125 mg/L, and PrP caused acute toxicity at the nominal concentration 12.5 mg/L. However, no evident indications of specific thyroid-disrupting effects caused by TBBPA or PrP were observed.

Chronic toxicity of 1,3,5-triazine herbicides in the postembryonic development of the western clawed frog Silurana tropicalis

Seven 1,3,5- triazine (s-triazine) herbicides (ametryn, prometryn, dimethametryn, simazine, atrazine, propazine, and cyanazine) were tested using an amphibian (Silurana tropicalis) metamorphosis assay focusing on morphometric, gravimetric, and thyroid-histological endpoints. Premetamorphic tadpoles were exposed to each s-triazine at 2 concentrations between 1/1000 and 1/10 of the 96-h acute toxicity values, until all tadpoles in the control group reached either the late prometamorphosic stages or the initial stage of metamorphic climax. All s-triazines tested induced significant retardation in growth and development at the higher concentrations (0.2-1.0mg/L), and some of them induced similar effects even at the lower concentrations (0.02-0.1mg/L) while each showing a linear dose-response. Total size of the thyroid glands tended to be reduced corresponding to the delayed development, but without showing histomorphological lesions typical of anti-thyroid chemicals. These consistent results suggest that the s-triazines can act as a chemical stressor inhibiting tadpole growth and development, possibly without disrupting the thyroid axis. In addition, tadpoles exhibiting spinal curvatures appeared in either one or both of the lower and higher concentration groups for each s-triazine tested. The incidence rate in the s-triazine exposure groups where tadpoles with scoliosis were observed ranged from 3.3% to 63.3%, some of which were significantly higher than that in the respective control groups (0-6.7%). It is speculated that the s-triazines may promote to occur axial malformations in developing tadpoles.

Effects of nitrate on metamorphosis, thyroid and iodothyronine deiodinases expression in Bufo gargarizans larvae

Chinese toad (Bufo gargarizans) tadpoles were exposed to nitrate (10, 50 and 100mg/L NO3-N) from the beginning of the larval period through metamorphic climax. We examined the effects of chronic nitrate exposure on metamorphosis, mortality, body size and thyroid gland. In addition, thyroid hormone (TH) levels, type II iodothyronine deiodinase (Dio2) and type III iodothyronine deiodinase (Dio3) mRNA levels were also measured to assess disruption of TH synthesis. Results showed that significant metamorphic delay and mortality increased were caused in larvae exposed to 100mg/L NO3-N. The larvae exposed to 100mg/L NO3-N clearly exhibited a greater reduction in thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels. Moreover, treatment with NO3-N induced down-regulation of Dio2 mRNA levels and up-regulation of Dio3 mRNA levels, reflecting the disruption of thyroid endocrine. It seems that increased mass and body size may be correlated with prolonged metamorphosis. Interestingly, we observed an exception that exposure to 100mg/L NO3-N did not exhibit remarkable alterations of thyroid gland size. Compared with control groups, 100mg/L NO3-N caused partial colloid depletion in the thyroid gland follicles. These results suggest that nitrate can act as a chemical stressor inducing retardation in development and metamorphosis. Therefore, we concluded that the presence of high concentrations nitrate can influence the growth, decline the survival, impair TH synthesis and induce metamorphosis retardation of B. gargarizans larvae.

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

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 0.40mg/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 0.40mg/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 0.30mg/L) nor thiobencarb (0.008 and 0.080mg/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.