Effects of copper on growth, metamorphosis and endocrine disruption of Bufo gargarizans larvae

• Copper increased mortality of Bufo gargarizans larvae.
• Copper could delay metamorphosis and decrease body size.
• Copper increased tail resorption time.
• Copper caused follicular cell hyperplasia in the thyroid gland.
• Copper changed mRNA expressions of Dio2, Dio3, TRα and TRβ.

Chinese toad (Bufo gargarizans) tadpoles were exposed to copper (1, 6.4, 32 and 64 μg L−1 copper) from the beginning of larval period through completion of metamorphosis. We examined the effects of chronic copper exposure on mortality, growth, time to metamorphosis, tail resorption time, body size at the metamorphic climax (Gs 42) and completion of metamorphosis (Gs 46) and thyroid gland histology. In addition, type 2 and 3 iodothyronine deiodinase (Dio2 and Dio3), thyroid hormone receptors (TRα and TRβ) mRNA levels were also measured to assess disruption of TH synthesis. Our result showed that 6.4–64 μg L−1 copper concentration increased the mortality and inhibited the growth of B. gargarizans tadpoles. In addition, significant reduction in size at Gs 42 and a time delay to Gs 42 were observed at 6.4–64 μg L−1 copper treatments. Moreover, histological examinations have clearly revealed that 64 μg L−1 copper caused follicular cell hyperplasia in thyroid gland. According to real-time PCR results, exposure to 32 and 64 μg L−1 copper significantly up-regulated mRNA expression of Dio3, but down-regulated mRNA expression of TRα and TRβ mRNA level. We concluded that copper delayed amphibian metamorphosis through changing mRNA expression of Dio3, TRα and TRβ, which suggests that copper might have the endocrine-disrupting effect.