Methylmercury: Teratogenic and lethal effects in frog embryos

Abstract

Rana pipiens embryos at the cleavage, blastula, gastrula, and neural-plate stages of development were treated with methylmercuric chloride in concentrations of 0.5-200 parts per billion (ppb) to determine embryocidal and teratogenic effects. Concentrations of 40 ppb and above were lethal to embryos treated during the cleavage stage. Embryos at the blastula, gastrula, and neuralplate stages were treated for 5 days with 5-30 ppb; 5 ppb had only minor effects, 10, 15, and 20 ppb caused various defects (exogastrulae, poor tail development, poor general development) and increased death rates related to exposure time and concentration, and embryos treated with 30 ppb had a high frequency of defects after 24 h and all died by the end of the 3rd day. Individual tolerances to methylmercury of various degrees were observed.

The potential influence of environmental pollution on amphibian development and decline

“…However, in many cases, toxicant levels found in nature may be substantially lower than levels found to cause mortality (Cooke 1977). For example, leopard frog (Rana pipiens) embryos exposed to 10 ppb methylmercury begin to show adverse effects (deformities) in the laboratory, but environmental water concentrations rarely exceed 0.1 -1 ppb (Dial 1976 However, sublethal exposure may lead to mortality …”

Review of environmental factors influencing the decline of Australian frogs

“…Byrne et al (1975) did report that mercury was present at various levels in four American amphibian species but did not study toxicity or teratogenic effects. Dial (1976) examined the effects of 0.5-220 ppb methyl mercury upon R. pipiens embryos and observed that at cleavage (the most sensitive stage), animals died when exposed to 40 ppb. Concentrations of 5-20 ppb caused abnormalities and fatalities in varying stages of development, which were related to exposure time and concentrations.…”

Disappearing frogs

“…Duellman and Trueb (1986) found that the membranes surrounding frog eggs were highly absorptive to a number of water-borne substances, including toxic pollutants such as insecticides. Tadpoles were similarly vulnerable to the effects of contaminants in the water (e.g., Dial 1976;Ghate and Mulherlar 1980;Clark and Hall 1985). Even the adult frog, because of its soft permeable skin, is not safe from the effects of water and air pollution (e.g., Freda 1986;Freda and McDonald 1990).…”