Multi-Level Effects of Low Dose Rate Ionizing Radiation on Southern Toad, Anaxyrus [Bufo] terrestris

Abstract

Despite their potential vulnerability to contaminants from exposure at multiple life stages, amphibians are one of the least studied groups of vertebrates in ecotoxicology, and research on radiation effects in amphibians is scarce. We used multiple endpoints to assess the radiosensitivity of the southern toad (Anaxyrus [Bufo] terrestris) during its pre-terrestrial stages of development -embryonic, larval, and metamorphic. Toads were exposed, from several hours after oviposition through metamorphosis (up to 77 days later), to four low dose rates of 137Cs at 0.13, 2.4, 21, and 222 mGy d-1, resulting in total doses up to 15.8 Gy. Radiation treatments did not affect hatching success of embryos, larval survival, or the length of the larval period. The individual family variation in hatching success of embryos was larger than the radiation response. In contrast, newly metamorphosed individuals from the higher dose-rate treatments had higher mass and mass/length body indices, a measure which may relate to higher post-metamorphic survival. The increased mass and index at higher dose rates may indicate that the chronic, low dose rate radiation exposures triggered secondary responses. Additionally, the increases in growth were linked to a decrease in DNA damage (as measured by the Comet Assay) in red blood cells at a dose rate of 21 mGy d-1 and a total dose of 1.1 Gy. In conclusion, the complex effects of low dose rates of ionizing radiation may trigger growth and cellular repair mechanisms in amphibian larvae.

Fig 1. Hatching success of embryos.

Hatching success for three clutches of Anaxyrus [Bufo] terrestris embryos exposed for 10 days to low dose ionizing radiation from ¹³⁷Cs. The four different radiation treatments (mGy d^-1) are indicated on the x-axis. The error bars indicate standard errors of the means.

Fig 2. Larval survival until metamorphosis.

Percent larval survival until metamorphosis of Anaxyrus [Bufo] terrestris tadpoles exposed to four dose rates of ¹³⁷Cs presented as average (top figure) and per clutch (bottom figure). Because dragonfly predation caused density effects, only bucket with >20–30 tadpoles were included in the analysis. The error bars indicate standard errors of the means.

Fig 3. Body size after metamophosis.

Body Index (mass/snout-to-vent length; g/mm) of metamorphosed Anaxyrus [Bufo] terrestris tadpoles exposed continuously to radiation during their larval period presented as average per bucket (top figure) and as average per egg clutch with standard error (bottom figure). The four ¹³⁷Cs radiation treatments (0.13, 2.4, 21, and 222 mGy d^-1) are indicated on the x-axis. Because dragonfly predation caused density effects, only buckets with >20–30 tadpoles were included in the analysis.

Fig 4. DNA damage in blood cells.

Percentage of migrated DNA in red blood cells of a subsample of individuals of newly metamorphosed Anaxyrus [Bufo] terrestris exposed since embryos to four radiation dose rates of ¹³⁷Cs. Results are presented as average per sample (top figure) and as average per egg clutch with standard error (bottom figure).