Australasian Science: Australia's authority on science since 1938

Ultraviolet Light Exposure Damages Tadpoles

By Stephen Luntz

Depletion of the ozone layer has been revived as an explanation for the extinction of amphibians after the discovery that increased ultraviolet-B radiation makes striped marsh frog tadpoles more vulnerable to predators.

Since 1980 more than 150 species of amphibians have become extinct. This compares poorly with background extinctions of one every 250 years. “With amphibians being the most threatened of all vertebrates, and also important indicators of environmental health, understanding the causes of their declines is critical for their conservation, and possibly the conservation of other species,” says Lesley Alton, a PhD Student at the University of Queensland’s School of Biological Sciences.

There are many possible causes of amphibian decline, and plenty of efforts to allocate blame. UV radiation was initially popular but some trials showed limited damage, and explanations such as the chytrid fungus have become more popular.

In research published in the Proceedings of the Royal Society B, Alton found that “embryos exposed to the lower UV-B treatment hatched as well as those exposed to the higher UV-B treatment. These tadpoles were also the same size and shape, and were able to swim just as fast.”

However, when exposed to predators in the form of shrimp, the sunburnt tadpoles were 30% less likely to survive. Disturbingly this occurred even though the eggs were exposed to just 5% more UV-B than the controls. Increases in ultra­violet radiation vary around the Earth, but ozone depletion has increased UV-B levels by 30% in some regions.

Alton says that the mechanism for the lower survival rate is unclear given that their speed was unaffected. However, she speculates that the cellular damage caused by the UV light creates a greater energy price from the exertion required to escape the shrimp. Repeated exposure may eventually wear the tadpoles down.

Skeptics note that many frog species lay eggs in shaded areas. However, Alton says that the first global amphibian assessment found that extinction rates were greater in high altitude species, where UV light is more intense and tree cover is usually less. She is not aware of any studies examining how much sunlight frogs’ eggs are exposed to in the natural environment, but estimates that the intensity of light used would have been less than one on a scale where UV warnings are not issued below three.

Alston adds that other studies have also found longer-term consequences from UV exposure in terms of growth or developmental abnormalities during metamorphosis.