A study in Japan has found that butterflies exposed to radiation from the Fukushima nuclear power plant accident in Japan have higher rates of mutation, according to a report from the BBC. The study was conducted by a team of Japanese researchers examining wild butterflies caught near the power plant and then bred in radiation-free laboratories far from the accident site.

Japan was struck by a magnitude 9.0 earthquake on March 11, 2011. The earthquake caused damage at the Fukushima Daiichi nuclear power plant in Fukushima, Japan, which released radiation into the environment surrounding the plant.

In an effort to study the effects of this radiation, butterflies were collected from ten sites in Japan, including sites near Fukushima, by a team led by researcher Joji Otaki of the University of the Ryukyus in Okinawa. The butterflies collected at this point would have been in a winter larval stage at the time of the accident. The researchers then bred the butterflies in laboratories far from the accident site, and far from any chance of subsequent radiation exposure. They found that succeeding generations bred from the Fukushima butterflies showed a higher rate of genetic abnormalities and mutations, including malformed antennae.

The team also collected butterflies from the ten sites six months later, and found that in this later collection the Fukushima butteflies still showed a higher rate of mutations. In fact, the second set of butterflies collected later had  more than double the mutation rate of the original set. The researchers attributed this higher rate of mutations to a combination of consumption of radioactively contaminated food and  to the expression of radiation-induced mutations that were caused, but not expressed, in the parent generation.

Professor Otaki says that the butterflies, a species of grass blue butterflies, was chosen because it has a long and well-studied history of being used as an environmental indicator. He says, “We had reported the real-time field evolution of color patterns of this butterfly in response to global warming before, and [because] this butterfly is found in artificial environments – such as gardens and public parks – this butterfly can monitor human environments.”

Tim Mousseau of the University of South Carolina, who specializes in the impact of radiation accidents on animals and plants, told the BBC, “This study is important and overwhelming in its implications for both the human and biological communities living in Fukushima. These observations of mutations and morphological abnormalities can only be explained as having resulted from exposure to radioactive contaminants.”

The paper is available from the journal Nature Scientific Reports.