The results show the butterflies were deteriorating both physically and genetically, with the share of those showing abnormalities increasing from 12 percent in the first generation to 18 percent in the second and 34 percent in the third.
To study the genetic changes, the scientists raised the new generations of the butterflies in Okinawa, which has not been affected by the radiation releases, mating each abnormal butterfly with one unaffected by such changes.
The researchers also demonstrated the effects of internal exposure to radiation by feeding leaves from plants from the area near the Fukushima nuclear plant to the butterfly larvae.
"The possible risk of internal exposure from ingestion should be investigated more accurately in the near future," it said.
Although the damage is irreversible, the species could develop resistance to the radiation, Otaki said. "In that case, we will observe adaptive evolution," he said.
The research appears to be "a very thorough study," said Jim T. Smith of Britain's University of Portsmouth, also another outsider. The replication of the mutations under lab conditions further supports the report's findings, he said in a telephone interview.
However, he said he would be "very, very wary of trying to extrapolate those results to humans."
Even in the area near Chernobyl in the Ukraine, the absence of humans who left after the 1986 nuclear accident there has actually benefited local fauna in the long run, said Smith, who has conducted research on radiation's effects on aquatic insects.
Associated Press writer Mari Yamaguchi in Tokyo contributed to this report.
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