Radiation from Japan's leaking Fukushima nuclear plant has caused mutations in some butterflies and damaged the local environment, though humans seem relatively unaffected, researchers say.
The mutations - including dented eyes and stunted wings - are the first evidence the radiation following last year's tsunami has caused genetic changes in living organisms.
The catastrophic meltdowns in three reactors of Fukushima Dai-ichi power plant after it was damaged by the earthquake and tsunami on March 11, 2011 prompted a public backlash against nuclear power, and forced the government to reassess resource-scarce Japan's entire energy strategy.
But the most visible example of the radiation's effect was claimed by a group of Japanese researchers who found radical physical changes in successive generations of a type of butterfly.
They said the threat to humans was unclear.
"Our findings suggest that the contaminants are causing ecological damage. I do not know its implication to humans," said Joji Otaki of the University of the Ryukyus in Okinawa, a member of the team that conducted the research.
A separate study, released this week, found very low levels of radioactivity in people who were living near the Fukushima plant when it suffered the meltdowns.
The study, published in the Journal of the American Medical Association, measured caesium levels in 8066 adults and 1432 children and found average doses of less than 1 millisievert, a level considered safe.
It was the first such study measuring internal exposures to caesium in a large number of people.
The research shows contamination decreased over time, particularly among children, in part because more precautions were taken with their food, water and outdoor activity.
"No case of acute health problems has been reported so far. However, assessments of the long-term effect of radiation requires ongoing monitoring of exposure and the health conditions of the affected communities," the report said.
So far, the actual radiation doses inflicted just after the accident are not exactly known, though exposure is thought to be very small, said David Brenner, a radiation physicist at Columbia University, who was not part of the research.
"We do need improved estimates of the radiation dose that people in and near Fukushima prefecture actually received," he said.
"Right now our estimates are based on very, very rough calculations."
The research on the butterflies was published in Scientific Reports, an open-access online journal by the Nature publication group, which provides faster publication and peer review by at least one scientist.
It says pale grass blue butterflies, a common species in Japan, collected from several areas near the Fukushima plant showed signs of genetic mutations.
Other experts said they viewed the research as significant.
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.