TOKYO >> Radiation that leaked from the Fukushima nuclear plant following last year’s tsunami caused mutations in some butterflies — including dented eyes and stunted wings — though humans seem relatively unaffected, researchers say.

The mutations are the first evidence that the radiation has caused genetic changes in living organisms. They are likely to add to concerns about potential health risks among humans though there is no evidence of it yet. Scientists say more study is needed to link human health with the Fukushima disaster.

The catastrophic meltdowns in three reactors of Fukushima Dai-ichi power plant after it was damaged by the 9.0-magnitude 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, which they said was caused by radiation exposure. They also said that the threat to humans — a much larger and longer-lived species — remains unclear.

“Our findings suggest that the contaminants are causing ecological damage. I do not know its implication to humans,” Joji Otaki of the University of the Ryukyus in Okinawa, a member of the research team, told The Associated Press in an email.

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 paper, published in the Journal of the American Medical Association, measured cesium levels in 8,066 adults and 1,432 children and found average doses of less than 1 millisievert, which are considered safe. It was the first such study measuring internal exposures to cesium in a large number of people from the disaster.

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.

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, such as dented eyes, malformed legs and antennae, and stunted wings.

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.

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.