Japanese scientists say "abnormalities" detected in the country's butterflies may be a result of radioactive fallout from the Fukushima nuclear disaster last year. In a study published in Scientific Reports, an online journal, researchers say "artificial radionuclides" from the Fukushima Daiichi power plant caused "physiological and genetic damage" to pale grass blue butterflies.

Scientists first began tracking common butterflies around the nuclear plant two months after the disaster. They collected 121 insects, and found 12 percent of them had unusually small wings. That number jumped more than 5 percent when butterflies collected from the plant site had offspring of their own.

In another group of butterflies collected six months after the disaster, scientists found 28 percent had "abnormal" traits. That number nearly doubled among the second generation born.

"At the time of the accident, the populations of this species were overwintering as larvae and were externally exposed to artificial radiation," the researchers wrote in their study. "It is possible that they ate contaminated leaves during the spring and were thus also exposed to internal radiation."

It has been 17 months after the worst nuclear disaster since Chernobyl, and its effects on human health have largely been considered minimal, with no radiation-related deaths or illnesses reported so far. But traces of radioactive cesium exceeding government safety levels have been detected in seafood off the Fukushima coast, limiting the catch for fisherman there.

Tiny amounts of cesium of 137 and cesium 134 were detected in more than a dozen bluefin tuna caught near San Diego in August last year. The levels were 10 times higher than tuna found in previous years, but well below those the Japanese and US governments considered harmful to human health.