Radioactive fallout from the Fukushima nuclear disaster caused mutations to butterflies, researchers show in a new study.

The study, published Thursday in the journal Scientific Reports, documented physiological and genetic damage to the pale grass blue Zizeeria maha, which was overwintering as larvae when the nuclear disaster began.

The researchers write that "the Zizeeria maha population in the Fukushima area is deteriorating physiologically and genetically. Most likely, this deterioration is due to artificial radionuclides from the Fukushima Dai-ichi NPP, as suggested by our field work and laboratory experiments."

"It has been believed that insects are very resistant to radiation," lead researcher Joji Otaki from the University of the Ryukyus, Okinawa, told BBC news.

"In that sense, our results were unexpected," said Otaki.

As ABC News explains the experiment,

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.

Researchers noted other abnormalities including malformed antennae and appendages.

The researchers say the information found in their study "has invaluable implications for the possible future effects of radiation on animals."