One of the most controversial environmental issues of the past decade now seems to have been solved thanks to the consolidated efforts of one U.S. and one U.K. researcher.

In the late 1980s and early 1990s, researchers started getting reports of numerous deformed wild frogs and toads. Many of them missed a limb partly or completely, while others – even more strikingly – had extra legs or extra arms.

The reason behind the deformities became a hot-potato, with some people suspecting chemical pollution or increased UV-B radiation (brought on by the thinning of the ozone layers), while others leaned towards predators or parasites.

“There was a veritable media firestorm, with millions of dollars of grant money at stake,” says Stanley Sessions, an amphibian specialist and professor of biology at Hartwick College, in Oneonta, New York.
Eventually, professor Sessions and other researchers managed to show that many amphibians with extra limbs were actually infected by small parasitic flatworms called Riberoria trematodes. These nematodes burrow into the hindquarters of tadpoles and rearrange the limb bud cells. This interferes with limb development, and in some cases the result is an extra arm or leg.

While these findings explained the conspicuous presences of additional limbs, it wasn’t enough to solve the mystery of the leg- and armless amphibians.

“Frogs with extra limbs may have been the most dramatic-looking deformities, but they are by far the least common deformities found,” Sessions explains. “The most commonly found deformities are frogs or toads found with missing or truncated limbs, and although parasites occasionally cause limblessness in a frog, these deformities are almost never associated with the trematode species known to cause extra limbs.”

To investigate the conundrum, Sessions teamed up with UK researcher Brandon Ballengee of the University of Plymouth. As a part of a larger research project, the two scientists placed tadpoles in aquariums and added various predators to see if any of them could be responsible for this type of injuries.

As it turned out, three different species of dragonfly nymph happily attacked and nicked of the hind legs of the tadpoles; feasting on the tasty legs without actually killing the tadpoles.

“Once they grab the tadpole, they use their front legs to turn it around, searching for the tender bits, in this case the hind limb buds, which they then snip off with their mandibles,” says Sessions. “Often the tadpole is released […],” says Sessions. “If it survives it metamorphoses into a toad with missing or deformed hind limbs, depending on the developmental stage of the tadpole.”

Eating just a leg instead of trying to kill the entire tadpole is beneficial for the dragonfly, since tadpoles develop poison glands in their skin much earlier than those in their hind legs.

Through surgical experiments, Sessions and Ballengee confirmed that losing a limb at a certain stage of a tadpole’s life can lead to missing or deformed limbs in the adult animal. Really young tadpoles are capable of growing a new limb, but they loose this ability with age.

Sessions stresses that the results of his study doesn’t completely rule out chemicals as the cause of some missing limbs, but says that this type of “selective predation” by dragonfly nymphs is now by far the leading explanation.

“Are parasites sufficient to cause extra limbs?,” he asks. “Yes. Is selective predation by dragonfly nymphs sufficient to cause loss or reduction of limbs. Yes. Are chemical pollutants necessary to understand either of these phenomena? No.”

You can find Sessions and Ballengee’s study in the Journal of Experimental Zoology Part B: Molecular and Developmental Evolution.

A number of Japanese citizens living in the Ishikawa Prefecture have made some strange observations during the last few days.

Nanao, Japan, June 4

During the evening of June 4, a man suddenly heard a plopping sound in a parking lot of the Nakajima citizens centre in Nanao. When he looked back, he was surprised to see tadpoles scattered over a car and on the ground. According to Kiwamu Funakura, 36, an official at the centre who went to the parking lot at the time, about 100 tadpoles, each 2 or 3 centimetres long, were scattered over an area measuring about 200 square meters.

Hakusan, Japan, June 6

Two days later and roughly 70 kilometres southwest of Nanao, a similar event occurred in another parking lot. In the morning of June 6, between 20 and 30 dead tadpoles were found on a car windshield and other places in a Hakusan parking lot, with some reportedly having lost their original shape.

Nanao, Japan, June 8

Back in Nano, Takeshi Kakiuchi, 62, a member of the Nanao Municipal Assembly, found six tadpoles on his car and on the ground around his home Monday morning. Kakiuchi’s home is located roughly 4 km from the Nakajima citizens centre.

Nakanotomachi, Japan, June 9

On Tuesday evening, Yukio Oumi, 78, found 13 fish on the back of his truck and on the ground around his home in Nakanotomachi. The fish are believed to be crucian carps, each measuring about 3 centimetres.

Fish and frogs falling from the sky?

The reason behind the strange events has not yet been determined, and the Kanazawa Local Meteorological Observatory says it has no information that any tornadoes occurred on the days when the animals appeared.

Susumu Aiba, professor at the Kanazawa Institute of Technology, says that small-scale wind gusts may have swept over limited areas, swirling up water and any creatures living in it. If the gusts were small enough, they may have been able to avoid meteorological detection.

Another suggestion comes from Kimimasa Tokikuni, the head of the Ishikawa prefectural branch of the Japanese Society for Preservation of Birds. “Birds such as herons or umineko that had these tadpoles in their mouths or gorges might have dropped them because they were startled by something while flying,” he says. All the places where animals seem to have fallen from the sky during the last few days are located in close vicinity to flooded rice paddies, so birds may have caught tadpoles and small fish there in an attempt to feed their young. Herons and other water fowl are in the middle of their breeding period right now.