Deformed toads, each a product of 'selective predation'
Scientists think they have resolved one of the most controversial environmental issues of the past decade: the curious case of the missing frogs' legs.
Around the world, frogs are found with missing or misshaped limbs, a striking deformity that many researchers believe is caused by chemical pollution.
However, tests on frogs and toads have revealed a more natural, benign cause.
The deformed frogs are actually victims of the predatory habits of dragonfly nymphs, which eat the legs of tadpoles.
In the late 1980s and early 1990s, researchers started getting reports of numerous wild frogs or toads being found with extra legs or arms, or with limbs that were partly formed or missing completely.
The cause of these deformities soon became a hotly contested issue.
Some researchers believed they might be caused naturally, by predators or parasites.
Others thought that was highly unlikely, fearing that chemical pollution, or UV-B radiation caused by the thinning of the ozone layer, was triggering the deformations.
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
Biologist Stanley Sessions describes the dining habits of dragonfly nymphs
"Deformed frogs became one of the most contentious environmental issues of all time, with the parasite researchers on one side, and the 'chemical company' as I call them, on the other," says Stanley Sessions, an amphibian specialist and professor of biology at Hartwick College, in Oneonta, New York.
"There was a veritable media firestorm, with millions of dollars of grant money at stake."
After a long period of research, Sessions and other researchers established that many amphibians with extra limbs were actually infected by small parasitic flatworms called Riberoria trematodes.
These creatures burrow into the hindquarters of tadpoles where they physically rearrange the limb bud cells and thereby interfere with limb development.
"But that was not end of the story," says Sessions.
"Frogs with extra limbs may have been the most dramatic-looking deformities, but they are by far the least common deformities found," he 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."
The mystery of what causes frogs to have missing or deformed limbs remained unsolved until Sessions teamed up with colleague Brandon Ballengee of the University of Plymouth, UK. They report their findings in the Journal of Experimental Zoology Part B: Molecular and Developmental Evolution.
For a decade, Ballengee and Sessions have collaborated on a series of art and science projects that image amphibians' bodies to show the detail within, the most recent of which is funded by the Arts Catalyst organisation, based in London.
A dragonfly nymph eats the leg of a tadpole
As part of this work, Ballengee and Richard Sunter, the official Recorder of Reptiles and Amphibians in Yorkshire, spent time during the summers of 2006 to 2008 surveying the occurrence of deformities in wild amphibians at three ponds in the county.
In all, they found that between 1.2% and 9.8% of tadpoles or metamorphosed toads at each location had hind limb deformities. Three had missing eyes.
"We were very surprised when we found so many metamorphic toads with abnormal limbs, as it was thought to be a North American phenomenon," says Ballengee.
While surveying, Ballengee also discovered a range of natural predators he suspected could be to blame, including stickleback fish, newts, diving beetles, water scorpions and predatory dragonfly nymphs.
So Ballengee and Sessions decide to test how each predator preyed upon the tadpoles, by placing them together in fish tanks in the lab.
None did, except three species of dragonfly nymph.
Crucially though, the nymphs rarely ate the tadpoles whole. More often than not, they would grab the tadpole and chew at a hind limb, often removing it altogether.
"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.
Remarkably, many tadpoles survive this ordeal.
"Often the tadpole is released and is able to swim away to live for another day," says Sessions. "If it survives it metamorphoses into a toad with missing or deformed hind limbs, depending on the developmental stage of the tadpole."
If tadpoles are attacked when they are very young, they can often regenerate their leg completely, but this ability diminishes as they grow older.
The researchers confirmed this by surgically removing the hind limbs of some tadpoles and watching them grow. These tadpoles developed in an identical way to those whose limbs had been removed by dragonflies, confirming that losing a limb at a certain stage of a tadpole's development can lead to missing or deformed limbs in adulthood.
Adult amphibians with one one hind limb appear able to live for quite a long time, Sessions says, explaining why so many deformed frogs and toads are discovered.
Why do the dragonflies like to eat the hind legs only?
A normal and a legless toad stained to reveal details of the skeletal deformities (bones purple, cartilage blue, soft tissues transparent).
As toad tadpoles mature, they develop poison glands in their skin much earlier than those in their hind legs, which could make the hind legs a far more palatable meal.
The front legs of tadpoles also develop within the gill chamber, where they are protected.
Sessions is careful to say that he doesn't completely rule out chemicals as the cause of some missing limbs. But 'selective predation' by dragonfly nymphs is now by far the leading explanation, he says.
"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."