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.

Deteriorating water quality, invasive species and the practise of draining lakes is now bringing the axolotl, a neotenic mole salamander native to central Mexico, to the brink of extinction. According to researchers the species could disappear in just five years and it is currently listed as Critically Endangered on the IUCN Red List of Threatened Species.

“If the axolotl disappears, it would not only be a great loss to biodiversity but to Mexican culture, and would reflect the degeneration of a once-great lake system,” says Luis Zambrano, a biologist at the Universidad Nacional Autónoma de México (UNAM).

The axolotl (Ambystoma mexicanum) is native to no more than two lakes, Lake Xochimilco and Lake Chalco, and Lake Chalco was drained in the 1970s to prevent flooding. The only surviving wild population of axolotl is therefore to be found in Lake Xochimilco; a lake that is rapidly vanishing due to draining efforts. Today, Lake Xochimilco can not even be accurately described as a lake anymore; it is just a series of canals and scattered lagoons. As if this wasn’t bad enough for the axolotl, this salamander also has to combat severe pollution and the introduction of alien species. Mexico City has been pumping its wastewater into the remains of Lake Xochimilco since the 1980s, and both African tilapia and Asian carp have been deliberately introduced to serve as food fish. These alien species compete for prey items with the axolotl and are also fond of eating its eggs.

The exact number of axolotls left in the wild remains unknown, but a survey conducted by Zambrano shows a sharp drop from roughly 1,500 specimens per square mile in 1998 to no more than 25 per square mile in 2008.

One way of improving the situation for the remaining axolotls may be to create a series of axolotl sanctuaries in canals cleared of invasive species, but this would of course require some type of barrier to keep the aliens from returning. A pilot sanctuary is planned to open in Mexico City in 2009.

Repopulating Lake Xochimilco with captive reared specimes has also been suggested, since the axolotl is a popular house pet known to readily breed in captivity. Unfortunately, it is always risky to re-introduce a species into the wild since captive specimens may be carriers of genetic problems or hosts of malicious organisms like parasites, viruses and bacteria.

During the Aztec empire, the axolotl was an important food item as well as an integral part of numerous myths and legends. According to legend, the Aztec god Xolotl – who was in charge of death, lighting and monstrosities – suspected the other gods of plotting to banish or even kill him, and turned himself into a salamander in order to fool them. He moved to Lake Xochimilco where he could stay hidden from the other gods and Xolotl became a-xolotl, blessing the Aztecs with an important source of food. If you visit markets in Mexico City today you can still find axolotls being sold as food. Cooked axolotl is however becoming a more and more scarce dish on Mexican dinner tables, mainly because fishermen almost never find them.

Outside Mexico, the axolotl is more popular as a pet than a snack. It can today be found in pet shops world-wide, sometimes being offered under other names such as Wooper Rooper or Mexican Water Monster. The axolotl is also famous for its ability to regenerate most body parts and is extensively used in scientific research on regeneration and evolution.