Today’s axolotls are experiencing extirpation, but scientists and pet enthusiasts are saving them from true extinction. Why? Axolotls have long fascinated the learned and laymen alike, thanks to the animal’s powers of regenerating and self-healing. While all organisms can regenerate to some degree, the axolotl’s capabilities are far more advanced.
Historical documentation cites Spallanzani, in 1768, as the first Western observer of an axolotl’s complete regeneration of tail and limb. Then in 1804, renowned naturalist Alexander von Humboldt collected the first wild specimens, shipping them to Europe. By 1863, axolotls first debuted in official science laboratories when a French expedition shipped 34 of them to the Natural History Museum in Paris. French zoologist Auguste Duméril received six of those original 34. His successfully bred lines launched the global axolotl diaspora as he shared line progeny with international colleagues.
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Present-day wild axolotls have not fared well. Despite its status as “the most widely distributed amphibian around the world in pet shops and labs,” the wild axolotl is nearing extirpation, said Richard Griffiths, an ecologist at the University of Kent in Canterbury, UK, to Scientific American. The International Union for the Conservation of Nature (IUCN) catalogs axolotls on the Red List, delineating threat risks from habitat loss, water pollution, fierce competition with non-native species, predation by invasive species, climate change-induced droughts, disease and inbreeding. Only popularity in both the pet and laboratory industries keeps axolotls from all-out extinction.
But what are axolotls exactly? They are amphibians in the Ambystomatidae family of mole salamanders and are identified by neoteny. That is, they retain larval traits or juvenile characteristics. What distinguishes them from juveniles of other salamander species is retention of their unmetamorphosed larva appearance, even as adults.
Peculiarly, they appear as “sexually mature tadpoles,” spending entire lives underwater, veritably breeding in that form, unlike other salamanders that metamorphose and crawl onto land. Of course, with iodine, axolotls can be induced into metamorphosis, even developing into bona fide salamanders that phenotypically resemble tiger salamanders. However, metamorphosed axolotls experience much-diminished lifespans compared to neotenous counterparts.
The most widely recognized axolotl is Ambystoma mexicanum, whose only remaining natural habitat is Mexico City’s canal system. Legend says while the Aztecs built Tenochtitlan, their capital, they discovered, in the lake, a large, feathery-gilled salamander. They named it after Xolotl, their fire and lightning deity. As Quetzalcoatl’s twin brother, Xolotl enjoyed shapeshifting powers. Live Science explains how Xolotl apparently “transformed into a salamander, among other forms, to avoid being sacrificed so the sun and moon could move in the sky,” showcasing even then how axolotls captivated the fancy of this ancient civilization and garnered placement in their pantheon.
Regrettably, Mexico’s endemic axolotls are dwindling drastically. From JSTOR Daily, the “first robust count of axolotls” in their natural habitat amounted to an estimated 6,000 axolotls per square kilometer. That population survey transpired in 1998. By 2015, the population plummeted, numbering “only 35 per square kilometer,” therefore revealing precipitous extirpation.
Formerly at the food chain apex, what changed for axolotls? Development sprawl, tourism and recreational use drained the natural water levels of axolotl habitats. Whatever water has remained is polluted by litter and offal, pesticides and non-organic fertilizers, heavy metals and toxic chemical runoff. Because axolotls breathe through their highly permeable skin, they are incredibly susceptible to pollution, which adversely affects axolotl health, growth and development.
Moreover, from the 1970s to 1980s, tilapia and carp, non-native fish that reproduce faster than can be caught, were released into the canals, disrupting local food webs and ecosystems. Tilapia and carp also forage around the canals’ aquatic plants, where axolotls lay eggs, further reducing offspring numbers even more in a prelude to the grim species-level extirpation of wild axolotls.
Additionally, climate change and severe weather, which the IUCN acknowledges as threats to the axolotl, in turn, perpetuate drought conditions, again decimating the axolotls’ natural habitat. It is feared only a few hundred axolotls remain in the wild.
Whereas wild axolotls might not all be rescued from Mexico’s canals, the species are nonetheless thriving in captive breeding programs at universities and scientific laboratories as well as in private aquariums of pet owners. Indeed, Scientific American documented that “tens of thousands can be found in home aquariums and research laboratories around the world. They are bred so widely in captivity that certain restaurants in Japan even serve them up deep-fried.”
In fact, from Duméril’s progeny lines, scientists continue to successfully breed axolotls to this day. This accounts for the Journal of Experimental Zoology’s assertion of axolotls being “the oldest self-sustaining laboratory animal population.” Duméril’s generosity ignited Europe and America’s axolotl breeding craze, says Scientific American, giving way to the 1930s breeding stock at the University at Buffalo, New York. That stock was then hybridized with both wild axolotls and tiger salamanders (Ambystoma tigrinum). As that hybridized lineage population flourished, it was then relocated to the University of Kentucky – Lexington, where the current Ambystoma Genetic Stock Center has evolved into academia’s epicenter of global axolotl breeding. From there, tens of thousands of axolotl embryos are sent to contemporary research labs.
For more than 150 years, the scientific community has remained intrigued by axolotl regenerative abilities, which are quite unlike those in mammals. Axolotls, for one, can completely regenerate amputated limbs, even after multiple amputations, with each new limb as functional as the original.
From American Zoologist, their cells ‘know’ what to regrow, even supernumerary limbs when regenerating tissue grafted onto other body quadrants. Should axolotls have damaged internal organs, they would be regrown. Crushed spinal cords can be fully repaired as well. In other words, no other animal comes close to axolotl regeneration and self-healing. Likewise, Science journal has documented axolotls readily receiving transplanted heads.
By the same token, back in 1865, “Duméril’s second generation of axolotls spontaneously transformed into air-breathing adults.” This hidden developmental stage led to 20th-century researchers discovering thyroid hormones, explains Nova.
Nowadays, the axolotl genome has been sequenced, and Max Planck Institute reveals it “is more than ten times larger than the human genome.” Besides being the largest genome decoded so far, the axolotl’s genome contains an “enormous number of large repetitive sequences.” Could these account for axolotl plasticity in developmental, regenerative and evolutionary assets, such as why it retains its tadpole-like qualities into adulthood? By studying the axolotl genome, scientists hope for ample opportunities to understand the gene regulation processes that make regeneration possible. These findings would revolutionize medicine and aging science.
Meanwhile, laboratory-bred axolotls are still vulnerable to loss — from inbreeding, disease or laboratory disasters like fires. Crossbreeding is not without its challenges. For more genetic diversity, the lab-bred stock must be crossbred with wilds, but wilds are being extirpated, making their collection difficult. That leaves hybridization with tiger salamanders, but the true axolotl gene pool gets diluted as a result.
With numbers in the wild not likely to rebound without help, it’s imperative to strengthen axolotl conservation efforts. Perhaps establishing sanctuaries and ecological refuges in the wild, as well as enacting more enforceable legislation, can help save axolotls from extirpation. No matter the case, axolotl conservation will require heavy human involvement. Once this exceptional organism goes extinct, the world loses out on all the knowledge they can provide.
Via Nova and Scientific American
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