The textbook example of neoteny: it breeds while still in its larval body
Sexually mature yet retaining gills, tail fin and an entirely aquatic life
Science · Biology
Axolotl
Described 1798; genome sequenced 2018
Biology — amphibians, neoteny and regeneration
The axolotl is a Mexican salamander that never grows up. Unlike almost all other amphibians it keeps its larval form for life — feathery external gills, a finned tail, an aquatic existence — while becoming sexually mature, a condition called neoteny or paedomorphosis. It is famous for regeneration: it regrows amputated limbs, tail, jaws, skin, parts of the heart and even sections of the brain and spinal cord, complete and without scarring. Wild animals survive only in the remnant canals and wetlands of Xochimilco on the southern edge of Mexico City, where they are critically endangered, even as hundreds of thousands live in laboratories and aquaria worldwide. Its genome, decoded in 2018, was the largest ever assembled at the time, around ten times the size of a human's.
How to spot them
- ▸A salamander that keeps its feathery external gills and larval body for life while breeding — the textbook example of neoteny
- ▸Regrows amputated limbs, tail, jaw, heart tissue and spinal cord without scarring
- ▸Wild population confined to the canals of Xochimilco, Mexico City; critically endangered
- ▸Its genome, about 32 billion base pairs, was the largest ever sequenced when published in 2018
- ▸Named for an Aztec god who, in the myth, fled the sacrifice by turning into a salamander
Key facts
Regrows an amputated limb complete with bone, muscle and nerves, and without a scar
The wound forms a blastema of dedicated progenitor cells that rebuilds the missing part
Dosing it with thyroid hormone forces it to metamorphose into a land-going salamander
It becomes a form resembling the tiger salamander, its close relative
Named after the dog-headed Aztec god of fire and lightning, twin of Quetzalcoatl
Xolotl fled sacrifice by transforming into a salamander in the water
Restricted in the wild to the canals and chinampas of Xochimilco, in southern Mexico City
Critically endangered; surveys have counted only a handful per square kilometre
Its 32-billion-base-pair genome, published in 2018, was the largest ever assembled (2018)
About ten times the human genome, decoded in Vienna, Dresden and Heidelberg
Julio Cortázar wrote a story about a man who becomes one while staring into a Paris aquarium (1956)
Published in the collection Final del juego
Key ideas
- Neoteny is caused by a failure of the thyroid axis: axolotl tissues respond to thyroid hormone normally, but the animal never produces the surge of it that drives metamorphosis in other amphibians.
- Regeneration works through a blastema — cells at the amputation site dedifferentiate into a proliferating mass that carries positional memory of where on the limb it sits, and rebuilds only the missing part.
Quick-fire facts
- •Ambystoma mexicanum (Shaw & Nodder, 1798); family Ambystomatidae, the mole salamanders
- •Its closest relative is the tiger salamander, Ambystoma tigrinum, which metamorphoses normally
- •Grows to about 15–30 cm; wild animals are dark brown-green mottled, while the familiar pink-white leucistic and albino laboratory strains are captive-bred mutants
- •It can regenerate a limb dozens of times over without loss of fidelity, and accepts transplanted tissue readily
- •Listed as Critically Endangered on the IUCN Red List; threats include urban water pollution, habitat loss and introduced carp and tilapia
- •The 2018 genome, published in Nature, runs to roughly 32 billion base pairs, more than ten times the human genome
- •A colony has been maintained at the University of Kentucky's Ambystoma Genetic Stock Center, descended largely from animals shipped to Paris in 1864
- •It appears on Mexico's 50-peso banknote issued in 2021
- •Common misconception: it is not a fish. It is an amphibian, and the 'Mexican walking fish' name is a misnomer
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Sources
- Sergej Nowoshilow et al., Nature 554 (2018) — The axolotl genome and the evolution of key tissue formation regulators
- Max Planck Society — Decoding the axolotl genome