Axolotl
The axolotl is a species of paedomorphic mole salamander. They mature into the terrestrial adult form without undergoing metamorphosis; the adults remain fully aquatic with obvious external gills. This trait, although somewhat unusual among the majority of amphibians, is not unique. Axolotls may be difficult to distinguish from the larval stage of other neotenic adult mole salamanders, such as mudpuppies or the occasionally paedomorphic tiger salamander.
Axolotls originally inhabited a system of interconnected wetlands and lakes in the Mexican highlands. They were known to inhabit the smaller lakes of Xochimilco and Chalco, and are also presumed to have inhabited the larger lakes of Texcoco and Zumpango. These waterways were mostly drained by Spanish settlers after the conquest of the Aztec Empire, leading to the destruction of much of the axolotl's natural habitat, which is now largely occupied by Mexico City. Despite this, they remained abundant enough to form part of the staple in the diet of native Mexica during the colonial era. Today, due to continued urbanization in Mexico City, which causes water pollution in the remaining waterways, as well as the introduction of invasive species such as tilapia and carp, the axolotls are now near extinction. This can be seen as the species has been listed as critically endangered in the wild, with a decreasing population of around 50 to 1,000 adult individuals, by the International Union for Conservation of Nature, and is listed under Appendix II of the Convention on International Trade in Endangered Species.
A large captive population of axolotls currently exists, with the specimens being used extensively in scientific research for their remarkable ability to regenerate parts of their body, including limbs, gills and parts of their eyes and brains. In general, they are model organisms that are also used in other research matters, and as aquarium technology has developed, they have become a common exhibit in zoos and aquariums, and as an occasional pet in home aquaria. Axolotls are also a popular subject in contemporary culture, inspiring a number of works and characters in the media.
Nomenclature
The term "axolotl" is a Nahuatl word which has been translated variably; it may be interpreted as "water slave", "water servant", "water sprite", "water player", "water monstrosity", "water twin", or "water dog". The word refers to the Aztec God, Xolotl, who holds dominion over multiple aspects such as fire, lightning, the dead and those resurrected, dogs, games, grotesque or ugly beings, and lastly, twins, as he is the twin of Quetzalcōātl.Some sources prefer the term "Mexican axolotl" to refer to this species unambiguously, as "axolotl" may be used for unmetamorphosed individuals of other Ambystoma species, although the word is most commonly used to refer to wild A. mexicanum and captive individuals.
Within Ambystomatidae, the closest relative of the axototl is the Eastern tiger salamander, A. tigrinum..
Description
A sexually mature adult axolotl, at age 18–27 months, ranges in length from ; a size close to is most common and any greater than is rare. Axolotls possess features typical of salamander larvae, including external gills and a caudal fin extending from behind the head to the vent. External gills are usually lost when salamander species mature into adulthood, however, axolotls retain this feature. This is a type of neoteny.Axolotls have wide heads and lidless eyes. Their limbs are underdeveloped and possess long, thin digits. Three pairs of external gill stalks originate behind their heads and are used to move oxygenated water. These are lined with filaments to increase surface area for gas exchange. Four-gill slits lined with gill rakers are hidden underneath the external gills, which prevent food from entering and allow particles to filter through. Males can be identified by their swollen cloacae lined with papillae, while females have noticeably wider bodies when gravid and full of eggs.
Axolotls have barely visible vestigial teeth; other salamanders only develop these during metamorphosis. Their primary method of feeding is by suction, during which their rakers interlock to close the gill slits. Axolotls use their external gills for respiration; buccal pumping may also be used to provide oxygen to their lungs. Buccal pumping can occur in a two-stroke manner that pumps air from the mouth to the lungs or a four-stroke manner which reverses this pathway using compression forces.
The wild type animal is brown or tan with gold speckles and an olive undertone. They also possess the ability to subtly alter their color by changing the relative size and thickness of their melanophores, presumably for camouflage.
Axolotls have four pigmentation genes; when mutated, they create different color variants. The five most common mutant colors are listed below:
- Leucistic: pale pink with black eyes.
- Xanthic: grey, with black eyes.
- Albinism: pale pink or white, with red eyes.
- Melanism: all black or dark blue with no gold speckling or olive tone.
The 32 billion base pair long sequence of the axolotl's genome was published in 2018 and was the largest animal genome completed at the time. It revealed species-specific genetic pathways that may be responsible for limb regeneration. Although the axolotl genome is about 10 times as large as the human genome, it encodes a similar number of proteins, namely 23,251. The size difference is mostly explained by a large fraction of repetitive sequences, but such repeated elements also contribute to increased median intron sizes which are 13, 16 and 25 times that observed in human, mouse and Tibetan frog, respectively.
Physiology
Regeneration
The feature of the axolotl that attracts most attention is its healing ability: the axolotl does not heal by scarring, but is capable of tissue regeneration; entire lost appendages such as limbs and the tail can regrow over a period of months, and, in certain cases, more vital structures, such as the tissues of the eye and heart can be regrown. They can even restore parts of their central nervous system, such as less vital parts of their brains. Moreover, they can also readily accept transplants from other individuals, including eyes and parts of the brain—restoring these "alien organs" to full functionality. In some special cases, axolotls have been known to repair a damaged limb, as well as regenerating an additional one, ending up with an extra appendage that makes them attractive to pet owners as a novelty. Their ability to regenerate declines with age but does not disappear, though in metamorphosed individuals, the ability to regenerate is greatly diminished. Axolotls experience indeterminate growth, meaning their bodies continue to grow throughout their life, and some consider this trait to be a direct contributor to their regenerative abilities. The axolotl is therefore used as a model for the development of limbs in vertebrates. There are three basic requirements for regeneration of the limb: the wound epithelium, nerve signaling, and the presence of cells from the different limb axes. A wound epidermis is quickly formed by the cells to cover up the site of the wound. In the following days, the cells of the wound epidermis divide and grow, quickly forming a blastema, which means the wound is ready to heal and undergo patterning to form the new limb.It is believed that during limb generation, axolotls have a different system to regulate their internal macrophage level and suppress inflammation, as scarring prevents proper healing and regeneration. However, this belief has been questioned by other studies. The axolotl's regenerative properties leave the species as the perfect model to study the process of stem cells and its own neoteny feature. Current research can record specific examples of these regenerative properties through tracking cell fates and behaviors, lineage tracing skin triploid cell grafts, pigmentation imaging, electroporation, tissue clearing and lineage tracing from dye labeling. The newer technologies of germline modification and transgenesis are better suited for live imaging the regenerative processes that occur for axolotls. In a 2025 study, scientists found a new way to insert and activate the genes inside the axolotl's brain and nervous system using special, harmless viruses called . Before this, it was hard for researchers to make specific genes work inside the axolotl, but this discovery allows them to explore how the axolotl's nervous system helps it regrow body parts like its brain and spinal cord. Additionally, they found that the axolotl's nervous system has a unique two-way communication between the brain and eye.
Neoteny
Most amphibians begin their lives as aquatic animals which are unable to live on dry land, often being dubbed as tadpoles. To reach adulthood, they go through a process called metamorphosis, in which they lose their gills and start living on land. The axolotl is unusual in that it has a lack of thyroid-stimulating hormone, which is needed for the thyroid to produce thyroxine for the axolotl to go through metamorphosis; it keeps its gills and lives in water all its life, even after it becomes an adult and is able to reproduce. Neoteny is the term for reaching sexual maturity without undergoing metamorphosis.The genes responsible for neoteny in laboratory axolotls may have been identified; they are not linked to the genes of wild populations, suggesting artificial selection is the cause of complete neoteny in laboratory and pet axolotls. The genes responsible have been narrowed down to a small chromosomal region called met1, which contains several candidate genes.
Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations. Sirens, Necturus mudpuppies, and the troglobitic olm are other examples of neotenic salamanders, although unlike axolotls, they cannot be induced to metamorphose by an injection of iodine or thyroxine hormone.
Neoteny has been observed in all salamander families in which it seems to be a survival mechanism, in aquatic environments only of mountain and hill, with little food and, in particular, with little iodine. In this circumstance, salamanders can reproduce and survive in the form of a smaller larval stage, which is aquatic and requires a lower quality and quantity of food compared to the big adult, which is terrestrial. If the salamander larvae ingest a sufficient amount of iodine, directly or indirectly through cannibalism, they quickly begin metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements, but an ability to disperse across dry land. In fact, in some high mountain lakes, dwarf forms of salmonids can be identified, which are caused by deficiencies in food and, in particular, iodine, which leads to causes cretinism and dwarfism due to hypothyroidism, as it does in humans.