Axolotls originally inhabited a system of interconnectedwetlands and lakes in the Mexicanhighlands; they were known to inhabit the smaller lakes ofXochimilco andChalco, and are also presumed to have inhabited the larger lakes ofTexcoco andZumpango. These waterways were mostlydrained bySpanish settlers after theconquest of the Aztec Empire, leading to thedestruction of much of the axolotl's natural habitat, which is now largely occupied byMexico City. Despite this, they remained abundant enough to form part of thestaple in the diet of nativeMexica during thecolonial era.[5] Due to continued urbanization in Mexico City, which causeswater pollution in the remaining waterways, as well as the introduction of invasive species such astilapia andcarp, the axolotl is nearextinction, the species being listed ascritically endangered in the wild, with a decreasing population of around 50 to 1,000 adult individuals, by theInternational Union for Conservation of Nature (IUCN) and is listed underAppendix II of the Convention on International Trade in Endangered Species (CITES).[2]
A large captive population of axolotls currently exist, with the specimens beingused extensively in scientific research for their remarkable ability toregenerate parts of their body, including limbs, gills and parts of their eyes and brains. In general, they aremodel organisms that are also used in other research matters, and asaquarium technology developed, they have become a common exhibit inzoos andaquariums, and as an occasional pet in home aquaria. Axolotls are also a popular subject in contemporary culture, inspiring a number ofworks and characters in media.
This sectionneeds expansion with: taxonomic history (look at those synonyms). You can help byadding to it.(June 2025)
The term "axolotl" is aNahuatl 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".[6][7][8] The word refers to theAztec God,Xolotl, who holds dominion over multiple aspects such asfire,lightning,the dead and thoseresurrected, dogs, games, grotesque or ugly beings, and twins, as he is the twin ofQuetzalcōātl.[9][6]
Some sources prefer the term "Mexican axolotl" to refer to this species unambiguously, as "axolotl" may be used for unmetamorphosed individuals of otherAmbystoma species,[8][10] though the word is most commonly used to refer to wildA. mexicanum and captive individuals.
Head of a dark-colored, perhaps wild type, axolotl
A sexually mature adult axolotl, at age 18–27 months, ranges in length from 15 to 45 cm (6 to 18 in), although a size close to 23 cm (9 in) is most common and greater than 30 cm (12 in) is rare. Axolotls possess features typical of salamander larvae, includingexternal gills and a caudal fin extending from behind the head to the vent.[11][12] External gills are usually lost when salamander species mature into adulthood, although the axolotl maintains this feature.[13] This is due to their neoteny, where axolotls are much more aquatic than other salamander species.[14]Their heads are wide, and their eyes arelidless. Their limbs are underdeveloped and possess long, thin digits. Three pairs ofexternal gill stalks (rami) originate behind their heads and are used to move oxygenated water. The external gill rami are lined with filaments (fimbriae) to increase surface area for gas exchange.[13] Four-gill slits lined withgill rakers are hidden underneath the external gills, which prevent food from entering and allow particles to filter through. Males can be identified by their swollencloacae lined with papillae, while females have noticeably wider bodies whengravid and full of eggs.
Buccal pumping
Axolotls have barely visiblevestigial teeth, which develop during metamorphosis. The primary method of feeding is bysuction, during which their rakers interlock to close the gill slits. External gills are used for respiration, althoughbuccal pumping (gulping air from the surface) may also be used to provide oxygen to their lungs.[13] Buccal pumping can occur in a two-stroke manner that pumps air from the mouth to the lungs, and with four-stroke that reverses this pathway with compression forces.
Thewild type animal (the "natural" form) is brown or tan with gold speckles and an olive undertone, and possess an ability to subtly alter their color by changing the relative size and thickness of theirmelanophores, presumably forcamouflage.[15] Axolotls have four pigmentation genes; when mutated, they create different color variants.[citation needed] The five most common mutant colors are listed below:[clarification needed]
Melanism: all black or dark blue with no gold speckling or olive tone.
In addition, there is wide individual variability in the size, frequency, and intensity of the gold speckling, and at least one variant develops a black and whitepiebald appearance upon reaching maturity.[16] Becausepet breeders frequentlycross the variant colors, doublehomozygous mutants are common in thepet trade, especially white/pink animals with pink eyes that are double homozygous mutants for both the albino and leucistic genes.[17]
Melanophores of a larva axolotl
The 32 billionbase pair long sequence of the axolotl'sgenome was published in 2018 and was the largest animal genome completed at the time. It revealed species-specificgenetic pathways that may be responsible for limb regeneration.[18] Although the axolotl genome is about 10 times as large as thehuman genome, it encodes a similar number of proteins, namely 23,251[18] (the human genome encodes about 20,000 proteins). The size difference is mostly explained by a large fraction ofrepetitive sequences, but such repeated elements also contribute to increased medianintron sizes (22,759 bp) which are 13, 16 and 25 times that observed in human (1,750 bp), mouse (1,469 bp) andTibetan frog (906 bp), respectively.[18]
The feature of the axolotl that attracts most attention is its healing ability: the axolotl does not heal byscarring, but is capable oftissue 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 andheart can be regrown.[19][20] They can restore parts of theircentral nervous system, such as less vital parts of their brains. They can also readily accepttransplants from other individuals, including eyes and parts of the brain—restoring these alien organs to full functionality. In some 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 anovelty. Their ability to regenerate declines with age but does not disappear, though in metamorphosed individuals, the ability to regenerate is greatly diminished. Axolotls experienceindeterminate growth, their bodies continuing to grow throughout their life, and some consider this trait to be a direct contributor to their regenerative abilities.[21] The axolotl is therefore used as a model for the development of limbs in vertebrates.[22] There are three basic requirements for regeneration of the limb: the woundepithelium, nerve signaling, and the presence of cells from the different limb axes.[23] 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 ablastema, 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 internalmacrophage level and suppressinflammation, as scarring prevents proper healing and regeneration.[24] However, this belief has been questioned by other studies.[25] The axolotl's regenerative properties leave the species as the perfect model to study the process ofstem cells and its own neoteny feature. Current research can record specific examples of these regenerative properties through tracking cell fates and behaviors, lineage tracing skintriploid cellgrafts, pigmentation imaging,electroporation, tissue clearing and lineage tracing from dye labeling. The newer technologies ofgermline modification andtransgenesis are better suited for live imaging the regenerative processes that occur for axolotls.[26] In a 2025 study, scientist found a new way to insert and activate the genes inside the axolotl’s brain and nervous system using special, harmless viruses calledAdeno-Associated Viruses (AAVs). 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.[27]
In animals with functioning thyroid glands, iodine in the form of iodide is selectively gathered into the colloid of the thyroid. Inside the colloid, iodide is reduced to elemental iodine (I2), which reacts with thetyrosyl residues ofthyroglobulin. Two iodinated tyrosyl residues are conjugated together. When they are cleaved from the thyroglobulin chain, thyroid hormone is obtained.[28]
Diiodotyrosine, an analogue of the iodinatedthyroglobulin precursor in thyroxine biosynthesis, causes metamorphosis in axolotls that have their thyroids removed.[29]Lugol's solution, which contains both iodide and I2, triggers metamorphosis when injected.[30] This is because diiodotyrosine and thyroxine is produced when I2 reacts with proteins other than thyroglobulin. If given in a bath instead of injected, I2 has no effect on axolotls.[31]Iodide, which does not react with proteins, does not trigger metamorphosis. It does speed up the rate of metamorphosis, once it has been triggered by thyroid hormone extract.[32]
Most amphibians begin their lives asaquatic animals which are unable to live on dry land, often being dubbed astadpoles. To reachadulthood, they go through a process calledmetamorphosis, in which they lose their gills and start living on land. The axolotl is unusual in that it has a lack ofthyroid-stimulating hormone, which is needed for thethyroid to producethyroxine 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 isable to reproduce. Neoteny is the term for reaching sexual maturity without undergoing metamorphosis.[33]
The genes responsible for neoteny in laboratory axolotls may have been identified; they are not linked to the genes of wild populations, suggestingartificial selection is the cause of complete neoteny in laboratory and pet axolotls.[34] The genes responsible have been narrowed down to a small chromosomal region calledmet1, which contains several candidate genes.[35]
Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations.Sirens,Necturus mudpuppies, and the troglobiticolm 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 allsalamander 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 way, 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 throughcannibalism, they quickly begin metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements, but an ability to disperse across dry land.[36] In fact, in some high mountain lakes there live dwarf forms ofsalmonids that are caused by deficiencies in food and, in particular, iodine, which causescretinism anddwarfism due tohypothyroidism, as it does in humans.
The axolotl's body has the capacity to go through metamorphosis if given the necessary hormone, but axolotls do not produce it, and must obtain it from an external source, after which an axolotl undergoes an induced metamorphosis and begins living on land.[37] Research on this phenomenon has been performed for over a century; in modern laboratory conditions, metamorphosis is reliably induced by administering either the thyroid hormonethyroxine or athyroid-stimulating hormone. The former is more commonly used.[35]
Metamorphosed axolotls
In the absence of induced metamorphosis, larval axolotls start absorbing iodide into their thyroid glands at 30 days post-fertilization. Larval axolotls do produce thyroid hormone from iodide, but the amount appears highly variable. Adult axolotls do not produce thyroid hormone unless metamorphism is triggered.[38]
An axolotl undergoing metamorphosis experiences a number of physiological changes that help them adapt to life on land. These include increased muscle tone in limbs, the absorption of gills and fins into the body, the development of eyelids, and a reduction in the skin's permeability to water, allowing the axolotl to stay more easily hydrated when on land. The lungs of an axolotl, though present alongside gills after reaching non-metamorphosed adulthood, develop further during metamorphosis.[39] Axolotl that complete their metamorphosis resembles an adultplateau tiger salamander, though the axolotl differs in its longer toes.[citation needed]
Lake Xochimilco, one of the lastrefuges of the wild axolotl
Axolotls are within the same genus as the tiger salamander (Ambystoma tigrinum), being part of itsspecies complex along with all other Mexican species ofAmbystoma.[40][41][42] Their habitat is like that of most[verify] neotenicAmbystoma species; a high-altitude body of water surrounded by a risky terrestrial environment, with these conditions thought to favor the development ofneoteny. However, a population of terrestrialMexican tiger salamanders occupies and breeds in the axolotl's habitat (beingsympatric).[citation needed] The axolotl is native to the freshwaterLakes Xochimilco andChalco in theValley of Mexico (though the species may have also inhabited the larger Lakes ofTexcoco andZumpango),[1] and is currently native only to the former two; Lake Chalco is an unstable ecosystem, often being drained as aflood control measure, and Lake Xochimilco remains a remnant of its former self, existing mainly ascanals. The water temperature in Xochimilco rarely rises above 20 °C (68 °F), although it may fall to 6–7 °C (43–45 °F) in the winter, and perhaps lower.[43] An additional population ofAmbystoma inhabiting theartificial lake atChapultepec was confirmed to contain axolotls; thus the extent of occurrence as of 23 October 2019 was 467 square kilometres (180 sq mi).[1] Overall, the wild axolotl prefers a system ofwater channels and deep-water lakes with abundantaquatic vegetation.[1]
The axolotl is carnivorous, consuming small prey such asmollusks,[44]worms,insects, other arthropods,[44] and small fish in the wild. Axolotls locate food by smell, and will "snap" at any potential meal,sucking the food into their stomachs with vacuum force.[45] The wild axolotl is thought to reachsexual maturity at 1.5 years of age, with ageneration length of around 5.5 years.[1]
Axolotls are only native to the Mexican Central Valley. Although the population once extended through most lakes and wetlands in this region, its habitat is now limited to Lake Xochimilco as a result of the expansion of Mexico City, and is under pressure by thecity's growth. Lake Xochimilco is not a large body of water, but a small series of artificialchannels, small lakes, and temporarywetlands. The axolotl is on the International Union for Conservation of Nature's Red List of threatened species.
Surveys in 1998, 2003, and 2008 found 6,000, 1,000, and 100 axolotls respectively, per square kilometer in Lake Xochimilco.[46] A four-month-long search in 2013 found no surviving individuals in the wild, but just a month later, two were spotted in a network of canals leading from Xochimilco.[47]
Lake Xochimilco has poorwater quality; tests reveal a low nitrogen-phosphorus ratio and a high concentration ofchlorophyll a, which are indicative of an oxygen-poor environment not well-suited to axolotls. This is caused by the demands of industries such asaquaculture and agriculture in the region, which maintain the water levels of the Lake through inputs of partiallytreated wastewater.[48] Agricultural pesticides from its intensive use eventually enters the lake throughrunoff; these pesticides contain chemical compounds that sharply increase mortality in axolotl embryos and larvae. Of the surviving embryo and larvae, there is also an increase of morphological, behavior, and activity abnormalities.[49]
With such a small native or wild population, there is a large loss ofgenetic diversity. This can be dangerous for the remaining population, causing an increase ininbreeding and decrease infitness andadaptive potential. Studies found indicators of a low interpopulationgene flow and higher rates ofgenetic drift. These are likely the result of multiple "bottleneck" incidents wherein the number of individuals in the population and its genetic diversity are sharply reduced. The offspring produced after bottleneck events have a greater risk of showing decreased fitness and are often less capable of adaptation, and multiple bottleneck events can have disastrous effects for the population, potentially leading to extinction. Studies have found high rates of relatedness that are indicative of inbreeding, which can be especially harmful as it can cause an increase in the presence ofdeleterious, or harmful, genes.[50] The detection ofintrogressed tiger salamander (A. tigrinum) DNA in the laboratory axolotl population raises concerns about the suitability of the captive population as an "ark" for potential reintroduction purposes.[51]
Another factor that threatens the population is theintroduction ofinvasive fish species such asNile tilapia andcommon carp. These eat the axolotls' young, as well as compete for their primary source of food.[52] The presence of these species has changed the behavior of axolotls, causing them to be less active to avoid predation. This reduction in activity greatly impacts the axolotl's foraging and mating opportunities.[53]
There has been little improvement in the conditions of the population of native axolotls over the years.[54][55] Many scientists are focusing their conservation efforts on translocation of captive-bred individuals into new habitats orreintroduction into Lake Xochimilco. TheLaboratorio de Restauracion Ecologica (transl. Laboratory of Ecological Restoration), of theNational Autonomous University of Mexico, has built up a population of 100 captive-bred individuals as of 2021. These axolotls are mostly used for research, but there are plans for a semi-artificial wetland inside the university, to establish a viable population of axolotls within it. Studies have shown that captive-bred axolotls that are raised in a semi-natural environment can catch prey, survive in the wild, and have moderate success in escaping predators. These captive-bred individuals can be introduced into unpolluted bodies of water or back into Lake Xochimilco itself.[56][57]
A 2025 study confirmed the viability of releasing captive-bred axolotls into the wild, with recaptured animals putting on weight compared to their release weight, though this practice risks the loss of the axolotls through predation, as several released axolotls were preyed upon bygreat egrets.[58][59][60]
Six adult axolotls (including a leucistic specimen) were shipped fromMexico City to theJardin des Plantes in Paris in 1863. Unaware of their neoteny,Auguste Duméril was surprised when, instead of the axolotl, he found in the vivarium a new species, similar to the salamander.[verification needed] This discovery was the starting point of research about neoteny. It is not certain thatAmbystoma velasci specimens were not included in the original shipment.[citation needed] Vilem Laufberger in Prague used thyroid hormone injections to induce an axolotl to grow into a terrestrial adult salamander. The experiment was repeated by EnglishmanJulian Huxley, who was unaware the experiment had already been done, using ground thyroids.[61] Since then, experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.[14]
Today, the axolotl is still used in research as amodel organism, and large numbers are bred in captivity. They are especially easy to breed compared to other salamanders in their family, which are rarely captive-bred due to thehusbandry demands of terrestrial life. One attractive feature for research is the large and easily manipulatedembryo, which allows viewing of thefull development of a vertebrate. Axolotls are used inheart defect studies due to the presence of a mutant gene that causes heart failure in embryos. Since the embryos survive almost to hatching with no heart function, the defect is very observable. Further research has been conducted to examine their heart as a model of a single humanventricle and excessive trabeculation.[62] The axolotl is also considered an ideal animal model for the study ofneural tube closure due to the similarities between human and axolotlneural plate and tube formation; the axolotl's neural tube, unlike afrog's, is not hidden under a layer of superficialepithelium.[63] There are also mutations affecting other organ systems some of which are not well characterized and others that are.[64] The genetics of the color variants of the axolotl have also been widely studied.[17]
This sectionis missing information about discussion on captive breeding and reproduction in general. Please expand the section to include this information. Further details may exist on thetalk page.(September 2025)
This animal was X-rayed several times as part of a research project over a period of two years. It was a normal healthy adult (26.3 cm; 159.5 gm) at the beginning of the project and lived several more years after the project ended.[65]
The axolotl is a popularexotic pet like its relative, the tiger salamander (Ambystoma tigrinum). As for allpoikilothermic organisms, lower temperatures result in slower metabolism and a very unhealthily reduced appetite. Temperatures at approximately 16 °C (61 °F) to 18 °C (64 °F) are suggested for captive axolotls to ensure sufficient food intake; stress resulting from more than a day's exposure to lower temperatures may quickly lead to disease and death, and temperatures higher than 24 °C (75 °F) may lead to metabolic rate increase, also causing stress and eventually death.[66][67]Chlorine, commonly added totapwater, is harmful to axolotls. A single axolotl typically requires a 150-litre (40-US-gallon) tank. Axolotls spend the majority of the time at the bottom of the tank.[68]
In captivity, axolotls eat a variety of readily available foods, including trout and salmon pellets, frozen or livebloodworms,earthworms, andwaxworms. Axolotls can also eatfeeder fish, but care should be taken as fish may contain parasites.[69]
Substrates are another important consideration for captive axolotls, as axolotls (like other amphibians and reptiles) tend to ingest bedding material together with food[70] and are commonly prone to gastrointestinal obstruction and foreign body ingestion.[71] Some common substrates used for animal enclosures can be harmful for amphibians and reptiles. Gravel (common in aquarium use) should not be used, and is recommended that any sand consists of smooth particles with a grain size of under 1mm.[70] One guide to axolotl care for laboratories notes that bowel obstructions are a common cause of death, and recommends that no items with a diameter below 3 cm (or approximately the size of the animal's head) should be available to the animal.[72]
There is some evidence that axolotls might seek out appropriately-sized gravel for use asgastroliths[73] based on experiments conducted at the University of Manitoba axolotl colony.[74][75] As there is no conclusive evidence pointing to gastrolith use, gravel should be avoided due to the high risk ofimpaction.[76]
Salts, such asHoltfreter's solution, are often added to the water to prevent infection.[77] Among hobbyists, the process of artificially inducing metamorphosis can often result in death during or even following a successful attempt, and so casual hobbyists are generally discouraged from attempting to induce metamorphosis in pet axolotls.[39] Morphed pet axolotls should be given solid footholds in their enclosure to satisfy their need for land. They should not be given live animals as food.[78]
The species is named after theAztec deityXolotl, the god of fire and lightning, who transformed himself into an axolotl to avoid being sacrificed by fellow gods. They continue to play an outsized cultural role in Mexico.[79] Axólotl also means water monster in theNahuatl language.
^Frost, Darrel R. (2018)."Ambystoma mexicanum (Shaw and Nodder, 1798)".Amphibian Species of the World: an Online Reference. Version 6.0. American Museum of Natural History. Retrieved10 August 2018.
^San Francisco Examiner (San Francisco, California) 7 August 1887, page 9, authored byYda Addis
^McIndoe, Rosemary; Smith, D. G. (1984), Seymour, Roger S. (ed.), "Functional morphology of gills in larval amphibians",Respiration and metabolism of embryonic vertebrates: Satellite Symposium of the 29th International Congress of Physiological Sciences, Sydney, Australia, 1983, Perspectives in vertebrate science, Dordrecht: Springer Netherlands, pp. 55–69,doi:10.1007/978-94-009-6536-2_4,ISBN978-94-009-6536-2
^Pietsch, Paul; Schneider, Carl W. (1985). "Vision and the skin camouflage reactions ofAmbystoma larvae: the effects of eye transplants and brain lesions".Brain Research.340 (1):37–60.doi:10.1016/0006-8993(85)90772-3.PMID4027646.S2CID22723238.
^abFrost, Sally K.; Briggs, Fran; Malacinski, George M. (1984). "A color atlas of pigment genes in the Mexican axolotl (Ambystoma mexicanum)".Differentiation.26 (1–3):182–188.doi:10.1111/j.1432-0436.1984.tb01393.x.
^Ingram, W. R. (1 December 1928). "Metamorphosis of the Colorado Axolotl by Injection of Inorganic Iodine".Experimental Biology and Medicine.26 (3): 191.doi:10.3181/00379727-26-4212.
^Krylov, O. A. (January 1961). "The role of haloids (bromine and iodine) in the metamorphosis of amphibia".Bulletin of Experimental Biology and Medicine.50 (1):724–727.doi:10.1007/BF00796048.
^Ley, Willy (February 1968)."Epitaph for a Lonely Olm". For Your Information.Galaxy Science Fiction. pp. 95–104.
^Wainwright, P. C.; Sanford, C. P.; Reilly, S. M.; Lauder, G. V. (1989). "Evolution of motor patterns: aquatic feeding in salamanders and ray-finned fishes".Brain, Behavior and Evolution.34 (6):329–341.doi:10.1159/000116519.PMID2611639.
^Ramos, A.G.; Mena-Gonzalez, H.; Zambrano, L (2021). "The potential of temporary shelters to increase survival of the endangered Mexican axolotl".Aquatic Conservation: Marine and Freshwater Ecosystems.31 (6):1535–1542.Bibcode:2021ACMFE..31.1535R.doi:10.1002/aqc.3520.S2CID235587173.
^Kulbisky, Gordon P; Rickey, Daniel W; Reed, Martin H; Björklund, Natalie; Gordon, Richard (1999). "The axolotl as an animal model for the comparison of 3-D ultrasound with plain film radiography".Ultrasound in Medicine and Biology.25 (6):969–975.doi:10.1016/s0301-5629(99)00040-x.PMID10461726.
^Björklund, N.K. (1993). Small is beautiful: economical axolotl colony maintenance with natural spawnings as if axolotls mattered. In: Handbook on Practical Methods. Ed.: G.M. Malacinski & S.T. Duhon. Bloomington, Department of Biology, Indiana University: 38–47.
^Minecraft (October 3, 2020).""Minecraft Live: Caves & Cliffs – First Look"".YouTube."And then we also found out that axolotls are endangered in the real world, and we think it's good to add endangered animals to Minecraft to create awareness about that." – Agnes Larsson
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