The species epithet, “mexicanum”, refers to where the species is found in the wild.

Description
Ambystoma mexicanum, also known as the Mexican Axolotl, is a large, long, cylindrical salamander that can reach lengths up to about 40 centimeters (Chaparro-Herrera et al. 2011). A fully aquatic, paedomorphic salamander, its most notable physical feature is its feathery gills, which protrude from the back of its wide head and remain there throughout adulthood. Its legs are short. It has four fingers on each of its front legs and five toes on each of its back legs (Utah's Hogle Zoo 2003).

Wild A. mexicanum can be distinguishable from most other Ambystoma by being the only dark-colored, obligate paedomorphic or neotenic salamander in the vicinity of Lake Xochimilco and its surrounding waterways (H. B. Shaffer pers. comm.). The wide ranging A. velasci has a range that extends to the vicinity of Xochimilco, but the species has not been collected at the site (Recuero et al. 2010).

In the wild, its coloration in life is dark or black (Humphrey 1975) with white flecks and black spots (Shaw and Nodder 1798), but several color mutations have been created in laboratory settings (Voss et al. 2009). Wild-type individuals have variable color patterns that are dependent on three types of pigment cells: melanophores (black or brown), xanthophores (yellow), and iridophores (iridescent white or yellowish). The iridophores are in limited concentration in the skin but are characteristic of the irises (Humphrey 1975).

Sexually mature males and females can be distinguished by males having an enlarged cloaca and females having rounder, more plump bodies (Majchrzak 2004).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Mexico

	View distribution map in BerkeleyMapper.
	View Bd and Bsal data (38 records).

Wild A. mexicanum can only be found in Lake Xochimilco and Lake Chalco, both adjacent to Mexico City at an elevation of 2240 m. Xochimilco and Chalco were, historically, part of a complex of five lakes, among which the Aztecs built Mexico City and around which the city has since expanded. Historically, the species was thought to be found in all the lakes, but it has been extirpated from most of this range. The species is currently only found on the southern edge of Mexico City, in the canals and wetlands of Xochimilco, including outside the Xochimilco city limits, and around the Chalco wetland. Records from close to downtown Mexico City in the Chapultepec Lake could refer to either this species or Ambystoma velasci, and requires confirmation (Smith 1989a, Griffiths and Thomas 1988, IUCN 2020, Ramos et al. 2021). The most recent estimated extent of occurrence is 468 km² (IUCN 2020).

Life History, Abundance, Activity, and Special Behaviors
Ambystoma mexicanum is most known for displaying an extreme form of paedomorphosis or neoteny; it retains an aquatic larval morphology for its entire life and lives in water permanently in the wild. The species is also well known as a biological model organism. In its native habitat, the species requires deepwater lakes with wetlands and aquatic vegetation, which provide structure for egg laying (IUCN 2020).

Lab-reared A. mexicanum can be traced back to 34 stock animals brought from Mexico City to Paris, France in 1864 as an exotic curiosity (Reiβ et al. 2015). Since then, scientific and hobbyist colonies have also cross-bred some lines with A. tigrinum tigrinum and/or selected for specific traits in captive A. mexicanum. As expected, given the initial bottleneck, cross-species outbreeding, and targeted selection, the genetics of lab-reared and wild A. mexicanum vary from each other (Voss and Shaffer 2000, Vance 2017). Much of their behavioral observations are based on lab individuals and may not be entirely accurate for wild individuals.

During the species' first year of life, individuals grow rapidly, reaching up to 20 cm in total length in the wild. In its second year of life wild individuals can grow an additional 10 cm, after which the growth rate decreases (Zambrano et al. 2007). Sexual maturity in the lab occurs between 8 – 15 months (Humphrey 1975, Voss et al. 2009). However, mortality in the wild is high during the first year (Zambrano 2006).

Ambystoma mexicanum is able to reproduce year round and have larger clutches as they age (Voss et al. 2015). In the lab, females can have 2 - 5 clutches a year with hundreds of eggs in each clutch (Voss et al. 2009). However in the wild, females spawn less frequently (Park et al. 2004). Captive males are capable of mating every 2 - 3 weeks. A single pair can produce up to several hundred fertile eggs (Park et al. 2004, Voss et al. 2009). In the lab, spawning occurs 10 – 24 hours after pairs are placed in the same environment (Humphrey 1975), with the best success being in the dark (Voss et al. 2009). Eggs are internally fertilized (Park et al. 2004, Voss et al. 2009) and laid singly (Majchrzak 2004).

Individuals mate readily in the lab as long as substrate, such as thin layers of coarse gravel or crushed stone, is provided for spermatophores to attach to (Humphrey 1975). Both males and females respond to chemosensory cues of the opposite sex. Sexually mature males increase their general activity when gravid or recently spawned female odorants are introduce, however, response to gravid females appears to be dependent on if the male has prior experience with a gravid female. Sexually mature females do not significantly increase their activity, but performed courtship displays when exposed to male odorants, particularly males that have not recently laid a spermatophore. These responses are also reflected in electro-olfactograms, which show larger responses to whole-body odorants from the opposite sex than from the same sex (Park et al. 2004).

Their courtship has been described as a “waltz” with a male and female pair first nudging each other with their snouts to the other’s flanks and cloaca leading to a circling behavior. After several minutes, the male moves away while undulating the posterior portion of his body in a “hula motion”. The female follows, often touching her snout to his tail and shaking her tail as she follows. After a short distance, the male deposits a cone-shaped spermatophore with a jelly-like base and white, sperm-containing cap by vigorously shaking his abdomen and tail. The female walks over the spermatophore and lowers her cloaca over the spermatophore. She then displays the same vigorous shaking of her abdomen and tail as the male when he deposits the spermatophore. The two move independently for a short while afterwards, but then repeat the whole process several times again (Eisthen 1989).

In the lab, males have been observed to lay spermatophores on top of other spermatophores they encounter. Females can only pick up the upper spermatophore and it is hypothesized that males display this behavior to ensure that females are inseminated by the courting male. Physical contact appears to be necessary for male spermatophore deposition and for females to accurately position themselves above spermatophores (Eisthen 1989).

Captive-reared, wild individuals can successfully mate and lay eggs in the wild, however, water properties must be within optimal conditions (see below) for eggs to hatch (Ramos et al. 2021).

In the lab at 20°C, embryos take approximately 20 days to develop. However, eggs can tolerate temperatures ranging from 5°C to 25°C, thus slowing or speeding up development (Voss et al. 2009). Shaffer (1989) reported that wild A. mexicanum are found in water temperatures ranging from 16 - 20°C and pHs between 7.4 - 8.2 in the 1980s. Additionally, Ramos et al. (2021) report the optimal hatching pH is between 7 and 8 and the optimal temperature for the species is 18 – 20°C.

In the lab, males can be aggressive towards one another and are often housed in low densities or individually (Voss et al. 2009).

In the lab, individuals can live over 10 years (Voss et al. 2009, IUCN 2020).

In rare instances, typically less than 1% in the lab setting, A. mexicanum will metamorphose (Voss et al. 2009) under stressful conditions, but they are unable to reproduce after metamorphosis (Chaparro-Herrera et al. 2011). Lab animals can also be induced to metamorphose from live-stock thyroid hormones (Vance 2017).

Historically, in the wild, adult A. mexicanum was a top predator with a generalist diet, eating any animal prey they could capture including, small minnow-sized fish, snails, crayfish, small invertebrates, and other salamanders (Shaffer 1989). However, individuals smaller than 2.6 - 3.5 cm could be preyed upon by native crayfish, Cambarellus montezumae, with the majority of A. mexicanum eaten by crayfish being hatchings (Zambrano et al. 2015). Unfortunately, since the 1970's, introduced fish have acted as both competitors and predators of A. mexicanum (Griffiths and Thomas 1988, Zambrano et al. 2007, Zambrano et al. 2010b, IUCN 2020; see Trends and Threats for more details).

Larva
Hatchling larvae look similar to adults but with very small forelimbs that are just beginning digit differentiation and with only hind limb buds (Duellman and Trube 1994, A. Chang per. comm.).

During the species' first year of life, individuals grow rapidly, reaching up to 20 cm in total length in the wild (Zambrano et al. 2007). However, mortality in the wild is high during the first year (Zambrano 2006).

In the wild, larvae feed on zooplankton (Chaparro-Herrera et al. 2011). In the lab, larvae can be fed on brine shrimp until they reach a total length of ~ 3 cm, after which they can be fed larger food items such as blackworms, beef liver, or salmon pellets (Voss 2009). Chaparro-Herrera et al. (2011) experimented with feeding genetically wild-type larvae more natural diets of phytoplankton with a variety of rotifers, cladocerans, and ostracods. They found that feeding rates varied with age and size. Specifically, A. mexicanum fed more on cladocerans, and ostracods, with increased feeding on the largest cladocerans after their fifth week.

The species is not an active hunter, having more of a sit-and-wait predator style, and only has rudimentary teeth that are only able to grasp prey but not tear or chew it. Larvae are also benthic - rarely moving up or down the water column for food (Chaparro-Herrera et al. 2011).

Larva have poor eyesight (Chaparro-Herrera et al. 2011), which may prevent them from effectively competing with introduced tilapia (Oreochromis niloticus) larvae for food in turbid environments (Chaparro-Herrera et al. 2020)

In the lab, larvae are often reared at low densities or separately because high density environments typically have feeding frenzies that result in the biting off conspecifics’ limbs and tails. Additionally, once individuals reach 6 cm, they are housed separately as males can be aggressive towards one another (Voss et al. 2009).

Larva from a captive population of wild parents can survive up to one year in semi-natural ponds, however, behavioral experiments are still needed to determine if they effectively perform antipredator behavior (Ramos et al. 2021).

Trends and Threats
Despite being a common pet species and scientific model organism, A. mexicanum is listed as “Critically Endangered” in the wild because of a dramatic decline, by 80%, since 2003 (IUCN 2020). Wild population numbers are so low that some researchers believe they are effectively “Extinct in the Wild” (Ramos et al. 2021). Surveys indicate that population size may be as low as 0.001 individuals/m², and the decline can be attributed to loss of habitat, water pollution, and introduced tilapia (Oreochromis niloticus) and carp (Cyprinus carpio), which act as both competitors and predators (Griffiths and Thomas 1988, Zambrano et al. 2007, Zambrano et al. 2010b, IUCN 2020).

More specifically, Lopez-Lopez et al. (2023) estimate that the lakes’ area has been reduced to 2.83% of its historic area and has been converted into only wetland ecosystems rather than deep lakes with wetlands. An ecological niche model conducted by Contreras et al. (2009) found only 11 sites, across six isolated and scattered areas dominated by traditional agricultural methods, as appropriate for the species’ distribution (IUCN 2020). Previous surveys showed the population structure is mainly composed of 1-year-old individuals, which indicates there is some recruitment, however, the lack of suitable habitat may still prevent species recovery (Zambrano et al. 2007).

Pollution and invasive species are likely contributing to an increased disease prevalence in the species (Zambrano et al. 2010a). A survey in 2008 found infections of Batrachochytrium dendrobatidis, a pathogenetic chytrid fungus also known as Bd, in captive individuals but did not detect the fungus in two wild-caught individuals (Frías-Alvarez et al. 2008). A later study showed that A. mexicanum can be successfully cleared of Bd using Itrafungol, however, studies are necessary to improve the protocol and determine the long-term effects of antifungal treatment (Michaels et al. 2018). However, García-Feria et al. (2017) reported that A mexicanum is not susceptible to Bd despite carrying the fungus and another study indicates it is resistant to the pathogenetic salamander chytrid fungus Batrachochytrium salamandrivorans (IUCN 2020). Ambystoma mexicanum is susceptible to bacterial infections from Pseudomonas and Aeromonas and to parasites, such as Eustrongylides, Lernaea and Ribeiroia ondatrae, found in polluted waters (Recuero et al. 2010). Captive-bred lab and pet populations are also vulnerable to disease because of inbreeding and loss of genetic diversity (Vance 2017).

Historically, no predatory fish were found in the lake systems that A. mexicanum inhabited, and A. mexicanum was the top predator (Shaffer 1989). However, in the 1970’s and 80’s, tilapia (Oreochromis niloticus) and carp (Cyprinus carpio) were introduced to the lakes as an alternative protein source by the Food and Agriculture Organization of the United Nations (Vance 2017). Larval diet experiments show that larval tilapia outcompete A. mexicanum larvae, particularly in turbid waters, and shift the phytoplankton community to smaller rotifers that are not commonly consumed by larval A. mexicanum. Additionally, adult tilapia also eat A. mexicanum eggs and larvae, and cause turbidity when feeding, further impacting A. mexicanum survival (Chaparro-Herrera et al. 2020). Both carp and tilapia both occupy a larger niche area than A. mexicanum, and the two exotic fish species cause changes to the tropic web structure that in turn create erosion problems (Zambrano 2006, Zambrano et al. 2010b).

In the past, a lesser threat was that A. mexicanum was illegally captured for medicinal purposes, human consumption, and the pet trade by locals. However, wild population sizes are so small, as of 2020, that current trade is likely to be with closely related species or captive-bred individuals. Consuming A. mexicanum as a traditional food seems to have also stopped (IUCN 2020).

The main focus of conservation for A. mexicanum is public education, nature tourism, and habitat restoration. Captive populations of wild A. mexicanum have been considered as re-introduction stock (Zambrano et al. 2010a) and 10,000 individuals were released in 2012 (IUCN 2020), however there is some hesitancy as re-introductions often fail due to predation or low-adaptability from the source/captive population. To increase the odds of successful re-introductions, between the summer and fall of 2017, members of the Laboratorio de Restauración Ecológica, which maintains a captive population of wild A. mexicanum at the Universidad Nacional Autónoma de México, experimented with reproduction and raising A. mexicanum larvae from eggs in semi-natural ponds near the university. They were able to show that captive-reared, wild individuals could successfully mate and lay eggs in the wild, however water properties play a major role in success. Parental age may also play a role in hatching success. Larvae from these experiments were able to survive for up to a year with higher survival estimates than from previous wild population viability analysis. However, their results indicated that careful selection of reintroduction sites is needed (Ramos et al. 2021).

The species is listed in the Special Protection category by the Mexican Government (NOM-059-SEMARNAT-2001) and has a CITES listing of Appendix II. The species has a special action plan and several captive colonies of wild populations (Contreras et al. 2009, IUCN 2020, CITES 2025). The species can also be found in Ejidos de Xochimilco y San Gregorio Atlapulco, a nature preserve in Mexico City (IUCN 2020).

Relation to Humans
Ambystoma mexicanum is kept as an aquarium pet and is used widely in laboratory experiments (Shaffer 1989, Voss et al. 2009, Ramos et al. 2021, Vance 2017). While there was some illegal wild harvest of individuals for human consumption, medicinal uses, and for pets, the assumption is that all traded individuals are now from closely related species or captive-bred animals due to their decline in the wild (IUCN 2020). Wild-caught individuals are also legally caught for captive breeding programs to be released back into the wild (Recuero et al. 2010, Zambrano et al. 2010a, Vance 2017).

In pre-Columbian times, A. mexicanum and local Aztecs co-habitated the lake, with humans creating a system of canals and wetlands that increased the amount of shoreline and allowed A. mexicanum to thrive. Axolotls were also eaten, incorporated into art, and part of Aztec creation myths (Voss et al. 2015). The axolotls are still a source of national pride, is the official emoji for Mexico City (Vance 2017) and, as of Feb 2020, is featured on the Mexican 50 peso bill (Yucatan Times 2020).

This species is also an important research model for studying regeneration and tissue repair. While many adult salamanders are known for their ability to regrow an astonishing amount and variety of body parts, the axolotl is easily kept in the lab and has the most complete genetic, genomic and transgenic tools available thus making it an ideal study animal (Voss et al. 2009). Ambystoma mexicanum is also valued in developmental biology, because they have large cells, a larger volume of neural plate cells - which are precursors to nerve tissues, and different pigmentation in different types of cells (Vance 2017).

Possible reasons for amphibian decline

General habitat alteration and loss
Urbanization
Drainage of habitat
Habitat fragmentation
Intentional mortality (over-harvesting, pet trade or collecting)

Comments
Ambystoma mexicanum is part of the A. tigrinum complex (Shaffer and McKnight 1996, Samuels et al. 2005, Recuero et al. 2010, Evans et al. 2021). However, the phylogenetics of the complex has been problematic due to a lack of reliable morphological characters, low genetic diversity, and possible incomplete lineage sorting or hybridization. Samuels et al. (2005) used Bayesian, Maximum Likelihood, and Maximum Parsimony methods on the complete mitochondrial genome of five members of the A. tigrinum complex: A. andersoni, A. californiense, A. dumerilii, A. mexicanum, and A. tigrinum. They found that A. mexicanum was sister to A. andesrsoni, with the next most closely related species being A. dumerilii followed by A. tigrinum. However, when Recuero et al. (2010) analyzed a mtDNA fragment that included tRNA-proline and D-loop with Bayesian Inference they found that A. mexicanum had haplotypes that were most closely related to A. amblycephalum, A. ordinarium and A. velasci causing A. mexicanum to be non-monophyletic. These results could be the result of incomplete lineage sorting between A. mexicanum and the first two species, or hybridization with A. velasci. More recently, Everson et al. (2021) analysed 95 nuclear loci for 347 individuals across 19 Ambystoma species along with life history data using Principal Components Analysis, Population Genetic Structure Analysis, and Bayesian and Maximum Likelihood methods of phylogenetic analysis. Their findings place A. mexicanum as sister to the clade composed of A. taylori and A. velasci and found that geographic isolation was a better predictor of speciation in the genus than having different life histories. Further investigation is needed to determine more accurate phylogenetic relationships in the complex.

Ambystoma mexicanum is assumed to have evolved from metamorphic tiger salamanders that colonized the stable lake system in the central highlands (Voss et al. 2015).

Metamorphic failure in captive A. mexicanum lines are attributed to single, major gene effect. However, in wild populations, the quantitative effect of the same gene region was lower, indicating that other factors were more important to the loss of metamorphosis in wild populations. This finding highlighted the need to study wild A. mexicanum separately from captive populations, especially with regards to conservation (Voss and Shaffer 2000).

According to Humphrey (1975) female A. mexicanum are the heterogametic sex.

Ambystoma mexicanum has 28 diploid chromosomes (Humphery 1975).

Ambystoma was incorrectly written as “Amblystoma” for many years because of misinterpretations of the spelling of the genus by the authority, Tschudi 1838. The misinterpretation was recognized by taxonomists as early as 1907 by Stejneger, however experimental biologists still used Amblystoma until Hobart M. Smith, a taxonomist, complained to Rufus Richard Humphrey, an experimentalist, about the misuse in 1943. After that, Humphrey started to regularly use the spelling “Ambystoma” and other experimental biologists and zoologists began to adopt the correct spelling (Smith 1989b).

The common name, “axolotl,” originates from the Aztec language, and its meaning can be attributed to several interpretations including: water dog, water twin, water sprite, or water slave. However the term, “axolotl,” has also been loosely used to refer to unmetamorphosed or larval stages of other Ambystoma species (Humphrey 1975).

Lab reared A. mexicanum can be traced back to 34 stock animals brought from Mexico City to Paris, France in 1864 as an exotic curiosity. Initially, they were kept as attractions at the Jardin zoologique d'acclimatation, but this changed when Auguste Duméril, ​​a professor of ichthyology and herpetology, began to study and breed them from his stock of five males and one female. Duméril gave specimens to interested researchers all over Europe, leading to the self-sustaining colonies of today (Reiβ et al. 2015).

As of December 2023, Universidad Nacional de Autónoma de México has launched an “Adopt an Axolotl” fundraiser for A. mexicanum conservation.

This species was mentioned in News of the Week on 15 January 2018:

Among vertebrates, adult salamanders are unique in their ability to regenerate complex tissues, such as limbs. The model organisms for limb regeneration are the Axolotl (Ambystoma mexicanum, family Ambystomatidae) and species from the family Salamandridae. The two families use different cellular mechanisms of regeneration and axolotls have very different life histories from newts as the former is fully aquatic while the latter exhibits a traditional, biphasic life cycle. Arenas Gomez et al. (2017) characterized limb regeneration in Bolitoglossa ramosi (family Plethodontidae), a fully terrestrial salamander with direct development. Among the differences in regeneration, they found that B. ramosi takes longer than other species to regenerate limbs, leading to questions regarding the relationship between limb regeneration and direct vs indirect development (Written by Ann T. Chang).

This species was featured in News of the Week on 19 February 2018:

The axolotl is the most common salamander used in biological research; they are easily bred, and thousands live in home aquariums and labs. Its long association with humans is fascinating. In the 13th century, the indigenous Mexica people built an island city in Lake Texcoco in the Central Valley of Mexico. They also built floating gardens and canals, which the native axolotls invaded. Eventually, the lakes were drained and the salamanders were cut off. Their numbers declined; a 1998 census found 6,000 axolotls per square kilometer. In 2000, Luis Zambrano, a biologist at the Universidad Nacional Autónoma de Mexico, found only 1,000 animals/km². By 2008, the census registered 100/km², and currently the estimate is only 35/km². With isolation, reduction in numbers, invasive predators, and environmental contaminants, the axolotl is almost extinct in the wild (Written by David Cannatella).

This species was mentioned in News of the Week on 31 May 2021

The Mexican axolotl (Ambystoma mexicanum) is a famous example of a paedomorphic salamander – one that retains an aquatic, larval phenotype into adulthood. But the axolotl isn’t the only salamander with a strange life history! Across the tiger salamander species complex, life history is highly variable, including several species that are always paedomorphic, some that are always metamorphic, and many others that can exhibit either state depending on the population. In a study published in PNAS, Everson et al. (2021) examine the extent to which this variation in life history has influenced speciation in tiger salamanders. By analyzing a large genetic dataset from 19 Ambystoma species, they shed new light on species limits and gene flow across the complex. A major finding is that gene flow is common among species with different life history traits, and geography seems to be the best predictor of species limits. Apparently obligate paedomorphosis is not the species delimiter that one might assume. These new insights into the evolution of tiger salamanders chart a course for more informed use of these species in experimental, ecological, and conservation research (Written by Katie Everson).

This species was mentioned in News of the Week on 2 May 2022:

Paedomorphosis, the retention of juvenile characteristics into adulthood, is well known in salamanders. In salamanders, paedomorphosis occurs via neoteny, the truncation of somatic development even as the gonads become mature. Extreme paedomorphic (obligatorily neotenic) forms typically look like larva and never transform, remaining aquatic throughout their lives. Perhaps the most famous of neotenic salamanders is the critically endangered Axolotl (Ambystoma mexicanum), known only from Lakes Xochimilco and Chalco in Central Mexico. This species has become an important model organism for developmental biology and is widely available in the pet trade, and thus has been distributed around the world. In Japan, the axolotl goes by the humorous name "Wooper Looper", ascribed to the species from a 1985 TV commercial that caught the fancy of the public. There is even a Wooper Looper Pokémon. Now Japan has its own Wooper Looper– a facultatively neotenic hynobiid salamander (the Ezo salamander, Hynobius retardatus). In this species, most individuals transform into terrestrial adults, but they are developmentally flexible and sometimes remain in the water, failing to transform while nevertheless achieving reproductive maturity. Neotenic adults were first collected in 1924 and observed again in 1932 in Lake Kuttara in Hokkaido, but neotenic adults were not seen again until Dr. Hisanori Okamiya rediscovered the phenomenon in the form of three specimens collected in a pond in south Hokkaido in December 2020 and April 2021 (Okamiya et al. 2021). Hynobius retardatus and Batrachuperus londongensis are the only hynobiid species (out of 87 total) that exhibit facultative neoteny (Written by Jim McGuire).

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Species Account Citation: AmphibiaWeb 2025 Ambystoma mexicanum: Mexican Axolotl <https://amphibiaweb.org/species/3842> University of California, Berkeley, CA, USA. Accessed Apr 19, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 19 Apr 2025.

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