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{{Short description|Species of salamander}}
{{Taxobox
{{Other uses}}
| color = pink
{{pp-move}}
{{pp-vandalism|small=yes}}

{{Speciesbox
| name = Axolotl
| name = Axolotl
| image = Ambystoma mexicanum 1zz.jpg
| image_caption = In the [[National Aquarium (Washington, D.C.)|National Aquarium]] in [[Washington, D.C.]]
| status = CR
| status = CR
| status_system = IUCN3.1
| trend = down
| status_ref = <ref name="iucn status 12 November 2021">{{cite iucn |author=IUCN SSC Amphibian Specialist Group |date=2020 |title=''Ambystoma mexicanum'' |volume=2020 |page=e.T1095A53947343 |doi=10.2305/IUCN.UK.2020-3.RLTS.T1095A53947343.en |access-date=12 November 2021}}</ref>
| image = Axolotl.jpg
| image_width = 250px
| status2 = CITES_A2
| status2_system = CITES
| regnum = [[Animal]]ia
| status2_ref = <ref name = "CITES">{{Cite web|title=Appendices {{!}} CITES|url=https://cites.org/eng/app/appendices.php|access-date=2022-01-14|website=cites.org}}</ref>
| phylum = [[Chordate|Chordata]]
| classis = [[Amphibia]]
| genus = Ambystoma
| ordo = [[Caudata]]
| species = mexicanum
| authority = ([[George Shaw (biologist)|Shaw]] and [[Frederick Polydore Nodder|Nodder]], 1798)
| familia = [[Ambystomatidae]]
| synonyms = {{species list
| genus = ''[[Ambystoma]]''
| Gyrinus mexicanus | Shaw and Nodder, 1798
| species = '''''A. mexicanum'''''
| Siren pisciformis | Shaw, 1802
| binomial = ''Ambystoma mexicanum''
| Siredon axolotl | Wagler, 1830
| binomial_authority = ([[George Shaw|Shaw]], 1789)
| Axolotes guttata | Owen, 1844
| Siredon Humboldtii | Duméril, Bibron, and Duméril, 1854
| Amblystoma weismanni | Wiedersheim, 1879
| Siredon edule | Dugès, 1888 }}
| synonyms_ref = <ref name="Frost"/>
| range_map = {{maplink|frame=yes|plain=yes|from=Ambystoma mexicanum range.map}}
| range_map_caption = IUCN range of the axolotl.
{{leftlegend|#FF0000|Axolotl (''Ambystoma mexicanum'')}}
}}
}}


The '''axolotl''' ({{IPAc-en|ˈ|æ|k|s|ə|l|ɒ|t|əl|audio=en-us-axolotl.oga}}; from {{langx|nci|āxōlōtl}} {{IPA-nah|aːˈʃoːloːtɬ||Axolotl.ogg}}) ('''''Ambystoma mexicanum''''')<ref name="Frost"/> is a [[neoteny|paedomorphic]] [[salamander]] closely related to the [[tiger salamander]].<ref name="Frost"/><ref name=aquariumindustries>{{cite web |url= http://www.aquariumindustries.com.au/wp-content/uploads/2012/07/Mexican-Walking-Fish.pdf |url-status = dead|archive-url= https://web.archive.org/web/20180315144303/https://www.aquariumindustries.com.au/wp-content/uploads/2017/07/Mexican-Walking-Fish.pdf |archive-date= 15 March 2018 |title=Mexican Walking Fish, Axolotls ''Ambystoma mexicanum''}}</ref><ref name="aquariumonline">{{cite web |url= http://www.aquariumonline.com.au/topics/creatures/mexican.htm |archive-date=10 April 2013 |url-status = dead|archive-url= https://web.archive.org/web/20130410234310/http://www.aquariumonline.com.au/topics/creatures/mexican.htm |title=Axolotols (Walking Fish) |publisher= Aquarium Online |access-date=2013-09-12}}</ref> It is unusual among [[amphibian]]s in that it reaches adulthood without undergoing [[metamorphosis]]. Instead of taking to the land, adults remain aquatic and [[external gills|gilled]]. The species was originally found in several lakes underlying what is now [[Mexico City]], such as [[Lake Xochimilco]] and [[Lake Chalco]].<ref name="iucn status 12 November 2021" /> These lakes were drained by Spanish settlers after the [[Spanish conquest of the Aztec Empire|conquest of the Aztec Empire]], leading to the destruction of much of the axolotl's natural habitat.
The '''Axolotl''' (''Ambystoma mexicanum'') is an aquatic [[salamander]] native to [[Mexico]]. In the [[Spanish language]] it is called the '''''ajolote'''''.
The name is from the [[Aztec]] [[Nahuatl language]], from "atl" meaning water, and "xolotl" meaning dog. It also derives its name from the Aztec God of Death [[Xolotl]], who &mdash; according to myth &mdash; assumed the amphibian's form in an effort to escape exile, and was killed while in this form.


{{As of|2020}}, the axolotl was near [[extinction]]<ref>{{cite news|url= http://news.bbc.co.uk/earth/hi/earth_news/newsid_8220000/8220636.stm |publisher= BBC |title= Axolotl verges on wild extinction |author= Matt Walker |date=2009-08-26 |access-date=2010-06-28}}</ref><ref>{{Cite web|last=PetAquariums.com|title=Are Axolotls Endangered? You Need To Be Careful…|url=https://www.petaquariums.com/2020/10/21/are-axolotls-endangered-you-need-to-be-careful/|access-date=2021-06-26|website=PetAquariums.com|date=22 April 2020 |language=en-US}}</ref> due to urbanization in Mexico City and consequent [[water pollution]], as well as the introduction of invasive species such as [[tilapia]] and [[perch]]. It is 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|International Union for Conservation of Nature and Natural Resources]] (IUCN) and is listed under Appendix II of the [[Convention on International Trade in Endangered Species]] (CITES).<ref name="CITES"/> Axolotls are used extensively in scientific research due to their ability to [[Regeneration (biology)|regenerate]] limbs, gills and parts of their eyes and brains.<ref name="nickbaker">{{cite video |date= 2009-11-11 |title= Weird Creatures with Nick Baker |medium= Television series |publisher= [[The Science Channel]] |location= Dartmoor, England, UK <!--|access-date= 2009-12-04 -->|time= 00:25}}</ref> Notably, their ability to regenerate declines with age, but it does not disappear. Axolotls keep modestly growing throughout their life and some consider this trait to be a direct contributor to their regenerative abilities.<ref>{{Cite journal |last=Sandoval-Guzmán |first=Tatiana |date=August 2023 |title=The axolotl |url=https://www.nature.com/articles/s41592-023-01961-5 |journal=Nature Methods |language=en |volume=20 |issue=8 |pages=1117–1119 |doi=10.1038/s41592-023-01961-5 |pmid=37553398 |s2cid=260699417 |issn=1548-7091}}</ref> Further research has been conducted to examine their heart as a model of human single ventricle and excessive trabeculation.<ref>{{Cite journal |last1=Meyer |first1=Sophie |last2=Lauridsen |first2=Henrik |last3=Pedersen |first3=Kathrine |last4=Andersson |first4=Sofie Amalie |last5=van Ooij |first5=Pim |last6=Willems |first6=Tineke |last7=Berger |first7=Rolf M. F. |last8=Ebels |first8=Tjark |last9=Jensen |first9=Bjarke |date=2022-11-28 |title=Opportunities and short-comings of the axolotl salamander heart as a model system of human single ventricle and excessive trabeculation |journal=Scientific Reports |language=en |volume=12 |issue=1 |pages=20491 |doi=10.1038/s41598-022-24442-9 |issn=2045-2322 |pmc=9705478 |pmid=36443330|bibcode=2022NatSR..1220491M }}</ref> Axolotls were also sold as food in Mexican markets and were a staple in the [[Aztec]] diet.<ref>{{cite news|title=Mythic Salamander Faces Crucial Test: Survival in the Wild|url=https://www.nytimes.com/2012/10/31/world/americas/struggle-of-axolotls-mexicos-mythical-salamander.html?pagewanted=all&_r=1|access-date=30 July 2015|newspaper=The New York Times| date=30 October 2012 | last1=Tickell | first1=Sofia Castello Y. }}</ref>
It is noted for its appearance and its demonstration of [[neoteny]], remaining in its aquatic larval form even as a sexually mature adult, and not undergoing metamorphosis into a terrestrial form.

Axolotls may be confused with the larval stage of the closely related tiger salamander (''A. tigrinum''), which are widespread in much of North America and occasionally become paedomorphic, or with [[mudpuppy|mudpuppies]] (''Necturus'' spp.), fully aquatic salamanders from a different family that are not closely related to the axolotl but bear a superficial resemblance.<ref name="axolotl bio">{{Cite journal| last= Malacinski| first= George M. |journal=American Zoologist |title=The Mexican Axolotl, ''Ambystoma mexicanum'': Its Biology and Developmental Genetics, and Its Autonomous Cell-Lethal Genes |date= Spring 1978 |doi=10.1093/icb/18.2.195 |volume=18 |issue=2 |pages=195–206|doi-access=free }}</ref>


==Description==
==Description==
[[File:AxolotlBE.jpg|thumb|left|A captive leucistic axolotl, perhaps the most well known form of the axolotl]]
A fully grown axolotl, at age 18&ndash;24 months, ranges in length from 150&ndash;450&nbsp;mm (6&ndash;18 inches), although a size close to 230&nbsp;mm (9 inches) is most common and greater than 300&nbsp;mm (12 inches) is rare. They have distinctive "fern-like" gill structures that are not covered (as are gills in fish and in frog tadpoles) &mdash; usually 3 stalks on each side of the head; the colour of the fern-like part varies although the usual colour is red and it has been known that species with a diet high in red meat gain red gills; axolotls also breathe through the skin and possess lungs. Axolotls have tiny teeth which are hardly noticable and are used to grip food rather then to tear and chew it. They are also known to have a fast regeneration rate where they can regrow limbs and organs.
[[File:Axolot's head (Ambystoma mexicanum).jpg|thumb|Face of a common or wild type axolotl]]
[[File:Axolotl ganz.jpg|thumb|left|The speckled [[wild type]] form]]
[[File:Axolot's_gills_(Ambystoma_mexicanum).jpg|thumb|Axolotl's gills (''Ambystoma mexicanum'')|left]]
A sexually mature adult axolotl, at age 18–27 months, ranges in length from {{convert|15| to |45|cm|0|abbr=on}}, although a size close to {{convert|23|cm|0|abbr=on}} is most common and greater than {{convert|30|cm|abbr=on}} is rare. Axolotls possess features typical of salamander larvae, including [[external gills]] and a caudal fin extending from behind the head to the vent.<ref>San Francisco Examiner (San Francisco, California) 7 August 1887, page 9, authored by [[Yda Addis]]</ref><ref>{{Citation|last1=McIndoe|first1=Rosemary|title=Functional morphology of gills in larval amphibians|date=1984|url=https://doi.org/10.1007/978-94-009-6536-2_4|work=Respiration and metabolism of embryonic vertebrates: Satellite Symposium of the 29th International Congress of Physiological Sciences, Sydney, Australia, 1983|pages=55–69|editor-last=Seymour|editor-first=Roger S.|series=Perspectives in vertebrate science|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-009-6536-2_4|isbn=978-94-009-6536-2|access-date=2021-05-13|last2=Smith|first2=D. G.}}</ref> External gills are usually lost when salamander species mature into adulthood, although the axolotl maintains this feature.<ref name=":1">{{Cite book|last=Kardong|first=Kenneth V|url=https://www.worldcat.org/oclc/1053847969|title=Vertebrates: comparative anatomy, function, evolution|date=2019|publisher=McGraw-Hill Education |isbn=978-1-259-70091-0|language=English|oclc=1053847969}}</ref> This is due to their neoteny evolution, where axolotls are much more aquatic than other salamander species.<ref name=":5"/>


Their heads are wide, and their eyes are lidless. Their limbs are underdeveloped and possess long, thin digits. Males are identified by their swollen [[cloaca]]e lined with papillae, while females are noticeable for their wider bodies full of eggs. Three pairs of [[External gills|external 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.<ref name=":1" /> Four-gill slits lined with [[gill rakers]] are hidden underneath the external gills, which prevent food from entering and allow particles to filter through.
Axolotls are very closely related to [[Tiger Salamander]]s (''Ambystoma tigrinum''). In colour they range from [[Albinism|albino]] or white (leucistic) to black, through greys, tans and browns. Wild axolotls are rarely white, though all leucistic (white with black eyes) axolotls are descended from a single mutant male brought to Paris in 1863 in a shipment to the [[Jardin des Plantes]] in the city. The true albino mutant common in labs and pet stores was created in an American laboratory in the [[1950s]] through hybridization with an albino [[Tiger Salamander]]. No axolotls commercially available today are wild caught (since doing so is strictly forbidden), the majority being descended from these same research specimens, even in the pet trade.


Axolotls have barely visible [[Vestigiality|vestigial]] teeth, which develop during metamorphosis. The primary method of feeding is by [[suction feeding|suction]], during which their rakers interlock to close the gill slits. External gills are used for respiration, although [[buccal pumping]] (gulping air from the surface) may also be used to provide oxygen to their lungs.<ref name=":1" /> 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.[[File:Buccal pumping.jpg|thumb|428x428px|Buccal pumping]]
==Habitat==
Only the axolotl is native to [[Lake Xochimilco]] and [[Lake Chalco]] in central [[Mexico]], however, axolotls have cousins that appear similar to the axolotl and are found through out Mexico as well as many places in the southern United States, often refered to as "mud puppies". Only the axolotl's cousins are found to live in "axalapascos" (volcanic craters filled with water, [[maare]]). The water temperature in Xochimilco rarely rises above 20°C (68°F), though it may fall to 6 or 7°C (43°F) in the winter, and perhaps lower.


[[File: Three Colors of Axolotl.jpg|thumb|right|Axolotls displaying variations in color]]
The wild population has been put under heavy pressure by the growth of [[Mexico City]]. Axolotls are also sold as food in Mexican markets, and were a staple in the Aztec diet. It is currently listed by [[CITES]] as an [[endangered species]], and by [[IUCN]] as [[critically endangered]] in the wild with a decreasing population.
Axolotls have four pigmentation genes; when mutated, they create different color variants. The normal wild-type animal is brown or tan with gold speckles and an olive undertone. The five most common mutant colors are listed below.


# Leucistic: pale pink with black eyes.
==Axolotl's neoteny==
# Xanthic: grey, with black eyes.
Axolotls exhibit a property called [[neoteny]], meaning that they reach adulthood without undergoing [[Metamorphosis (biology)|metamorphosis]]. Some axolotls metamorphose, although they do so rarely, and usually only after with [[iodine]] or shots of [[thyroxine]] hormone. Another method for inducing transformation, though one that is very rarely successful, involves removing an axolotl in good condition to a shallow tank in a vivarium and slowly reducing the water level so that the axolotl has difficulty submerging, and eventually is unable submerge at all; it will then stop using its gills to breathe and use its lungs. It will then, over a period of weeks, slowly metamorphose into an adult salamander. During transformation, the air in the vivarium must remain moist, and the maturing axolotl sprayed with a fine mist of pure water. The odds of the animal being able to metamorphose via this method are extremely small, and most attempts at inducing metamorphosis lead to death. Spontaneous metamorphosis has been known to occur very rarely, but attempts to do so artificially are best left to trained scientists. An inability to produce the necessary hormones required for metamorphosis on their own results in such a [[Neoteny|neotenic]] existence. Artificial metamorphosis also dramatically shortens the axolotl's lifespan, if they survive the process. A neotenic axolotl will live an average of 10&ndash;15 years (though an individual in Paris is credited with achieving 25 years), while a metamorphosed specimen will scarcely live past the age of five. The adult form resembles the adult [[Tiger Salamander]], supporting the theory that the two species are closely related.
# Albino: pale pink or white, with red eyes, which is more common in axolotls than other species.
# Melanoid: 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 white piebald appearance upon reaching maturity.<ref>{{Cite web|url=https://exopetguides.com/axolotl/axolotl-colors/|title=18 Types of Axolotl Colors You Can Own (Axolotl Color Guide)|date=August 14, 2019}}</ref> Because pet breeders frequently cross the variant colors, double homozygous mutants are common in the pet trade, especially white/pink animals with pink eyes that are double homozygous mutants for both the albino and leucistic trait.<ref name="Color Atlas of Pigment Genes">{{Cite journal |last1=Frost |first1=Sally K. |last2=Briggs |first2=Fran |last3=Malacinski |first3=George M. |date=1984 |title=A color atlas of pigment genes in the Mexican axolotl (''Ambystoma mexicanum'') |journal=Differentiation |volume=26 |issue=1–3 |pages=182–188 |doi=10.1111/j.1432-0436.1984.tb01393.x}}</ref> Axolotls also have some limited ability to alter their color to provide better camouflage by changing the relative size and thickness of their melanophores.<ref>{{Cite journal |last1=Pietsch |first1=Paul |last2=Schneider |first2=Carl W. |date=1985 |title=Vision and the skin camouflage reactions of ''Ambystoma'' larvae: the effects of eye transplants and brain lesions |journal=Brain Research |volume=340 |issue=1 |pages=37–60 |doi=10.1016/0006-8993(85)90772-3|pmid=4027646 |s2cid=22723238 }}</ref>
====Jardin des Plantes story====
The first imported axolotls in Europe during the 19th Century were placed in the "[[Jardin des Plantes]]" in Paris. 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. This discovery was the starting point of research about neoteny.


==Habitat and ecology==
====Vilem Laufberger's experiment====
[[File:Sunrise_in_Xochimilco_(Amanecer_en_Xochimilco)_2.jpg|thumb|Lake Xochimilco, Mexico City (Amanecer en Xochimilco). The native habitat of axolotls is important to the study of preservation and conservation. |left]]
[[Vilem Laufberger]] did a famous experiment in Germany concerning the axolotl. Using hormone injections, he persuaded an axolotl to grow into a fully adult [[salamander]] of a species that no one had ever seen. This is because the axolotl is a juvenile form of an amphibian that evolved to be sexually mature still in the 'infant' stage. The experiment was repeated by the Englishman [[Julian Huxley]], who was unaware the experiment had already been done.
[[File:Axolotl Wild Type.jpg|thumb|left|Wild form]]
The axolotl is native only to the freshwater of [[Lake Xochimilco]] and [[Lake Chalco]] in the [[Valley of Mexico]]. Lake Chalco no longer exists, having been drained as a flood control measure, and Lake Xochimilco remains a remnant of its former self, existing mainly as canals. The water temperature in Xochimilco rarely rises above {{convert|20|C|F}}, although it may fall to {{convert|6|-|7|C|F}} in the winter, and perhaps lower.<ref name=":2">{{Cite web|title=Lake Xochimilco, Borough of Xochimilco in southern México City, 162 L • Biotope Aquarium|url=http://biotope-aquarium.info/aquariums/lake-xochimilco-borough-of-xochimilco-in-southern-mexico-city-162-l/|access-date=2021-04-30|website=Biotope Aquarium|language=en-GB}}</ref>


Surveys in 1998, 2003, and 2008 found 6,000, 1,000, and 100 axolotls per square kilometer in its Lake Xochimilco habitat, respectively.<ref name = "Stevenson2014">{{cite web
==Uses in research==
| last = Stevenson | first = M.
The axolotl is used in research and large numbers are bred in captivity. The feature of the salamander that attracts most attention is its healing ability: the axolotl does not heal by scarring and is capable of the [[Regeneration (biology)|regeneration]] of entire lost appendages in a period of months, and, in certain cases, more vital structures. Some have indeed been found restoring the less vital parts of their brains. They can also readily accept transplants from other individuals, including eyes and parts of the brain &mdash; 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 a novelty. In metamorphosed individuals, however, the ability to regenerate is greatly diminished. Another attractive feature, for research, is the large and robust [[embryo]]s. Axolotls are especially easy to breed compared to other salamanders in their family, which are almost never captive bred.
| title = Mexico's 'water monster' may have disappeared
| agency= Associated Press| website= SFGate.com
| date = 2014-01-28
| url = http://www.sfgate.com/news/science/article/Mexico-s-water-monster-may-have-disappeared-5182823.php
| access-date = 2014-01-29}}</ref> A four-month-long search in 2013, however, turned up no surviving individuals in the wild. Just a month later, two wild ones were spotted in a network of canals leading from Xochimilco.<ref>{{Cite news|url=https://www.independent.co.uk/environment/axolotl-found-in-mexico-city-lake-after-scientists-feared-it-only-survived-in-captivity-9148775.html|title=Endangered 'water monster' Axolotl found in Mexico City lake|date=2014-02-24|work=The Independent|access-date=2017-06-02|language=en-GB}}</ref>


The wild population has been put under heavy pressure by the growth of [[Mexico City]]. The axolotl is currently on the International Union for Conservation of Nature's annual Red List of threatened species. Non-native fish, such as [[tilapia as exotic species|African tilapia]] and [[Asian carp]], have also recently been introduced to the waters. These new fish have been eating the axolotls' young, as well as their primary source of food.<ref>{{cite news|url=http://www.uswaternews.com/archives/arcglobal/8mexicity11.html |title=Mexico City's 'water monster' nears extinction |date=November 2008 |access-date=2010-06-28 |url-status = dead|archive-url=https://web.archive.org/web/20110723194731/http://www.uswaternews.com/archives/arcglobal/8mexicity11.html |archive-date=2011-07-23 }}</ref>
==Feeding==
The axolotl is carnivorous, consuming small prey such as worms, insects, and small fish in the wild.


Axolotls are members of the tiger salamander, or ''[[Ambystoma tigrinum]]'', [[species complex]], along with all other Mexican species of ''Ambystoma''. Their habitat is like that of most neotenic species—a high-altitude body of water surrounded by a risky terrestrial environment. These conditions are thought to favor [[neoteny]]. However, a terrestrial population of Mexican tiger salamanders occupies and breeds in the axolotl's habitat.{{Citation needed|date=January 2017}}
In captivity, they can be fed a variety of readily available foods, including trout and salmon pellets, frozen or live [[bloodworm]]s, [[earthworm]]s, and [[waxworm]]s on occasion. Beef heart is sometimes recommended, but is not a good choice for the animal as mammal proteins are not readily digestible. Small juveniles can be fed with [[Daphnia]] or [[brine shrimp]] until old enough for the larger foodstuffs. Axolotls locate food by smell, and will "snap" at any potential meal, sucking the food into their stomachs with vacuum force. This is why other animals may not be housed with axolotls (excluding adults with adults) as anything smaller will invariably end up in their mouths &mdash; even siblings. Fish can also nibble an axolotl's gill stalks and appendages while it is asleep, leading to infection.


==Elevage conditions==
=== Diet ===
The axolotl is carnivorous, consuming small prey such as mollusks,<ref name="animaldiversity.org">{{Cite web|url=https://animaldiversity.org/accounts/Ambystoma_mexicanum/|title=Ambystoma mexicanum (Salamandra ajolote)|website=[[Animal Diversity Web]] }}</ref> worms, insects, other arthropods,<ref name="animaldiversity.org"/> 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.<ref>{{Cite journal |last1=Wainwright |first1=P. C. |last2=Sanford |first2=C. P. |last3=Reilly |first3=S. M. |last4=Lauder |first4=G. V. |date=1989 |title=Evolution of motor patterns: aquatic feeding in salamanders and ray-finned fishes |journal=Brain, Behavior and Evolution |volume=34 |issue=6 |pages=329–341 |doi=10.1159/000116519 |pmid=2611639}}</ref>
[[Image:Ambystoma mexicanum.jpg|right|250px]]
Axolotls live at temperatures of 14&ndash;20°C (54&ndash;68°F), preferably 17&ndash;18°C (62&ndash;64°F). Lower temperatures slow down their metabolism, although temperatures around 10 C (50 F) are a panacea when it comes to treating disease. Higher temperatures can lead to stress and increased appetite; use of a cooling aggregate is strongly recommended. Tapwater should be treated with a [[dechlorinator]] before the animal is introduced into it, as chlorine is harmful. A single typical axolotl should be kept in a normal 10 U.S. gallon tank (44 litres) with around 150&nbsp;mm (6 inches) of water. A full tank without a lid can lead to the animal "jumping ship" as it were. A power filter or underwater filter run by an airpump can be used, though water flow should be kept to a minimum to avoid stressing the animal. A 20% water change should be done every week, with debris being siphoned from the bottom. Small gravel should be avoided, as the axolotl can mistakenly ingest the rocks, leading to stress and gas. Floating plants should be provided for juveniles as a hiding place, and small caves should likewise be provided for adults. No light is necessary as axolotls are nocturnal. Axolotls will spend a majority of the time on the bottom of the tank (floating is a sign of stress and illness), so design accordingly.


==Use as a model organism==
==Cultural references==
{{See also|Model organism}}
*Argentinian writer [[Julio Cortazar]] wrote a short story named after the salamander.
[[File:Axolotl-2193331 1280.webp|thumb|right|Leucistic axolotl in captivity]]
*The word ''axolotl'' is used frequently in [[crossword puzzle]]s.
Today, the axolotl is still used in research as a [[model 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 the demands of terrestrial life. One attractive feature for research is the large and easily manipulated [[embryo]], which allows viewing of the full development of a vertebrate. Axolotls are used in [[heart defects|heart 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. The axolotl is also considered an ideal animal model for the study of [[neural tube]] closure due to the similarities between human and axolotl [[neural plate]] and tube formation; the axolotl's neural tube, unlike the frog's, is not hidden under a layer of superficial [[epithelium]].<ref>{{Cite journal |last=Gordon |first=R. |date=1985 |title=A review of the theories of vertebrate neurulation and their relationship to the mechanics of neural tube birth defects |url= https://journals.biologists.com/dev/content/89/Supplement/229 |journal= [[Journal of Embryology and Experimental Morphology]] |volume=89 |issue=Supplement |pages= 229–255 |pmid=3913733}}</ref> There are also mutations affecting other organ systems some of which are not well characterized and others that are.<ref>{{Cite journal |last=Armstrong |first=John B. |date=1985 |title= The axolotl mutants |journal= [[Developmental Genetics]] |volume=6 |issue=1 |pages=1–25 |doi=10.1002/dvg.1020060102}}</ref> The genetics of the color variants of the axolotl have also been widely studied.<ref name= "Color Atlas of Pigment Genes"/>
*[[Axolotl tank]]s are frequently mentioned in [[Frank Herbert]]'s ''[[Dune (novel)|Dune]]'' series of books.
*It was often used in the early years of [[Mad (magazine)|''Mad'' magazine]] as a [[running gag]] and [[in-joke]], for instance in a [[parody]] of [[William Wordsworth|Wordsworth's]] "[[I Wandered Lonely as a Cloud]]":
:"I wandered lonely as a clod,
:Just picking up old rags and bottles,
:When onward on my way I plod,
:I saw a host of axolotls."
*The axolotl is mentioned in the musical ''[[Children of Eden]]'' by [[Stephen Schwartz]]. In the aptly titled song called "The Naming", Adam and Eve run through a long list of animals that start with the letter "a", at one point going mentioning the axolotl.
*The fictional creatures [[Wooper]] and [[Quagsire]] from the ''[[Pokémon]]'' franchise are both based on axolotls.
*The fictional Dr. Shrunk from ''[[Animal Crossing: Wild World]]'' is an axolotl. If talked to, he will grant you emotions to use ingame.
*In [[Rene Daumal]]'s book ''[[A Night of Serious Drinking]]'' there is an extended metaphor relating mankind to axolotls. He states that while an axolotl may be forced into maturity, man cannot be forced but must desire such a transformation.
*Axolotls are also mentioned in Primo Levi's short story, ''Angelica Farfalla''.
*The band [[Hum (band)|Hum]] titled a song ''Afternoon With the Axolotls'' on their [[1997]] album [[Downward is Heavenward]].
*The webcomic 5ideways has a character that is referred to as "Axolotl"


=== Regeneration ===
The band 'Nautilis' have a 2002 electronica album entitled 'Are You an Axolotl' out on the record label 'Planet Mu'
The feature of the axolotl that attracts most attention is its healing ability: the axolotl does not heal by [[scar]]ring and is capable of the [[regeneration (biology)|regeneration]] of entire lost appendages in a period of months, and, in certain cases, more vital structures, such as tail, limb, central nervous system, and tissues of the eye and heart.<ref>{{Cite journal|last1=Caballero-Pérez|first1=Juan|last2=Espinal-Centeno| first2= Annie|last3=Falcon|first3=Francisco|last4=García-Ortega|first4=Luis F.|last5=Curiel-Quesada|first5=Everardo|last6=Cruz-Hernández| first6= Andrés| last7=Bako|first7=Laszlo|last8=Chen|first8=Xuemei|last9=Martínez|first9=Octavio|last10=Alberto Arteaga-Vázquez| first10= Mario| last11= Herrera-Estrella|first11=Luis|date=January 2018|title=Transcriptional landscapes of Axolotl (Ambystoma mexicanum)|journal= [[Developmental Biology]]| language= en| volume=433|issue=2|pages=227–239|doi=10.1016/j.ydbio.2017.08.022|pmid=29291975|doi-access=}}</ref> They can even restore less vital parts of their brains. 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 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. In metamorphosed individuals, however, the ability to regenerate is greatly diminished. The axolotl is therefore used as a model for the development of limbs in vertebrates.<ref name=PMID18814845>{{cite journal |last1=Roy |first1=S |last2=Gatien |first2=S |title=Regeneration in axolotls: a model to aim for! |journal= [[Experimental Gerontology]] |date=November 2008 |volume=43 |issue=11 |pages=968–73 |pmid=18814845 |doi=10.1016/j.exger.2008.09.003 |s2cid=31199048 }}</ref> 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.<ref>{{cite journal |last1=Vieira |first1=Warren A. |last2=Wells |first2=Kaylee M. |last3=McCusker |first3=Catherine D. |title=Advancements to the Axolotl Model for Regeneration and Aging |journal=Gerontology |date=2020 |volume=66 |issue=3 |pages=212–222 |doi=10.1159/000504294 |pmid=31779024|pmc=7214127 |doi-access=free}}</ref> 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.<ref>{{cite journal |last1=Goodwin |first1=James W. |last2=Pinto |first2= Alexander R. |last3=Rosenthal |first3=Nadia A. |editor-first= Eric N.| editor-last= Olson |title=Macrophages are required for adult salamander limb regeneration |journal= [[Proceedings of the National Academy of Sciences of the United States of America]]|date=June 4, 2013 |volume=110 |issue=23 |pages=9415–9420 |doi=10.1073/pnas.1300290110 |pmid=23690624 |pmc=3677454 |bibcode=2013PNAS..110.9415G |doi-access=free }}</ref> However, this belief has been questioned by other studies.<ref>{{cite journal |last1=Pedersen |first1=Katherine |last2=Rasmussen |first2=Rikke Kongsgaard |last3=Dittrich |first3=Anita |last4=Pedersen |first4= Michael |last5=Lauridsen |first5=Henrik |title=Modulating the immune response and the pericardial environment with LPS or prednisolone in the axolotl does not change the regenerative capacity of cryoinjured hearts |journal=[[The FASEB Journal]] |date=April 17, 2020 |volume=34 |issue= S1 |page=1 |doi=10.1096/fasebj.2020.34.s1.04015 |s2cid=218792957 |doi-access=free }}</ref> 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.<ref>Masselink, Wouter, and Elly M. Tanaka. "Toward Whole Tissue Imaging of Axolotl Regeneration." Developmental Dynamics, vol. 250, no. 6, 2020, pp. 800–806., https://doi.org/10.1002/dvdy.282.</ref>

=== Genome ===
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 pathway]]s that may be responsible for limb regeneration.<ref name=":0">{{Cite journal |last1=Nowoshilow |first1=Sergej |last2=Schloissnig |first2=Siegfried |last3=Fei |first3=Ji-Feng |last4=Dahl |first4=Andreas |last5=Pang |first5=Andy W. C. |last6=Pippel |first6=Martin |last7=Winkler |first7=Sylke |last8=Hastie |first8=Alex R. |last9=Young |first9=George |date=2018-01-24 |title=The axolotl genome and the evolution of key tissue formation regulators |journal=[[Nature (journal)|Nature]] |volume=554 |issue=7690 |pages=50–55 |doi=10.1038/nature25458 |pmid=29364872 |issn=1476-4687 |bibcode=2018Natur.554...50N |doi-access=free |hdl=21.11116/0000-0003-F659-4 |hdl-access=free }}</ref> Although the axolotl genome is about 10 times as large as the [[human genome]], it encodes a similar number of proteins, namely 23,251<ref name=":0" /> (the human genome encodes about 20,000 proteins). The size difference is mostly explained by a large fraction of [[repeated sequence (DNA)|repetitive sequences]], but such repeated elements also contribute to increased median [[intron]] sizes (22,759&nbsp;bp) which are 13, 16 and 25 times that observed in human (1,750&nbsp;bp), [[mouse]] (1,469&nbsp;bp) and [[Nanorana parkeri|Tibetan frog]] (906&nbsp;bp), respectively.<ref name=":0" />

=== Neoteny ===
{{Main article|Neoteny}}
Most amphibians begin their lives as [[aquatic animal]]s which are unable to live on dry land, often being dubbed as [[tadpole]]s. To reach [[adult]]hood, they go through a process called [[metamorphosis (biology)|metamorphosis]], in which they lose their gills and start living on land. However, the axolotl is unusual in that it has a lack of [[thyroid-stimulating hormone]], which is needed for the [[thyroid]] to produce [[thyroxine]] in order for the axolotl to go through metamorphosis; therefore, it keeps its gills and lives in water all its life, even after it becomes an adult and is [[sexual maturity|able to reproduce]]. Neoteny is the term for reaching sexual maturity without undergoing metamorphosis.<ref name="ley196802">{{Cite magazine
|last=Ley
|first=Willy
|date=February 1968
|title=Epitaph for a Lonely Olm
|department=For Your Information
|url=https://archive.org/stream/Galaxy_v26n03_1968-02_modified#page/n37/mode/2up
|magazine=Galaxy Science Fiction
|pages=95–104
}}</ref>

The genes responsible for neoteny in laboratory animals may have been identified; however, they are not linked in wild populations, suggesting [[artificial selection]] is the cause of complete neoteny in laboratory and pet axolotls.<ref name=":3">{{Cite journal|last=Malacinski |first=George M.|date=1978-05-01|title=The Mexican Axolotl, ''Ambystoma mexicanum'': Its Biology and Developmental Genetics, and Its Autonomous Cell-lethal Genes|journal=American Zoologist|volume=18|issue=2|pages=195–206|doi=10.1093/icb/18.2.195 |doi-access=free}}</ref> The genes responsible have been narrowed down to a small chromosomal region called ''met1'', which contains several candidate genes.<ref name=Crowner>{{cite journal |last1=Crowner |first1=Anne |last2=Khatri |first2=Shivam |last3=Blichmann |first3=Dana |last4=Voss |first4=S. Randal |title=Rediscovering the Axolotl as a Model for Thyroid Hormone Dependent Development |journal=Frontiers in Endocrinology |date=12 April 2019 |volume=10 |page=237 |doi=10.3389/fendo.2019.00237 |doi-access=free|pmid=31031711 |pmc=6473073 }}</ref>

==== Metamorphosis ====
The axolotl's body has the capacity to go through metamorphosis if given the necessary hormone, but axolotls do not produce it, and must be exposed to it from an external source, after which an axolotl undergoes an artificially-induced metamorphosis and begins living on land.<ref>{{Cite journal |last1=Demircan |first1=Turan |last2=Ovezmyradov |first2=Guvanch |last3=Yıldırım |first3=Berna |last4=Keskin |first4=İlknur |last5=İlhan |first5=Ayşe Elif |last6=Fesçioğlu |first6=Ece Cana |last7=Öztürk |first7=Gürkan |last8=Yıldırım |first8=Süleyman |date=2018-07-20 |title=Experimentally induced metamorphosis in highly regenerative axolotl (''Ambystoma mexicanum'') under constant diet restructures microbiota |journal=Scientific Reports |language=en |volume=8 |issue=1 |page=10974 |doi=10.1038/s41598-018-29373-y |pmid=30030457 |pmc=6054665 |bibcode=2018NatSR...810974D }}</ref> In laboratory conditions, metamorphosis is reliably induced by administering either the thyroid hormone [[thyroxine]] or [[thyroid-stimulating hormone]]. The former is more commonly used.<ref name=Crowner/>

<!-- This is the sort of stuff that gets put in a box in fancy journal articles. -->
{{Side box|metadata=no|text=
'''Role of iodine'''
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 (I<sub>2</sub>), which reacts with the [[tyrosyl]] residues of [[thyroglobulin]]. Two iodinated tyrosyl residues are conjugated together. When they are cleaved from the thyroglobulin chain, thyroid hormone is obtained.<ref name=boron2012>Chapter 49, "Synthesis of Thyroid Hormones" in: {{cite book |author1=Walter F. Boron |author2=Emile L. Boulpaep |title=Medical Physiology |edition= 2nd|publisher=Elsevier/Saunders |year=2012 |isbn=9781437717532}}{{page needed|date=April 2016}}</ref>

In the absence of induced metamorphosis, larval axolotls start absorbing iodide into their thyroid glands at 30 days postfertilization. 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.<ref name="pmid9371791">{{cite journal |last1=Brown |first1=Donald D. |title=The role of thyroid hormone in zebrafish and axolotl development |journal=Proceedings of the National Academy of Sciences |date=25 November 1997 |volume=94 |issue=24 |pages=13011–13016 |doi=10.1073/pnas.94.24.13011 |doi-access=free |pmid=9371791 |pmc=24254|bibcode=1997PNAS...9413011B }}</ref>

[[Diiodotyrosine]], an analogue of the iodinated [[thyroglobulin]] precursor in thyroxine biosynthesis, causes metamorphosis in axolotls that have their thyroids removed.<ref>{{cite journal |last1=Swingle |first1=W. W. |title=Iodine and Amphibian Metamorphosis |journal=The Biological Bulletin |date=November 1923 |volume=45 |issue=5 |pages=229–253 |doi=10.2307/1536749|jstor=1536749 }}</ref> [[Lugol's solution]], which contains both iodide and I<sub>2</sub>, triggers metamorphosis when injected.<ref>{{cite journal |last1=Ingram |first1=W. R. |title=Metamorphosis of the Colorado Axolotl by Injection of Inorganic Iodine. |journal=Experimental Biology and Medicine |date=1 December 1928 |volume=26 |issue=3 |pages=191 |doi=10.3181/00379727-26-4212}}</ref> This is because diiodotyrosine and thyroxine is produced when I<sub>2</sub> reacts with proteins other than thyroglobulin. If given in a bath instead of injected, I<sub>2</sub> has no effect on axolotls.<ref>{{cite journal |last1=Dvoskin |first1=Samuel |title=The Thyroxine-Like Action of Elemental Iodine in the Rat and Chick1 |journal=Endocrinology |date=May 1947 |volume=40 |issue=5 |pages=334–352 |doi=10.1210/endo-40-5-334|pmid=20245954 }}</ref> [[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.<ref>{{cite journal |last1=Krylov |first1=O. A. |title=The role of haloids (bromine and iodine) in the metamorphosis of amphibia |journal=Bulletin of Experimental Biology and Medicine |date=January 1961 |volume=50 |issue=1 |pages=724–727 |doi=10.1007/BF00796048}}</ref>
}}

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.<ref name=":6">{{Cite web|title=Axolotls - Metamorphosed & Tiger Salamanders|url=https://www.axolotl.org/tiger_salamander.htm|access-date=2022-01-25|website=www.axolotl.org}}</ref>

An axolotl that has gone through metamorphosis resembles an adult [[plateau tiger salamander]], though the axolotl differs in its longer toes.{{Citation needed|date=February 2011}} 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.<ref name=":6" /> 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.<ref>{{cite web |title=Transition & Feeding |url=https://www.morphedaxolotls.com/transition-and-feeding |website=Morphed Axolotls |language=en}}</ref>

==== History ====

Six adult axolotls (including a leucistic specimen) were shipped from [[Mexico City]] to the ''[[Jardin 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.{{verify source|reason=French language verification needed|date=November 2021}} This discovery was the starting point of research about neoteny. It is not certain that ''[[Ambystoma velasci]]'' specimens were not included in the original shipment.{{Citation needed|date=October 2021}} Vilem Laufberger in Prague used thyroid hormone injections to induce an axolotl to grow into a terrestrial adult salamander. The experiment was repeated by Englishman [[Julian Huxley]], who was unaware the experiment had already been done, using ground thyroids.<ref name=":4">{{Cite journal|last1=Reiß|first1=Christian|last2=Olsson|first2=Lennart|last3=Hoßfeld|first3=Uwe|date=2015|title=The history of the oldest self-sustaining laboratory animal: 150 years of axolotl research|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/jez.b.22617|journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution|language=en|volume=324|issue=5|pages=393–404|doi=10.1002/jez.b.22617|pmid=25920413|bibcode=2015JEZB..324..393R |issn=1552-5015}}</ref> Since then, experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.<ref name=":5">{{Cite journal|last1=Safi|first1=Rachid|last2=Bertrand|first2=Stéphanie|last3=Marchand|first3=Oriane|last4=Duffraisse|first4=Marilyne|last5=de Luze|first5=Amaury|last6=Vanacker|first6=Jean-Marc|last7=Maraninchi|first7=Marie|last8=Margotat|first8=Alain|last9=Demeneix|first9=Barbara|last10=Laudet|first10=Vincent|date=2004-02-01|title=The Axolotl (''Ambystoma mexicanum''), a Neotenic Amphibian, Expresses Functional Thyroid Hormone Receptors|journal=Endocrinology|volume=145|issue=2|pages=760–772|doi=10.1210/en.2003-0913|pmid=14576183 |doi-access=free}}</ref>

==== In other salamanders ====
Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations. [[Siren (amphibian)|Siren]]s and ''[[Necturus]]'' are other 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 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 through [[cannibalism]], they quickly begin metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements.<ref>{{cite web|last=Venturi|first= S.|year=2004 |url= https://sites.google.com/site/iodinestudies/morosini |archive-date=4 March 2017 |url-status = dead|archive-url=https://web.archive.org/web/20170304010444/https://sites.google.com/site/iodinestudies/morosini|access-date = 25 September 2020|title= Iodine and Evolution. DIMI-Marche}}</ref> In fact, in some high mountain lakes there live dwarf forms of [[salmonids]] that are caused by deficiencies in food and, in particular, iodine, which causes [[cretinism]] and [[dwarfism]] due to [[hypothyroidism]], as it does in humans.

== Online Model Organism Database ==
[[xenbase]] provides limited support (BLAST, JBrowse tracks, genome download) for Axolotls.

== Threats ==
Axolotls are only native to the Mexican Central Valley. Although the native axolotl population once extended through most of the lakes and wetlands that make up this region, the native habitat is now limited to Lake Xochimilco as a result of the expansion of Mexico City. Lake Xochimilco is not a large body of water, but rather a small series of artificial channels, small lakes, and temporary wetlands.

[[File:Lake Xochimilco in Southern Mexico.jpg|thumb|Lake Xochimilco is the only native habitat left for the axolotl.]]

Lake Xochimilco has poor water quality, caused by the region's aquaculture and agriculture demands. It is also maintained by inputs of only partially treated wastewater. Water quality tests reveal a low nitrogen-phosphorus ratio and a high concentration of chlorophyll a, which are indicative of an oxygen-poor environment that is not well-suited for axolotls.<ref>{{Cite journal |last1=Nandini |first1=Sarma |last2=García |first2=Pedro Ramirez |last3=Sarma |first3=S. S. S. |date=2016 |title=Water quality in Lake Xochimilco, Mexico: zooplankton indicators and Vibrio cholerae |url=https://www.jlimnol.it/index.php/jlimnol/article/view/jlimnol.2015.1213 |journal=Journal of Limnology |language=en |volume=75 |issue=1 |doi=10.4081/jlimnol.2015.1213 |issn=1723-8633|doi-access=free }}</ref> In addition, the intensive use of pesticides from agriculture around Lake Xochimilco causes run off into the lake and a reduction of habitat quality for axolotls. The pesticides used contain chemical compounds that studies show to 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.<ref>{{Cite journal |last1=Robles-Mendoza |first1=C. |last2=García-Basilio |first2=C. |last3=Cram-Heydrich |first3=S. |last4=Hernández-Quiroz |first4=M. |last5=Vanegas-Pérez |first5=C. |date=2009-02-01 |title=Organophosphorus pesticides effect on early stages of the axolotl Ambystoma mexicanum (Amphibia: Caudata) |url=https://www.sciencedirect.com/science/article/pii/S004565350801223X |journal=Chemosphere |language=en |volume=74 |issue=5 |pages=703–710 |doi=10.1016/j.chemosphere.2008.09.087 |pmid=19012946 |bibcode=2009Chmsp..74..703R |issn=0045-6535}}</ref>

Another factor that threatens the native axolotl population is the introduction of invasive species such as the Nile tilapia and common carp. These invasive fish species threaten axolotl populations by eating their eggs or young and by out-competing them for natural resources. The presence of these species has also been shown to change the behavior of axolotls, causing them to be less active to avoid predation. This reduction in activity greatly impacts the axolotls foraging and mating opportunities.<ref>{{Cite journal |last1=Alcaraz |first1=Guillermina |last2=López-Portela |first2=Xarini |last3=Robles-Mendoza |first3=Cecilia |date=2015-07-01 |title=Response of a native endangered axolotl, Ambystoma mexicanum (Amphibia), to exotic fish predator |url=https://doi.org/10.1007/s10750-015-2194-4 |journal=Hydrobiologia |language=en |volume=753 |issue=1 |pages=73–80 |doi=10.1007/s10750-015-2194-4 |s2cid=254550469 |issn=1573-5117}}</ref>

With such a small native population, there is a large loss of genetic diversity. This lack of genetic diversity can be dangerous for the remaining population, causing an increase in inbreeding and a decrease in general fitness and adaptive potential. It ultimately raises the axolotl's risk for extinction, something that they are already in danger of. Studies have found indicators of a low interpopulation gene flow and higher rates of genetic drift. These are likely the result of multiple “bottleneck” incidents in which events that kill off several individuals of a population occur and sharply reduce the genetic diversity of the remaining population. The offspring produced after bottleneck events have a greater risk of showing decreased fitness and are often less capable of adaptation down the line. Multiple bottleneck events can have disastrous effects on a population. Studies have also found high rates of relatedness that are indicative of inbreeding. Inbreeding can be especially harmful as it can cause an increase in the presence of deleterious, or harmful, genes within a population.<ref>{{Cite journal |last1=Parra-Plea |first1=G |last2=Zamudio |first2=K.R. |last3=Recuero |first3=E. |last4=Aguilar-=Miguel |first4=X. |last5=Huaxuz |first5=D. |last6=Zambrano |first6=L. |date=2011 |title=Conservation genetics of threatened Mexican axolotls (Ambystoma) |url=https://doi.org/10.1111/j.1469-1795.2011.00488.x |journal=American Conservation |volume=15 |issue=1 |pages=61–72|doi=10.1111/j.1469-1795.2011.00488.x |s2cid=46992721 }}</ref> The detection of introgressed tiger salamander (''A. tigrinum'') DNA in the laboratory axolotl population raises further concerns about the suitability of the captive population as an ark for potential reintroduction purposes.<ref>{{Cite journal |last1=Woodcock |first1=M. Ryan |last2=Vaughn-Wolfe |first2=Jennifer |last3=Elias |first3=Alexandra |last4=Kump |first4=D. Kevin |last5=Kendall |first5=Katharina Denise |last6=Timoshevskaya |first6=Nataliya |last7=Timoshevskiy |first7=Vladimir |last8=Perry |first8=Dustin W. |last9=Smith |first9=Jeramiah J. |last10=Spiewak |first10=Jessica E. |last11=Parichy |first11=David M. |last12=Voss |first12=S. Randal |date=2017-01-31 |title=Identification of Mutant Genes and Introgressed Tiger Salamander DNA in the Laboratory Axolotl, Ambystoma mexicanum |journal=Scientific Reports |language=en |volume=7 |issue=1 |pages=6 |doi=10.1038/s41598-017-00059-1 |pmid=28127056 |issn=2045-2322|pmc=5428337 |bibcode=2017NatSR...7....6W }}</ref>

There has been little improvement in the conditions of the lake or the population of native axolotls. Many scientists are focusing their conservation efforts on translocation of captive-bred individuals into new habitats or reintroduction into Lake Xochimilco. The Laboratorio de Restauracion Ecologica (LRE) in the Universidad Nacional Autonoma de Mexico (UNAM) has built up a population of more than 100 captive-bred individuals. These axolotls are mostly used for research by the lab but plans of a semi-artificial wetland inside the university have been established and the goal is 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 to establish or re-establish a wild population.<ref>{{Cite journal |last1=Ramos |first1=A.G. |last2=Mena-Gonzalez |first2=H. |last3=Zambrano |first3=L |date=2021 |title=The potential of temporary shelters to increase survival of the endangered Mexican axolotl |url=https://doi.org/10.1002/aqc.3520 |journal=Aquatic Conservation: Marine and Freshwater Ecosystems |volume=31 |issue=6 |pages=1535–1542|doi=10.1002/aqc.3520 |bibcode=2021ACMFE..31.1535R |s2cid=235587173 }}</ref><ref>{{Cite news |last=Paúl |first=María Luisa |date=2023-12-01 |title=Mexico wants you to adopt an axolotl, the amphibian that never grows up |language=en-US |newspaper=Washington Post |url=https://www.washingtonpost.com/nation/2023/12/01/axolotl-mexican-salamander-adoption-campaign/ |access-date=2023-12-01 |issn=0190-8286}}</ref>

==Captive care==
{{See also|Herpetoculture}}
[[File: Ambystoma mexicanum at Vancouver Aquarium.jpg|thumb|left|These axolotls at [[Vancouver Aquarium]] are [[leucistic]], with less pigmentation than normal.]]

[[File:Axolotl in a Pet store in Melbourne.jpg|thumb|Axolotl in a pet store in Melbourne, Australia]] The axolotl is a popular exotic pet like its relative, the tiger salamander (''Ambystoma tigrinum''). As for all [[poikilotherm]]ic organisms, lower temperatures result in slower metabolism and a very unhealthily reduced appetite. Temperatures at approximately {{convert|16|C|F}} to {{convert|18|C|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 {{convert|24|C|F}} may lead to metabolic rate increase, also causing stress and eventually death.<ref>{{Cite web|url=http://www.axolotl.org/requirements.htm|title=Axolotls – Requirements & Water Conditions in Captivity|website=axolotl.org|access-date=2016-03-14}}</ref><ref>{{Cite web|url=http://www.caudata.org/cc/species/Ambystoma/A_mexicanum.shtml|title=Caudata Culture Species Entry – ''Ambystoma mexicanum'' – Axolotl|website=www.caudata.org|access-date=2016-03-14|archive-date=2016-03-15|archive-url=https://web.archive.org/web/20160315031956/http://www.caudata.org/cc/species/Ambystoma/A_mexicanum.shtml|url-status=dead}}</ref> [[Chlorine]], commonly added to [[tapwater]], is harmful to axolotls. A single axolotl typically requires a {{convert|150|L|gal|adj=on|abbr=off}} tank. Axolotls spend the majority of the time at the bottom of the tank.<ref>{{Cite web |url=https://www.tfhmagazine.com/articles/freshwater/axolotls-keeping-a-water-monster-full |last=Wiegert |first=Joshua |title=Axolotls: Keeping a Water Monster}}</ref>

[[File: Axolotl with Grit XRay.png|thumb|upright=0.6|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.<ref name=kulbisky>{{Cite journal |last1=Kulbisky |first1=Gordon P |last2=Rickey |first2=Daniel W |last3=Reed |first3=Martin H |last4=Björklund |first4=Natalie |last5=Gordon |first5=Richard |date=1999 |title=The axolotl as an animal model for the comparison of 3-D ultrasound with plain film radiography |journal=Ultrasound in Medicine and Biology |volume=25 |issue=6 |pages=969–975 |doi=10.1016/s0301-5629(99)00040-x|pmid=10461726 }}</ref>]]

Salts, such as [[Holtfreter's solution]], are often added to the water to prevent infection.<ref>{{cite web|last=Clare|first= John P. |url=http://www.axolotl.org/health.htm |title=Health and Diseases|website=axolotl.org}}</ref>

In captivity, axolotls eat a variety of readily available foods, including trout and salmon pellets, frozen or live [[Glycera (genus)|bloodworm]]s, [[earthworm]]s, and [[waxworm]]s. Axolotls can also eat [[feeder fish]], but care should be taken as fish may contain parasites.<ref>{{Cite journal | doi=10.1577/03632415.2011.10389070|title = The Aquarium Trade as an Invasion Pathway in the Pacific Northwest| journal=Fisheries| volume=36| issue=2| pages=74–85|year = 2011|last1 = Strecker|first1 = Angela L.| last2=Campbell| first2=Philip M.| last3=Olden| first3=Julian D.| bibcode=2011Fish...36...74S |url = https://pdxscholar.library.pdx.edu/esm_fac/40}}</ref>

Substrates are another important consideration for captive axolotls, as axolotls (like other amphibians and reptiles) tend to ingest bedding material together with food<ref name=Pough /> and are commonly prone to gastrointestinal obstruction and foreign body ingestion.<ref>{{cite journal|title=Amphibian Emergency Medicine|journal=Veterinary Clinics of North America: Exotic Animal Practice|volume=10|issue=2|pages=587–620|doi=10.1016/j.cvex.2007.02.004|pmid=17577564|year=2007|last1=Clayton|first1=Leigh Ann|last2=Gore|first2=Stacey R.}}</ref> 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.<ref name=Pough>{{cite web|last=Pough|first=F. H.|url=http://netvet.wustl.edu/species/reptiles/pough.txt|title=Recommendations for the Care of Amphibians and Reptiles in Academic Institutions|publisher=National Academy Press|location=Washington, D.C.|year=1992}}</ref> 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&nbsp;cm (or approximately the size of the animal's head) should be available to the animal.<ref name=Gresens>{{cite journal|last1=Gresens|first1=Jill|title=An Introduction to the Mexican Axolotl (''Ambystoma mexicanum'')|journal=Lab Animal|volume=33|issue=9|year=2004|pages=41–47|doi=10.1038/laban1004-41|pmid=15457201|s2cid=33299160}}</ref>

There is some evidence that axolotls might seek out appropriately-sized gravel for use as [[gastroliths]]<ref>Wings, O [http://www.dinosaurhunter.org/files/app-2007-wings-gastrolith_function_classification.pdf A review of gastrolith function with implications for fossil vertebrates and a revised classification] {{Webarchive|url=https://web.archive.org/web/20160304062304/http://www.dinosaurhunter.org/files/app-2007-wings-gastrolith_function_classification.pdf |date=2016-03-04 }} Acta Palaeontologica Polonica 52 (1): 1–16</ref> based on experiments conducted at the University of Manitoba axolotl colony.<ref>Gordon, N, [https://embryogenesisexplained.org/2015/09/10/gastroliths-how-i-learned-to-stop-worrying-and-love-gravel/ Gastroliths – How I Learned to Stop Worrying and Love Gravel.] {{Webarchive|url=https://web.archive.org/web/20200922142827/https://embryogenesisexplained.org/2015/09/10/gastroliths-how-i-learned-to-stop-worrying-and-love-gravel/ |date=2020-09-22 }}</ref><ref>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.</ref> As there is no conclusive evidence pointing to gastrolith use, gravel should be avoided due to the high risk of [[Impaction (animals)|impaction]].<ref>{{Cite journal|last=Loh|first=Richmond|date=2015-05-15|title=Common Disease Conditions in Axolotls|website=Vin.com |url=https://www.vin.com/apputil/content/defaultadv1.aspx?pId=14365&catId=73681&id=7259254|url-status=live|access-date=2022-01-21|archive-url=https://web.archive.org/web/20200804110231/https://www.vin.com/apputil/content/defaultadv1.aspx?pId=14365&catId=73681&id=7259254 |archive-date=2020-08-04 }}</ref>

== Cultural significance ==
The species is named after the [[Aztec deity]] [[Xolotl]], 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.<ref name="Reuters2018">{{Cite news |date=2018-11-20 |title=Mexico's axolotl, a cartoon hero and genetic marvel, fights for survival |language=en |work=Reuters |url=https://www.reuters.com/article/us-mexico-axolotl-idUSKCN1NP0F6 |access-date=2022-08-16}}</ref> Axólotl also means water monster in the [[Nahuatl]] language.

They appear in the works of Mexican muralist [[Diego Rivera]]. In 2021, [[Mexico]] released a new design for its 50-[[Mexican peso|peso]] banknote featuring an axolotl along with [[maize]] and [[chinampa]]s on its back.<ref>{{Cite web |date=2020-02-21 |title=Mexican axolotl will be the new image of the 50 peso bill |url=https://www.theyucatantimes.com/2020/02/mexican-axolotl-will-be-the-new-image-of-the-50-peso-bill/ |access-date=2020-03-04 |website=The Yucatan Times |language=en-US}}</ref><ref>{{Cite web |title=Billete de 50 pesos de la familia G |url=https://www.banxico.org.mx/billetes-y-monedas/billete-50-pesos-familia-g.html |access-date=2023-02-20 |website=www.banxico.org.mx |language=es}}</ref> It was recognized as "Bank Note of the Year" by the [[International Bank Note Society]].<ref>{{Cite web |title=Banknote of 2021 Nominations |url=https://www.theibns.org/joomla/index.php?option=com_content&view=article&id=809:banknote-of-2021-nominations&catid=39:banknote-of-2021&Itemid=51 |access-date=2023-02-20 |website=www.theibns.org}}</ref> [[HD 224693]], a [[star]] in the [[Celestial equator|equatorial]] [[constellation]] of [[Cetus]], was named Axólotl in 2019.<ref>{{Cite web |title=Approved names |url=http://www.nameexoworlds.iau.org/final-results |access-date=2020-01-02 |website=www.nameexoworlds.iau.org |language=en}}</ref><ref>{{Cite web |date=December 17, 2019 |title=100 000s of People from 112 Countries Select Names for Exoplanet Systems In Celebration of IAU's 100th Anniversary |url=https://www.iau.org/news/pressreleases/detail/iau1912/ |access-date=2020-01-02 |website=[[International Astronomical Union]]}}</ref>

The [[List of Pokémon|Pokémon]] [[Mudkip]] and its evolutions, added in ''[[Pokémon Ruby and Sapphire]]'' (2002), take some visual inspiration from axolotls.<ref name="Reuters2018" /> Additionally, the Pokémon [[Wooper]], added in ''[[Pokémon Gold, Silver, and Crystal|Pokémon Gold, Silver and Crystal]]'' (1999), is directly based on an axolotl.<ref name="Reuters2018" />{{Additional citation needed|date=May 2023}} The looks of the dragons Toothless and The Night Fury in the ''[[How to Train Your Dragon]]'' movies are based on axolotls.<ref name="Reuters2018" /> They were also added to the video game ''[[Minecraft]]'' in 2020. It is following [[Mojang Studios]]' trend of adding endangered species to the game to raise awareness.<ref>{{Cite web |last=Minecraft |date=October 3, 2020 |title="Minecraft Live: Caves & Cliffs - First Look" |url=https://www.youtube.com/watch?v=DBvZ2Iqmm3M&t=2486s |website=[[YouTube]] |quote="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}}</ref> They were also added to its spin-off ''[[Minecraft Dungeons|Minecraft: Dungeons]]'' in 2022 and are available in ''[[Lego Minecraft]]''.<ref>{{Cite web |title=The Guardian Battle 21180 |url=https://www.lego.com/en-us/product/the-guardian-battle-21180 |access-date=2023-02-20 |website=www.lego.com |language=en}}</ref> An anthropomorphic Axolotl named Axo was also added as a purchasable outfit in ''[[Fortnite Battle Royale]]'' on August 9, 2020.<ref>{{cite web |date=5 August 2020 |title=Fortnite v13.40 Leaked Skins: Axo, Castaway Jonesy, Crustina & More |url=https://culturedvultures.com/fortnite-v13-40-leaked-skins-axo-castaway-jonesy-crustina/}}</ref><ref>{{cite tweet|number=1425245581480448005|user=FortniteGame|title=Axo got a new style just in time for @maisie_williams & @reubenSelby's hand-picked Locker Bundle}}</ref>

==See also==
* [[Amphibious fish]]
* [[Barred tiger salamander]]
* [[Handfish]]
* [[Lake Patzcuaro salamander]]
* [[Mudpuppies]]
* [[Olm]]
* [[Regenerative biomedicine]]
* [[Texas blind salamander]]
* [[Texas salamander]]

==References==
{{Reflist|30em|refs=
<ref name=Frost>{{cite web |url=http://research.amnh.org/vz/herpetology/amphibia/Amphibia/Caudata/Ambystomatidae/Ambystoma/Ambystoma-mexicanum |title=''Ambystoma mexicanum'' (Shaw and Nodder, 1798) |author=Frost, Darrel R. |year=2018 |website=Amphibian Species of the World: an Online Reference. Version 6.0 |publisher=American Museum of Natural History |access-date=10 August 2018}}</ref>
}}


==External links==
==External links==
{{Commons category|Ambystoma mexicanum}}
*[http://www.easyamphibians.co.uk/site/content/view/13/26/ Care of axolotl as pets]
* [https://web.archive.org/web/20160805192629/http://www.fishtankforum.co.uk/forum/viewtopic.php?f=190&t=5263 Follow the Eggs, Hatchlings and Juveniles]
*[http://www.axolotl.org Axolotl.org]
* [https://web.archive.org/web/20160310105933/http://fishtankforum.co.uk/forum/viewtopic.php?f=190&t=6455 Mating Dance and Laying Eggs]
*[http://www.biocrawler.com/videos/displayimage.php?album=12&pos=0 Short narrated film about the Axolotl]
* [https://web.archive.org/web/20160307055355/http://fishtankforum.co.uk/forum/viewtopic.php?f=190&t=6509 Follow the Eggs and Hatchlings (2nd Batch)]
* [https://web.archive.org/web/20140517152123/http://www.biologydir.com/indiana-university-axolotl-colony-info-11384.html Indiana U Axolotl Colony]
* [https://ambystoma.uky.edu/genetic-stock-center/ University of KY Axolotl Colony]
* [http://america.aljazeera.com/articles/2015/2/15/mystical-amphibian-venerated-by-aztecs-nears-extinction.html Mystical amphibian venerated by Aztecs nears extinction]
* [https://www.vox.com/22877353/axolotl-salamander-pet-extinction-mexico The animal that’s everywhere and nowhere]
* {{cite EB1911|wstitle=Axolotl |volume=20 |page=63 |short=x}}
* [https://www.xenbase.org xenbase.org]


{{Taxonbar|from=Q22718}}
[[Category:Mole salamanders]]
{{Authority control}}
[[Category:In-jokes]]
[[Category:Fauna of Mexico]]
[[Category:Nahuatl words]]
[[Category:MAD Magazine]]


[[Category:Amphibians described in the 18th century]]
[[bg:Аксолотл]]
[[Category:Animal models]]
[[da:Axolotl]]
[[Category:Animal testing on amphibians]]
[[de:Axolotl]]
[[Category:Animals bred for albinism on a large scale]]
[[es:Ajolote]]
[[Category:Critically endangered biota of Mexico]]
[[fr:Axolotl]]
[[Category:Critically endangered fauna of North America]]
[[it:Axolotl]]
[[Category:Endemic amphibians of Mexico]]
[[he:אקסולוטל מקסיקני]]
[[Category:Mole salamanders]]
[[nah:Āxōlōtl]]
[[Category:Population genetics]]
[[nl:Axolotl]]
[[Category:Regenerative biomedicine]]
[[ja:アホロートル]]
[[Category:Taxa named by Frederick Polydore Nodder]]
[[pl:Ambystoma meksykańska]]
[[Category:Taxa named by George Shaw]]
[[pt:Ambystoma mexicanum]]
[[Category:Valley of Mexico]]
[[ru:Аксолотль]]
[[Category:Species that are or were threatened by pollution]]
[[uk:Аксолотль]]
[[Category:Species that are or were threatened by invasive species]]

Latest revision as of 02:28, 14 December 2024

Axolotl
In the National Aquarium in Washington, D.C.
CITES Appendix II (CITES)[2]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Urodela
Family: Ambystomatidae
Genus: Ambystoma
Species:
A. mexicanum
Binomial name
Ambystoma mexicanum
(Shaw and Nodder, 1798)
Map
IUCN range of the axolotl.
  Axolotl (Ambystoma mexicanum)
Synonyms[3]
  • Gyrinus mexicanus Shaw and Nodder, 1798
  • Siren pisciformis Shaw, 1802
  • Siredon axolotl Wagler, 1830
  • Axolotes guttata Owen, 1844
  • Siredon Humboldtii Duméril, Bibron, and Duméril, 1854
  • Amblystoma weismanni Wiedersheim, 1879
  • Siredon edule Dugès, 1888

The axolotl (/ˈæksəlɒtəl/ ; from Classical Nahuatl: āxōlōtl [aːˈʃoːloːtɬ] ) (Ambystoma mexicanum)[3] is a paedomorphic salamander closely related to the tiger salamander.[3][4][5] It is unusual among amphibians in that it reaches adulthood without undergoing metamorphosis. Instead of taking to the land, adults remain aquatic and gilled. The species was originally found in several lakes underlying what is now Mexico City, such as Lake Xochimilco and Lake Chalco.[1] These lakes were drained by Spanish settlers after the conquest of the Aztec Empire, leading to the destruction of much of the axolotl's natural habitat.

As of 2020, the axolotl was near extinction[6][7] due to urbanization in Mexico City and consequent water pollution, as well as the introduction of invasive species such as tilapia and perch. It is 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 Natural Resources (IUCN) and is listed under Appendix II of the Convention on International Trade in Endangered Species (CITES).[2] Axolotls are used extensively in scientific research due to their ability to regenerate limbs, gills and parts of their eyes and brains.[8] Notably, their ability to regenerate declines with age, but it does not disappear. Axolotls keep modestly growing throughout their life and some consider this trait to be a direct contributor to their regenerative abilities.[9] Further research has been conducted to examine their heart as a model of human single ventricle and excessive trabeculation.[10] Axolotls were also sold as food in Mexican markets and were a staple in the Aztec diet.[11]

Axolotls may be confused with the larval stage of the closely related tiger salamander (A. tigrinum), which are widespread in much of North America and occasionally become paedomorphic, or with mudpuppies (Necturus spp.), fully aquatic salamanders from a different family that are not closely related to the axolotl but bear a superficial resemblance.[12]

Description

A captive leucistic axolotl, perhaps the most well known form of the axolotl
Face of a common or wild type axolotl
The speckled wild type form
Axolotl's gills (Ambystoma mexicanum)

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, including external gills and a caudal fin extending from behind the head to the vent.[13][14] External gills are usually lost when salamander species mature into adulthood, although the axolotl maintains this feature.[15] This is due to their neoteny evolution, where axolotls are much more aquatic than other salamander species.[16]

Their heads are wide, and their eyes are lidless. Their limbs are underdeveloped and possess long, thin digits. Males are identified by their swollen cloacae lined with papillae, while females are noticeable for their wider bodies full of eggs. Three pairs of external 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.[15] Four-gill slits lined with gill rakers are hidden underneath the external gills, which prevent food from entering and allow particles to filter through.

Axolotls have barely visible vestigial teeth, which develop during metamorphosis. The primary method of feeding is by suction, during which their rakers interlock to close the gill slits. External gills are used for respiration, although buccal pumping (gulping air from the surface) may also be used to provide oxygen to their lungs.[15] 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.

Buccal pumping
Axolotls displaying variations in color

Axolotls have four pigmentation genes; when mutated, they create different color variants. The normal wild-type animal is brown or tan with gold speckles and an olive undertone. The five most common mutant colors are listed below.

  1. Leucistic: pale pink with black eyes.
  2. Xanthic: grey, with black eyes.
  3. Albino: pale pink or white, with red eyes, which is more common in axolotls than other species.
  4. Melanoid: 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 white piebald appearance upon reaching maturity.[17] Because pet breeders frequently cross the variant colors, double homozygous mutants are common in the pet trade, especially white/pink animals with pink eyes that are double homozygous mutants for both the albino and leucistic trait.[18] Axolotls also have some limited ability to alter their color to provide better camouflage by changing the relative size and thickness of their melanophores.[19]

Habitat and ecology

Lake Xochimilco, Mexico City (Amanecer en Xochimilco). The native habitat of axolotls is important to the study of preservation and conservation.
Wild form

The axolotl is native only to the freshwater of Lake Xochimilco and Lake Chalco in the Valley of Mexico. Lake Chalco no longer exists, having been drained as a flood control measure, and Lake Xochimilco remains a remnant of its former self, existing mainly as canals. 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.[20]

Surveys in 1998, 2003, and 2008 found 6,000, 1,000, and 100 axolotls per square kilometer in its Lake Xochimilco habitat, respectively.[21] A four-month-long search in 2013, however, turned up no surviving individuals in the wild. Just a month later, two wild ones were spotted in a network of canals leading from Xochimilco.[22]

The wild population has been put under heavy pressure by the growth of Mexico City. The axolotl is currently on the International Union for Conservation of Nature's annual Red List of threatened species. Non-native fish, such as African tilapia and Asian carp, have also recently been introduced to the waters. These new fish have been eating the axolotls' young, as well as their primary source of food.[23]

Axolotls are members of the tiger salamander, or Ambystoma tigrinum, species complex, along with all other Mexican species of Ambystoma. Their habitat is like that of most neotenic species—a high-altitude body of water surrounded by a risky terrestrial environment. These conditions are thought to favor neoteny. However, a terrestrial population of Mexican tiger salamanders occupies and breeds in the axolotl's habitat.[citation needed]

Diet

The axolotl is carnivorous, consuming small prey such as mollusks,[24] worms, insects, other arthropods,[24] 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.[25]

Use as a model organism

Leucistic axolotl in captivity

Today, the axolotl is still used in research as a model 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 the demands of terrestrial life. One attractive feature for research is the large and easily manipulated embryo, which allows viewing of the full development of a vertebrate. Axolotls are used in heart 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. The axolotl is also considered an ideal animal model for the study of neural tube closure due to the similarities between human and axolotl neural plate and tube formation; the axolotl's neural tube, unlike the frog's, is not hidden under a layer of superficial epithelium.[26] There are also mutations affecting other organ systems some of which are not well characterized and others that are.[27] The genetics of the color variants of the axolotl have also been widely studied.[18]

Regeneration

The feature of the axolotl that attracts most attention is its healing ability: the axolotl does not heal by scarring and is capable of the regeneration of entire lost appendages in a period of months, and, in certain cases, more vital structures, such as tail, limb, central nervous system, and tissues of the eye and heart.[28] They can even restore less vital parts of their brains. 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 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. In metamorphosed individuals, however, the ability to regenerate is greatly diminished. The axolotl is therefore used as a model for the development of limbs in vertebrates.[29] 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.[30] 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.[31] However, this belief has been questioned by other studies.[32] 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.[33]

Genome

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.[34] Although the axolotl genome is about 10 times as large as the human genome, it encodes a similar number of proteins, namely 23,251[34] (the human genome encodes about 20,000 proteins). The size difference is mostly explained by a large fraction of repetitive sequences, but such repeated elements also contribute to increased median intron sizes (22,759 bp) which are 13, 16 and 25 times that observed in human (1,750 bp), mouse (1,469 bp) and Tibetan frog (906 bp), respectively.[34]

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. However, the axolotl is unusual in that it has a lack of thyroid-stimulating hormone, which is needed for the thyroid to produce thyroxine in order for the axolotl to go through metamorphosis; therefore, 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.[35]

The genes responsible for neoteny in laboratory animals may have been identified; however, they are not linked in wild populations, suggesting artificial selection is the cause of complete neoteny in laboratory and pet axolotls.[36] The genes responsible have been narrowed down to a small chromosomal region called met1, which contains several candidate genes.[37]

Metamorphosis

The axolotl's body has the capacity to go through metamorphosis if given the necessary hormone, but axolotls do not produce it, and must be exposed to it from an external source, after which an axolotl undergoes an artificially-induced metamorphosis and begins living on land.[38] In laboratory conditions, metamorphosis is reliably induced by administering either the thyroid hormone thyroxine or thyroid-stimulating hormone. The former is more commonly used.[37]

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.[45]

An axolotl that has gone through metamorphosis resembles an adult plateau tiger salamander, though the axolotl differs in its longer toes.[citation needed] 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.[45] 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.[46]

History

Six adult axolotls (including a leucistic specimen) were shipped from Mexico City to the Jardin 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 that Ambystoma 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 Englishman Julian Huxley, who was unaware the experiment had already been done, using ground thyroids.[47] Since then, experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.[16]

In other salamanders

Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations. Sirens and Necturus are other 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 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 through cannibalism, they quickly begin metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements.[48] In fact, in some high mountain lakes there live dwarf forms of salmonids that are caused by deficiencies in food and, in particular, iodine, which causes cretinism and dwarfism due to hypothyroidism, as it does in humans.

Online Model Organism Database

xenbase provides limited support (BLAST, JBrowse tracks, genome download) for Axolotls.

Threats

Axolotls are only native to the Mexican Central Valley. Although the native axolotl population once extended through most of the lakes and wetlands that make up this region, the native habitat is now limited to Lake Xochimilco as a result of the expansion of Mexico City. Lake Xochimilco is not a large body of water, but rather a small series of artificial channels, small lakes, and temporary wetlands.

Lake Xochimilco is the only native habitat left for the axolotl.

Lake Xochimilco has poor water quality, caused by the region's aquaculture and agriculture demands. It is also maintained by inputs of only partially treated wastewater. Water quality tests reveal a low nitrogen-phosphorus ratio and a high concentration of chlorophyll a, which are indicative of an oxygen-poor environment that is not well-suited for axolotls.[49] In addition, the intensive use of pesticides from agriculture around Lake Xochimilco causes run off into the lake and a reduction of habitat quality for axolotls. The pesticides used contain chemical compounds that studies show to 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.[50]

Another factor that threatens the native axolotl population is the introduction of invasive species such as the Nile tilapia and common carp. These invasive fish species threaten axolotl populations by eating their eggs or young and by out-competing them for natural resources. The presence of these species has also been shown to change the behavior of axolotls, causing them to be less active to avoid predation. This reduction in activity greatly impacts the axolotls foraging and mating opportunities.[51]

With such a small native population, there is a large loss of genetic diversity. This lack of genetic diversity can be dangerous for the remaining population, causing an increase in inbreeding and a decrease in general fitness and adaptive potential. It ultimately raises the axolotl's risk for extinction, something that they are already in danger of. Studies have found indicators of a low interpopulation gene flow and higher rates of genetic drift. These are likely the result of multiple “bottleneck” incidents in which events that kill off several individuals of a population occur and sharply reduce the genetic diversity of the remaining population. The offspring produced after bottleneck events have a greater risk of showing decreased fitness and are often less capable of adaptation down the line. Multiple bottleneck events can have disastrous effects on a population. Studies have also found high rates of relatedness that are indicative of inbreeding. Inbreeding can be especially harmful as it can cause an increase in the presence of deleterious, or harmful, genes within a population.[52] The detection of introgressed tiger salamander (A. tigrinum) DNA in the laboratory axolotl population raises further concerns about the suitability of the captive population as an ark for potential reintroduction purposes.[53]

There has been little improvement in the conditions of the lake or the population of native axolotls. Many scientists are focusing their conservation efforts on translocation of captive-bred individuals into new habitats or reintroduction into Lake Xochimilco. The Laboratorio de Restauracion Ecologica (LRE) in the Universidad Nacional Autonoma de Mexico (UNAM) has built up a population of more than 100 captive-bred individuals. These axolotls are mostly used for research by the lab but plans of a semi-artificial wetland inside the university have been established and the goal is 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 to establish or re-establish a wild population.[54][55]

Captive care

These axolotls at Vancouver Aquarium are leucistic, with less pigmentation than normal.
Axolotl in a pet store in Melbourne, Australia

The axolotl is a popular exotic pet like its relative, the tiger salamander (Ambystoma tigrinum). As for all poikilothermic 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.[56][57] Chlorine, commonly added to tapwater, 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.[58]

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.[59]

Salts, such as Holtfreter's solution, are often added to the water to prevent infection.[60]

In captivity, axolotls eat a variety of readily available foods, including trout and salmon pellets, frozen or live bloodworms, earthworms, and waxworms. Axolotls can also eat feeder fish, but care should be taken as fish may contain parasites.[61]

Substrates are another important consideration for captive axolotls, as axolotls (like other amphibians and reptiles) tend to ingest bedding material together with food[62] and are commonly prone to gastrointestinal obstruction and foreign body ingestion.[63] 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.[62] 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.[64]

There is some evidence that axolotls might seek out appropriately-sized gravel for use as gastroliths[65] based on experiments conducted at the University of Manitoba axolotl colony.[66][67] As there is no conclusive evidence pointing to gastrolith use, gravel should be avoided due to the high risk of impaction.[68]

Cultural significance

The species is named after the Aztec deity Xolotl, 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.[69] Axólotl also means water monster in the Nahuatl language.

They appear in the works of Mexican muralist Diego Rivera. In 2021, Mexico released a new design for its 50-peso banknote featuring an axolotl along with maize and chinampas on its back.[70][71] It was recognized as "Bank Note of the Year" by the International Bank Note Society.[72] HD 224693, a star in the equatorial constellation of Cetus, was named Axólotl in 2019.[73][74]

The Pokémon Mudkip and its evolutions, added in Pokémon Ruby and Sapphire (2002), take some visual inspiration from axolotls.[69] Additionally, the Pokémon Wooper, added in Pokémon Gold, Silver and Crystal (1999), is directly based on an axolotl.[69][additional citation(s) needed] The looks of the dragons Toothless and The Night Fury in the How to Train Your Dragon movies are based on axolotls.[69] They were also added to the video game Minecraft in 2020. It is following Mojang Studios' trend of adding endangered species to the game to raise awareness.[75] They were also added to its spin-off Minecraft: Dungeons in 2022 and are available in Lego Minecraft.[76] An anthropomorphic Axolotl named Axo was also added as a purchasable outfit in Fortnite Battle Royale on August 9, 2020.[77][78]

See also

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