Axolotl

The axolotl (; from ) (''Ambystoma mexicanum'') is a paedomorphic salamander, one that matures without undergoing metamorphosis into the terrestrial adult form; adults remain fully aquatic with obvious external gills. This trait is somewhat unusual among amphibians, though this trait is not unique to axolotls, and this is apparent as they may be confused with the larval stage or other neotenic adult mole salamanders (''Ambystoma spp.''), such as the occasionally paedomorphic tiger salamander ('' A. tigrinum'') widespread in North America; or with mudpuppies (''Necturus spp.''), which bear a superficial resemblance but are from a different family of salamanders.

Axolotls originally inhabited a system of interconnected wetlands and lakes in the Mexican highlands; they were known to inhabit the smaller lakes of Xochimilco and Chalco, and are also presumed to have inhabited the larger lakes of Texcoco and Zumpango. These waterways were mostly drained by Spanish settlers after the conquest of the Aztec Empire, leading to the destruction of much of the axolotl's natural habitat, which is now largely occupied by Mexico City. Despite this, they remained abundant enough to form part of the staple in the diet of native Mexica during the colonial era. Due to continued urbanization in Mexico City, which causes water pollution in the remaining waterways, as well as the introduction of invasive species such as tilapia and carp, the axolotl is near extinction, the species being 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 (IUCN) and is listed under Appendix II of the Convention on International Trade in Endangered Species (CITES).

A large captive population of axolotls currently exist, with the specimens being used extensively in scientific research for their remarkable ability to regenerate parts of their body, including limbs, gills and parts of their eyes and brains. In general, they are model organisms that are also used in other research matters, and as aquarium technology developed, they have become a common exhibit in zoos and aquariums, and as an occasional pet in home aquaria. Axolotls are also a popular subject in contemporary culture, inspiring a number of works and characters in media.

Nomenclature

The term "axolotl" is a nahuatl word which has been translated variably; it may be interpreted as "water slave", "water servant", "water sprite", "water player", "water monstrosity", "water twin", or "water dog". The word refers to the Aztec God, Xolotl, who holds dominion over multiple aspects such as fire, lightning, the dead and those resurrected, dogs, games, grotesque or ugly beings, and twins, as he is the twin of Quetzalcōātl.

Some sources prefer the term "Mexican axolotl" to refer to this species unambiguously, as "axolotl" may be used for unmetamorphosed individuals of other ''Ambystoma'' species, though the word is most commonly used to refer to wild ''A. mexicanum'' and captive individuals.

Description

A sexually mature adult axolotl, at age 18–27 months, ranges in length from , although a size close to is most common and greater than is rare. Axolotls possess features typical of salamander larvae, including external gills and a caudal fin extending from behind the head to the vent. External gills are usually lost when salamander species mature into adulthood, although the axolotl maintains this feature. This is due to their neoteny, where axolotls are much more aquatic than other salamander species.
Their heads are wide, and their eyes are lidless. Their limbs are underdeveloped and possess long, thin digits. 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. Four-gill slits lined with gill rakers are hidden underneath the external gills, which prevent food from entering and allow particles to filter through. Males can be identified by their swollen cloacae lined with papillae, while females have noticeably wider bodies when gravid and full of eggs.

Axolotls have barely visible vestigial teeth, 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. 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.

The wild type animal (the "natural" form) is brown or tan with gold speckles and an olive undertone, and possess an ability to subtly alter their color by changing the relative size and thickness of their melanophores, presumably for camouflage.
Axolotls have four pigmentation genes; when mutated, they create different color variants. The five most common mutant colors are listed below;

# Leucistic: pale pink with black eyes.
# Xanthic: grey, with black eyes.
# Albinism: pale pink or white, with red eyes.
# Melanism: all black or dark blue with no gold speckling or olive tone.

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. 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 genes.

The 32 billion base pair long sequence of the axolotl's genome was published in 2018 and was the largest animal genome completed at the time. It revealed species-specific genetic pathways that may be responsible for limb regeneration. Although the axolotl genome is about 10 times as large as the human genome, it encodes a similar number of proteins, namely 23,251 (the 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.

Physiology

Regeneration

The feature of the axolotl that attracts most attention is its healing ability: the axolotl does not heal by scarring, but is capable of tissue regeneration; entire lost appendages such as limbs and the tail are regrow over a period of months, and, in certain cases, more vital structures, such as the tissues of the eye and heart can be regrown. They can restore parts of their central nervous system, such as 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. Their ability to regenerate declines with age but does not disappear, though in metamorphosed individuals, the ability to regenerate is greatly diminished. Axolotls experience indeterminate growth, their bodies continuing to grow throughout their life, and some consider this trait to be a direct contributor to their regenerative abilities. The axolotl is therefore used as a model for the development of limbs in vertebrates. There are three basic requirements for regeneration of the limb: the wound epithelium, nerve signaling, and the presence of cells from the different limb axes. A wound epidermis is quickly formed by the cells to cover up the site of the wound. In the following days, the cells of the wound epidermis divide and grow, quickly forming a blastema, which means the wound is ready to heal and undergo patterning to form the new limb.

It is believed that during limb generation, axolotls have a different system to regulate their internal macrophage level and suppress inflammation, as scarring prevents proper healing and regeneration. However, this belief has been questioned by other studies. The axolotl's regenerative properties leave the species as the perfect model to study the process of stem cells and its own neoteny feature. Current research can record specific examples of these regenerative properties through tracking cell fates and behaviors, lineage tracing skin triploid cell grafts, pigmentation imaging, electroporation, tissue clearing and lineage tracing from dye labeling. The newer technologies of germline modification and transgenesis are better suited for live imaging the regenerative processes that occur for axolotls.

Neoteny

Most amphibians begin their lives as aquatic animals which are unable to live on dry land, often being dubbed as tadpoles. To reach adulthood, they go through a process called metamorphosis, in which they lose their gills and start living on land. The axolotl is unusual in that it has a lack of thyroid-stimulating hormone, which is needed for the thyroid to produce thyroxine for the axolotl to go through metamorphosis; it keeps its gills and lives in water all its life, even after it becomes an adult and is able to reproduce. Neoteny is the term for reaching sexual maturity without undergoing metamorphosis.

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

Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations. Sirens, ''Necturus'' mudpuppies, and the troglobytic olm are other examples of neotenic salamanders, although unlike axolotls, they cannot be induced to metamorphose by an injection of iodine or thyroxine hormone.

Neoteny has been observed in all salamander families in which it seems to be a survival mechanism, in aquatic environments only of mountain and hill, with little food and, in particular, with little iodine. In this 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, but an ability to disperse across dry land. 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.

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 obtain it from an external source, after which an axolotl undergoes an induced metamorphosis and begins living on land. Research on this phenomenon has been performed for over a century; in modern laboratory conditions, metamorphosis is reliably induced by administering either the thyroid hormone thyroxine or a thyroid-stimulating hormone. The former is more commonly used.

In the absence of induced metamorphosis, larval axolotls start absorbing iodide into their thyroid glands at 30 days post-fertilization. Larval axolotls do produce thyroid hormone from iodide, but the amount appears highly variable. Adult axolotls do not produce thyroid hormone unless metamorphism is triggered.

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. Axolotl that complete their metamorphosis resembles an adult plateau tiger salamander, though the axolotl differs in its longer toes.

Wild population

Axolotls are within the same genus as the tiger salamander (''Ambystoma tigrinum''), being part of its species complex along with all other Mexican species of ''Ambystoma''. Their habitat is like that of most neotenic ''Ambystoma'' species; a high-altitude body of water surrounded by a risky terrestrial environment, with these conditions thought to favor the development of neoteny. However, a population of terrestrial Mexican tiger salamanders occupies and breeds in the axolotl's habitat (being sympatric). The axolotl is native to the freshwater Lakes Xochimilco and Chalco in the Valley of Mexico (though the species may have also inhabited the larger Lakes of Texcoco and Zumpango), and is currently native only to the former two; Lake Chalco is an unstable ecosystem, often being 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 , although it may fall to in the winter, and perhaps lower. An additional population of ''Ambystoma'' inhabiting the artificial lake at Chapultepec was confirmed to contain axolotls; thus the extent of occurrence as of 23 October 2019 was . Overall, the wild axolotl prefers a system of water channels and deep-water lakes with abundant aquatic vegetation.

Biology

The axolotl is carnivorous, consuming small prey such as mollusks, worms, insects, other arthropods, 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. The wild axolotl is thought to reach sexual maturity at 1.5 years of age, with a generation length of around 5.5 years.

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, and is continually under heavy pressure by the city's growth . Lake Xochimilco is not a large body of water, but rather a small series of artificial channels, small lakes, and temporary wetlands. 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 introduced fish have been eating the axolotls' young, as well as competing for their primary source of food.

Surveys in 1998, 2003, and 2008 found 6,000, 1,000, and 100 axolotls per square kilometer in its Lake Xochimilco habitat, respectively. 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.

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. 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.

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.

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. 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.

There has been little improvement in the conditions of the lake or the population of native axolotls over the years. 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.

A 2025 study confirmed the viability of releasing captive-bred axolotls into the wild, with recaptured animals putting on weight compared to their release weight, though this practice risks the loss of the axolotls through predation, as a number of released axolotls were preyed upon by great egrets.

The fungus '' Batrachochytrium dendrobatidis'' has been detected in axolotls; ''B. dendrobatidis'' is a fungus that causes chytridiomycosis in amphibians, and is a major concern for amphibian conservation worldwide. However, the axolotl displays resistance to both ''B. dendrobatidis'' and '' B. salamandrivorans'', so chytridiomycosis is thought to not be a threat to this species.

Relation to humans

Research history

Alexander von Humboldt noted that the Mexicans, having been vanquished by the Spanish Empire, lived "in great want, compelled to feed on roots of aquatic plants, insects and a problematical reptile called axolotl."

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. 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. 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. Since then, experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.

Use as a model organism

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 husbandry 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. Further research has been conducted to examine their heart as a model of a single human ventricle and excessive trabeculation. 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 a frog's, is not hidden under a layer of superficial epithelium. There are also mutations affecting other organ systems some of which are not well characterized and others that are. The genetics of the color variants of the axolotl have also been widely studied.

Captive care

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 to 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 may lead to metabolic rate increase, also causing stress and eventually death. Chlorine, commonly added to tapwater, is harmful to axolotls. A single axolotl typically requires a tank. Axolotls spend the majority of the time at the bottom of the tank.

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.

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

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

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

File: Ambystoma mexicanum at Vancouver Aquarium.jpg, These axolotls at Vancouver Aquarium are leucistic, with less pigmentation than normal.
File:Axolotl in a Pet store in Melbourne.jpg, Axolotl in a pet store in Melbourne, Australia
File:Axolotls in Kew Gardens.jpg, Axolotls in a pond with ''Pistia'', Kew Gardens

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. Axólotl also means water monster in the Nahuatl language.

Julio Cortázar wrote a book titled "''Axolotl''".

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. It was recognized as "Bank Note of the Year" by the International Bank Note Society. HD 224693, a star in the equatorial constellation of Cetus, was named Axólotl in 2019.

In the 21st century, axolotls became renowned as a cultural icon, with the species' likeness appearing in or inspiring various aspects of contemporary media, such as television shows, movies, or video games. The Pokémon Mudkip and its evolutions, added in '' Pokémon Ruby and Sapphire'' (2002), take some visual inspiration from axolotls. Additionally, the Pokémon Wooper, added in '' Pokémon Gold, Silver and Crystal'' (1999), is directly based on an axolotl. The dragon Toothless in the ''How to Train Your Dragon'' movies was modeled after axolotls as well. Following Mojang Studios' trend of adding endangered species to the game to raise awareness, axolotls were added to the video game ''Minecraft'' in 2020 (depicted as troglofauna in-game), and were included in its spin-offs '' Minecraft: Dungeons'' and ''Lego Minecraft''. An anthropomorphic axolotl named Axo was added as a purchasable outfit in ''Fortnite Battle Royale'' on August 9, 2020.

See also

* '' Ambystoma''
** Barred tiger salamander
** Lake Patzcuaro salamander
* Mudpuppies
* Olm
* Regenerative biomedicine
* Texas blind salamander
* Texas salamander
* Amphibious fish
* Handfish

References

External links

Follow the Eggs, Hatchlings and Juveniles

Mating Dance and Laying Eggs

Follow the Eggs and Hatchlings (2nd Batch)



University of KY Axolotl Colony



The animal that's everywhere and nowhere
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xenbase.org

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Amphibians described in the 18th century
Animal models
Animal testing on amphibians
Animals bred for albinism on a large scale
Critically endangered biota of Mexico
Critically endangered fauna of North America
Endemic amphibians of Mexico
Mole salamanders
Population genetics
Regenerative biomedicine
Taxa named by Frederick Polydore Nodder
Taxa named by George Shaw
Valley of Mexico
Species that are or were threatened by pollution
Species that are or were threatened by invasive species