What are facts about the Axolotl?
Axolotls are carniverous and native to only one place in the
entire world (Lake Xochimilco, Mexico), they also have both gills
and lungs. The axolotl can grow from 15 to 45 centimeters but most
commonly they stop growing at 23 cm. They are on the Critically
Endangered list and are of the kingdom Animalia, phylum Chordata,
class Amphibia, order Caudata, family Ambystomatidae, Genus
Ambystoma, scientific name Ambystoma mexicanum. In Japan the
axolotl is called the Wooper Rooper, and is often called the
Mexican Mole Lizard in other parts of the world. Axolotls are cold
blooded and typically live in a 10 gallon (38 l, 8.3 imp gal) tank
with about 5.1 fl oz of water inside it.
Axolotls live at temperatures of 14–20°C, higher temperature are considered to be harmful to the Axolotls. The Axolotl will spend a majority of the time on the bottom of a tank (floating is a sign of stress and illness).
Australians and New Zealanders frequently refer to the Axolotl as the Mexican Walking Fish, though the Axolotl is not a fish but an amphibian, a salamander, part of the order Caudata/Urodela. Because it's a salamander, it's part of one of the three branches of class Amphibia, which also includes the frogs and toads (the Anurans), and the mainly eel-like order, Gymnophiona, which are also known as the Caecilians. Have a look at the Biology Page for a short guide to the Axolotl's body and characteristics. One common misconception is that axolotls and other salamanders are lizards or reptiles. In fact, amphibians are a completely separate group of animals. For example, did you know that reptiles and human beings have a four-chambered heart? Well amphibians have only three chambers. That's just one example of how appearance can be deceiving: salamanders might look like lizards, but they are very different indeed.
This page is a brief introduction for those new to the Axolotl and salamanders. If you require specific information, you can search this site using the search facility at the top right of this page. Caudata.org also contains a wealth of axolotl information and it's a great place to buy axolotls or trade with other hobbyists. Caudata.org is the Internet's premier source of salamander and newt information and it places an emphasis on their maintenance in captivity. There is a very busy axolotl forum at Caudata.org, used by people just like you. I hope that you find this site useful, but most of all I hope you enjoy what you read and find here. If you're looking for information about metamorphosed axolotls, click here.
Axolotls of various colours occur in captivity, including grey, shades of brown, leucistic (white with black eyes), golden albino, white albino, as well as other varieties, such as the melanoid (a near-black animal). The normally coloured axolotl, the "wild type", can be near-black like the one in the group photo to the left, chocolate brown like the one in the site's logo, or even creamy in colour, and anywhere in between. There are even "piebald" axolotls in various colours, and a variety that is piebald in more than one colour, known as the "harlequin". You can learn more about how colour comes about and how it is passed on by taking a look at the Genetics Page. And why not take a look at the hundreds of photos of the weird and wonderful varieties of axolotls submitted by enthusiasts like yourself at the Axolotl Section of the Caudata.org User Photo Galleries?
The name "Axolotl" comes from the Aztec language, "Nahuatl". One of the most popular translations of the name connects the Axolotl to the god of deformations and death, Xolotl, while the most commonly accepted translation is "water-dog" (from "atl" for water, and "xolotl", which can also mean dog).
Prior to the growth of Mexico city in the basin of Mexico, the Axolotl was native to both Lake Xochimilco, and Lake Chalco. Of these two high altitude freshwater lakes, only the remnants of Xochimilco as canals can be seen today. Unfortunately many information sources mention these lakes as if they still exist (such as this ill-researched article about a metamorphosed axolotl on the BBC News Web site). If only this were still the case: sadly it is rarely caught in the wild but at least the Axolotl is now on the CITES endangered species list. There have been efforts to breed and release the animal, in order to re-establish its numbers. However the location of the remaining waterways where the animal may live (located in the Mexico City metropolitan area) are likely to be very threatened by the city's continuing expansion and the days of the species surviving in the wild are surely quite limited. Fortunately, due to the importance of the Axolotl in scientific research, it is unheard of for them to be taken from the wild for that purpose because of the huge numbers bred in captivity each year. There are related Mexican Ambystoma species that also remain gilled as adults. These species are located in water bodies further from Mexico city and may have a slightly brighter future in the wild than the Axolotl.
Despite its endangered status, the use of the Axolotl as a laboratory animal should ensure the species' survival, if only in captivity. It has long been known that the Axolotl is a worthy study due to its amazing healing and regeneration abilities. Normal wound healing in animals occurs through the growth of scar tissue, which is not the same as the original tissue, nor is it as robust. Normal wound healing also does not allow for most animals to re-grow a lost limb. However the axolotl is fully capable of complete limb re-growth. The animal has the added scientific attraction of having especially large embryos, making it easier to deal with under laboratory conditions. Its embryo is also very robust, and can be spliced and combined with different parts of other axolotl embryos with a high degree of success.
The Axolotl is a fascinating creature for a number of reasons, including its grotesque appearance, its ability to regenerate, and primarily the fact that it exhibits the phenomenon known as neoteny. Ordinarily, amphibians undergo metamorphosis from egg to larva (the tadpole of a frog is a larva), and finally to adult form. The Axolotl, along with a number of other amphibians, remains in its larval form throughout its life. This means that it retains its gills and fins, and it doesn't develop the protruding eyes, eyelids and characteristics of other adult salamanders. It grows much larger than a normal larval salamander, and it reaches sexual maturity in this larval stage. Another term to describe this state is "perennibranchiate". The animal is completely aquatic, and although it does possess rudimentary lungs, it breathes primarily through its gills and to a lesser extent, its skin.
It is generally accepted that neoteny is a "backward" step in evolution, because the Axolotl is descended from what were once terrestrial salamanders, like the closely related species, the Tiger Salamander, Ambystoma tigrinum and Ambystoma mavortium spp. (in fact, one likely theory suggests that the Axolotl is in fact a Tiger salamander off-shoot, as it can interbreed with that species with some success). Through some quirk of nature, a neotenous form developed and, probably due to environmental conditions, prospered. Neoteny is sometimes found in other amphibians, but tends to be caused by low levels of iodine (an essential element for animals to make thyroxine hormones, necessary for growth and development), or possibly by random genetic mutation. Research has also shown that very low temperatures can suppress the production of these hormones, thus also inducing neoteny.
In the Axolotl, neoteny is now totally genetic (click for more information on the Axolotl's genetics). When treated with hormones, the axolotl will usually begin to metamorphose, but in very rare cases it will metamorphose spontaneously, such as the metamorphosed wild type axolotl pictured here. The metamorphosed wild type axolotl bears a close resemblance to the Mexican race of the Tiger Salamander, Ambystoma velasci. There is a wonderful thread on the Caudata.org forum here about the metamorphosed axolotl in the photo.
The axolotl (pronounced /ˈæksəlɒtəl/), Ambystoma mexicanum, is a neotenic mole salamander belonging to the Tiger Salamander complex. Larvae of this species fail to undergo metamorphosis, so the adults remain aquatic and gilled. The species originates from the lake underlying Mexico City and is also called ajolote (which is also the common name for the Mexican Mole Lizard). Axolotls are used extensively in scientific research due to their ability to regenerate most body parts, ease of breeding, and large embryos. They are commonly kept as pets in the United States, Great Britain, Australia, Japan (sold under the name wooper looper (ウーパールーパー, Ūpā Rūpā?)) and other countries.
Axolotls should not be confused with waterdogs, the larval stage of the closely related Tiger Salamanders (Ambystoma tigrinum and Ambystoma mavortium), which are widespread in much of North America and also occasionally become neotenic, nor with mudpuppies (Necturus spp.), fully-aquatic salamanders which are not closely related to the axolotl but bear a superficial resemblance.
As of 2010[update], wild axolotls are near extinction due to urbanization in Mexico City and polluted waters. Nonnative 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 its primary source of food. The axolotl is currently on the International Union for Conservation of Nature's annual Red List of threatened species.
- 1 Description
- 2 Habitat and ecology
- 3 Axolotl's neoteny
- 4 Use as a model organism
- 5 Captivity
- 6 See also
- 7 References
- 8 External links
A sexually mature adult axolotl, at age 18--24 months, ranges in length from 15--45 cm (6--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. 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. Four gill slits lined with gill rakers are hidden underneath the external gills. Axolotls have barely visible vestigial teeth, which would have developed 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 in order to provide oxygen to their lungs. Axolotls have four different colours, two naturally occurring colours and two mutants. The two naturally occurring colours are wildtype (varying shades of brown usually with spots) and melanoid (black). The two mutant colors are leucistic (pale pink with black eyes) and albino (golden, tan or pale pink with pink eyes).
Axolotls are members of the Ambystoma tigrinum (Tiger salamander) 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.
The axolotl is carnivorous, consuming small prey such as worms, insects, 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.
Unlike some other neotenic salamanders (Sirens and Necturus), axolotls can be induced to metamorphose by an injection of iodine (used in the production of thyroid hormones) or by 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. 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. This is likely due to the strong genetic basis for neoteny in laboratory and pet axolotls, which means that few captive animals have the ability to metamorphose on their own. Artificial metamorphosis also dramatically shortens the axolotl's lifespan if it survives the process. A neotenic axolotl will live an average of 10--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 a terrestrial Mexican Tiger Salamander, but has several differences, such as longer toes, which support its status as a separate species.
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 Mexican Tiger Salamanders were not included in the original shipment.
Vilem Laufberger of Germany used thyroid hormone injections to induce an axolotl to grow into a terrestrial adult salamander. The experiment was repeated by the Englishman Julian Huxley, who was unaware the experiment had already been done, using ground thyroid hormones. Since then, experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.
Today, the axolotl is still used in research as a model organism, and large numbers are bred in captivity. Axolotls are especially easy to breed compared to other salamanders in their family, which are almost never 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 presence of several colour morphs has also been extensively studied.
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 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---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.
Axolotls live at temperatures of 12 °C (54 °F)-20 °C (68 °F), preferably 17 °C (63 °F)-18 °C (64 °F). As for all poikilothermic organisms, lower temperatures result in slower metabolism; higher temperatures can lead to stress and increased appetite. Chlorine, commonly added to tapwater, is harmful to axolotls. A single typical axolotl typically requires a 40 l (11 US gal) tank with a water depth of at least 15 cm (6 in). Axolotls spend a majority of the time at the bottom of the tank.
Salts, such as Holtfreter's solution, are usually added to the water to prevent infection.
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 left in the tank may graze on the axolotls' exposed gills.
It should also be noted that Axolotls may suffer from impaction related issues if not kept on the correct substrate with fine sand being the preferred option. Impaction can be caused by the digestion of gravel and could be severe enough to cause death, therefore they must never be kept on gravel or stones that are smaller than the axolotls' head.
The Mexican axolotl (pronounced ACK-suh-LAH-tuhl) salamander has the rare trait of retaining its larval features throughout its adult life. This condition, called neoteny, means it keeps its tadpole-like dorsal fin, which runs almost the length of its body, and its feathery external gills, which protrude from the back of its wide head.
Found exclusively in the lake complex of Xochimilco (pronounced SO-chee-MILL-koh) near Mexico City, axolotls differ from most other salamanders in that they live permanently in water. In extremely rare cases, an axolotl will progress to maturity and emerge from the water, but by and large, they are content to stay on the bottom of Xochimilco's lakes and canals.
Close relatives of the tiger salamander, axolotls can be quite large, reaching up to a foot (30 centimeters) in length, although the average size is closer to half that. They are typically black or mottled brown, but albino and white varieties are somewhat common, particularly among captive specimens.
Axolotls are long-lived, surviving up to 15 years on a diet of mollusks, worms, insect larvae, crustaceans, and some fish. Accustomed to being a top predator in its habitat, this species has begun to suffer from the introduction of large fish into its lake habitat. Natural threats include predatory birds such as herons.
Populations are in decline as the demands of nearby Mexico City have led to the draining and contamination of much of the waters of the Xochimilco Lake complex. They are also popular in the aquarium trade, and roasted axolotl is considered a delicacy in Mexico, further shrinking their numbers. They are considered a critically endangered species.