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Axolotl
Leucistic specimen
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Caudata
Family: Ambystomatidae
Genus: Ambystoma
Species: A. mexicanum
Binomial name
Ambystoma mexicanum
(Shaw, 1789)

The axolotl (pronounced /ˈæksəlɒtəl/), Ambystoma mexicanum, is the best known of the Mexican neotenic mole salamanders 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 2008, wild axolotls are near extinction [1] 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.[2] The axolotl is currently on the International Union for Conservation of Nature's annual Red List of threatened species.[3]

Contents

Description

A sexually mature adult axolotl, at age 18–24 months, ranges in length from 15–45 centimetres (5.9–18 in), although a size close to 23 centimetres (9.1 in) is most common and greater than 30 centimetres (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 colours are leucistic (pale pink with black eyes) and albino (golden, tan or pale pink with pink eyes).

Habitat and ecology

The axolotl is only native to Lake Xochimilco and Lake Chalco in central Mexico. Unfortunately for the axolotl, Lake Chalco no longer exists as it was drained by humans to avoid periodic flooding, and Lake Xochimilco remains a diminished glimpse of its former self, existing mainly as canals. The water temperature in Xochimilco rarely rises above 20 °C (68 °F), though it may fall to 6 or 7 °C (45 °F) in the winter, and perhaps lower. The wild population has been put under heavy pressure by the growth of Mexico City.[3] Axolotls are also sold as food in Mexican markets and were a staple in the Aztec diet.[2] They are currently listed by CITES as an endangered species and by IUCN as critically endangered in the wild, with a decreasing population.

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.

Axolotl's neoteny

Axolotls exhibit a property called neoteny, meaning that they reach sexual maturity without undergoing metamorphosis. Many species within the axolotl's genus are either entirely neotenic or have neotenic populations. In the axolotl, metamorphic failure is caused by a lack of thyroid stimulating hormone, which is used to induce the thyroid to produce thyroxine in transforming salamanders. 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.

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.

Use as a model organism

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 color 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.

Captivity

An axolotl in captivity

Axolotls live at temperatures of 14 °C (57 °F)-20 °C (68 °F), preferably 17 °C (63 °F)-18 °C (64 °F). As for all cold-blooded 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 litres (8.8 imp gal; 11 US gal) tank with a water depth of at least 15 centimetres (5.9 in). Axolotls spend a majority of the time at the bottom of the tank.

In laboratory colonies, adult axolotls are often housed three to a one-gallon container, and water changes are performed more regularly. 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.

See also

In contemporary fiction

  • Argentinian writer Julio Cortázar wrote a short story named "Axolotl", in which a man is turned into an axolotl he used to visit in an aquarium.
  • The Axlotl tank (sic) is used as a "device for reproducing a living human being from the cells of a cadaver" in works based on Dune, by Frank Herbert.
  • The Pokémon Wooper is directly named after the Japanese "Wooper Looper" axolotl marketing phenomenon, and looks much the same as an axolotl.

References

  1. ^ BBC - Earth News - Axolotl verges on wild extinction
  2. ^ a b Weird Creatures with Nick Baker. [Television series]. Dartmoor, England, U.K.: The Science Channel. 2009-11-11. Event occurs at 00:25.  
  3. ^ a b "Mexico City's 'water monster' nears extinction", David Koop, Yahoo News, November 2, 2008

External links

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

AXOLOTL, the Mexican name given to larvae salamanders of the genus Amblystoma. It required the extraordinary acumen of the great Cuvier at once to recognize, when the first specimens of the Gyrinus edulis . or Axolotl of Mexico were brought to him by Humboldt in the beginning of the 10th century, that these Batrachians were not really related to the Perennibranchiates, such as Siren and Proteus, with which he was well acquainted, but represented the larval form of some air-breathing salamander. Little heed was paid to his opinion by most systematists, and when, more than half a century later, the axolotl was found to breed in its branchiferous condition, the question seemed to be settled once for all against him, and the genus Siredon, as it was called by J. Wagler, was unanimously maintained and placed among the permanent gill-breathers.

It seemed impossible to admit that an animal which lives for years without losing its gills, and is able to propagate in that state, could be anything but a perfect form. And yet subsequent discoveries, which followed in rapid succession, have established that Siredon is but the larval form of the salamander Amblystoma, a genus long known from various parts of North America; and Cuvier's conclusions now read much better than they did half a century after they were published. Before reviewing the history of these discoveries, it is desirable to say a few words of the characters of the axolotl (larval form) and of the Amblystoma (perfect or imago form).

The axolotl has been known to the Mexicans from the remotest times, as an article of food regularly brought from neighbouring lakes to the Mexico market, its flesh being agreeable and wholesome. Francisco Hernandez (1514-1578) has alluded to it as Gyrinus edulis or atolocatl, and as lusus aquarum, piscis ludicrus, or axolotl, which latter name has remained in use, in Mexico and elsewhere, to the present day. But for its large size - it grows to a length of eleven inches - it is a nearly exact image of the British newt larvae. It has the same moderately long, plump body, with a low dorsal crest, the continuation of the membrane bordering the strongly compressed tail; a large thick head with small eyes without lids and with a large pendent upper lip; two pairs of well-developed limbs, with free digits; and above all, as the most characteristic feature, three large appendages on each side of the back of the head, fringed with filaments which, in their fullest development, remind one of black ostrich feathers. These are the external gills, through which the animal breathes the oxygen dissolved in the water. The jaws are provided with small teeth in several rows, and there is an elongate patch of further teeth on each side of the front of the palate (inserted on the vomerine and palatine bones). The colour is blackish, or of a dark olive-grey or brownish grey with round black spots or dots.

The genus Amblystoma was established by J. J. Tschudi in 1838 for various salamanders from North America, which had previously been described as Lacerta or Salamandra, and which, so far as general appearance is concerned, differ little from the European salamanders. The body is smooth and shiny, with vertical grooves on the sides, the tail is but feebly compressed, the eye is moderately large and provided with movable lids, and the upper lip is nearly straight. But the dentition of the palate is very different; the small teeth, which are in a single row, as in the jaws, form a long transverse, continuous or interrupted series behind the inner nares or choanae. The animal leaves the water after completing its metamorphosis, the last stage of which is marked by the loss of the gills. One of the largest and most widely distributed species of this genus, which includes about twenty, is the Amblystoma tigrinum, an inhabitant of both the east and west of the United States and of a considerable part of the cooler parts of Mexico. It varies much in colour, but it may be described as usually brown or blackish, with more or less numerous yellow spots, sometimes arranged in transverse bands. It rarely exceeds a length of nine inches. This is the Amblystoma into which the axolotl has been ascertained to transform. It is generally admitted that the axolotls which were kept alive in Europe and were particularly abundant between 1870 and 1880 are all the descendants of a stock bred in Paris and distributed chiefly by dealers, originally, we believe, by the late P. Carbonnier. Close in-breeding without the infusion of new blood is probably the cause of the decrease in their numbers at the present day, specimens being more difficult to procure and fetching much higher prices than they did formerly, at least in England and in France.

The original axolotls, from the vicinity of Mexico City, it is believed, arrived at the Jardin d'Acclimatation, Paris, late in 1863. They were thirty-four in number, among which was an albino, and had been sent to that institution, together with a few other animals, by order of Marshal Forey, who was appointed commander-in-chief of the French expeditionary force to Mexico after the defeat of General Lorencez at Puebla (May 5th, 1862), and returned to France at the end of 1863, after having handed over the command to Marshal (then General) Bazaine. Six specimens (five males and one female) were given by the Societe d'Acclimatation to Professor A. Dumeril, the administrator of the reptile collection of the Jardin des Plantes, the living specimens of which were at that time housed in a very miserable structure, situated at a short distance from the comparatively sumptuous building which was erected some years later and opened to the public in 1874. Soon after their arrival at the Jardin d'Acclimatation, some of the axolotls spawned, but the eggs, not having been removed from the aquarium, were devoured by its occupants. At the same time, in the Jardin des Plantes, the single female axolotl also spawned, twice in succession, and a large number of young were successfully reared. This, it then seemed, solved the often-discussed question of the perennibranchiate nature of these) Batrachians. But a year later, the second generation having reached sexual maturity, new broods were produced, and out of these some individuals lost their gills and dorsal crest, developed movable eyelids, changed their dentition, and assumed yellow spots, - in fact, took on all the characters of Amblystoma tigrinum. However, these transformed salamanders, of which twenty-nine were obtained from 1865 to 1870, did not breed, although their branchiate brethren continued to do so very freely. It was not until 1876 that the axolotl in its Amblystoma state, offspring of several generations of perennibranchiates, was first observed to spawn, and this again took place in the reptile house of the Jardin des Plantes, as reported by Professor E. Blanchard.

The original six specimens received in 1864 at the Jardin des Plantes, which had been carefully kept apart from their progeny, remained in the branchiate condition, and bred eleven times from 1865 to 1868, and, after a period of two years' rest, again in 1870. According to the report of Aug. Dumeril, they and their offspring gave birth to 0000 or io,000 larvae during that period. So numerous were the axolotls that the Paris Museum was able to distribute to other institutions, as well as to dealers and private individuals, over a thousand examples, which found their way to all parts of Europe, and numberless specimens have been kept in England from 1866 to the present day. The first specimens exhibited in the London Zoological Gardens, in August 1864, were probably part of the original stock received from Mexico by the Societe d'Acclimatation, but do not appear to have bred.

"White" axolotls, albinos of a pale flesh colour, with beautiful red gills, have also been kept in great numbers in England and on the continent. They are said to be all descendants of one albino male specimen received in the Paris Museum menagerie in 1866, which, paired with normal specimens in 1867 and 1868, produced numerous white offspring, which by selection have been fixed as a permanent race, without, according to L. Vaillant, showing any tendency to reversion. We are not aware of any but two of these albinos having ever turned into the perfect Amblystoma form, as happened in Paris in 1870, the albinism being retained.

Thus we see that in our aquariums most of the axolotls remain in the branchiate condition, transformed individuals being on the whole very exceptional. Now it has been stated that in the lakes near Mexico City, where it was first discovered, the axolotl never transforms into an Amblystoma. This the present writer is inclined to doubt, considering that he has received examples of the normal Amblystoma tigrinum from various parts of Mexico, and that Alfred Duges has described an Amblystoma from mountains near Mexico City; at the same time he feels very suspicious of the various statements to that effect which have appeared in so many works, and rather disposed to make light of the ingenious theories launched by biological speculators who have never set foot in Mexico, especially Weismann's picture of the dismal condition of the salt-incrusted surroundings which were supposed to have hemmed in the axolotl - the brackish Lago de Texcoco, the largest of the lakes near Mexico, being evidently in the philosopher's mind.

Thanks to the enthusiasm of H. Gadow during his visit to Mexico in the summer of 1902, we are now better informed on the conditions under which the axolotl lives near Mexico City. First, he ascertained that there are no axolotls at all in the Lago de Texcoco, thus disposing at once of the Weismannian explanation; secondly, he confirmed A. Duges's statement that there is a second species of Amblystoma, which is normal in its metamorphosis, near Mexico but at a higher altitude, which may explain Velasco's observation that regularly transforming Amblystomas occur near that city; and thirdly, he made a careful examination of the two lakes, Chalco and Xochimilco, where the axolotls occur in abundance and are procured for the market. The following is an abstract of Gadow's very interesting account. "Lakes Chalco and Xochimilco are a paradise, situated about 10 ft. higher than the Texcoco Lake and separated from it by several hills. High mountains slope down to the southern shores, with a belt of fertile pastures, with shrubs and trees and little streams, here and there with rocks and ravines. In fact, there are thousands of inviting opportunities for newts to leave the lake if they wanted to do so. Lake Xochimilco contains powerful springs, but away from them the water appears dark and muddy, full of suspended fresh and decomposing vegetable matter, teeming with fish, larvae of insects, Daphniae, worms and axolotl. These breed in the beginning of February. The native fishermen know all about them; how the eggs are fastened to the water plants, how soon after the little larvae swarm about in thousands, how fast they grow, until by the month of June they are all grown into big, fat creatures ready for the market; later in the summer the axolotls are said to take to the rushes, in the autumn they become scarce, but none have ever been known to leave the water or to metamorphose, nor are any perfect Amblystomas found in the vicinity of the two lakes." In Gadow's opinion, the reason why there are only perennibranchiate axolotls in these lakes is obvious. The constant abundance of food, stable amount of water, innumerable hidingplaces in the mud, under the banks, amongst the reeds and roots of the floating islands which are scattered all over them, - all these points are inducements or attractions so great that the creatures remain in their paradise and consequently retain all those larval features which are not directly connected with sexual maturity. There is nothing whatever to prevent them from leaving these lakes, but there is also nothing to induce them to do so. The same applies occasionally to European larvae, as in the case observed in the Italian Alps by F. de Filippi. Nevertheless, in the axolotl the latent tendency can still be revived, as we have seen above and as is proved by the experiments of Marie von Chauvin. When once sexually ripe the axolotl are apparently incapable of changing, but their ancestral course of evolution is still latent in them, and will, if favoured by circumstances, reappear in following generations.

BIBLIOGRAPHY.-G. Cuvier, Mem. Instit. Nation. (1807), p. 149, and in A. Humboldt and A. Bompland, Observ. zool. i. (1811), p. 93 L. Calori, Mem. Acc. Bologna, iii. (1851), p. 269; A. Dumeril, Comptes rendus, lx. (1865), p. 765, and N. Arch. Mus. ii. (1866), p. 265; E. Blanchard, Comptes rendus, lxxxii. (1876), p. 716; A. Weismann, Z. wiss. Zool. xxv. (Suppl. 18 75), p. 2 97; M. von Chauvin, Z. wiss. Zool. xxvii. (1876), p. 522; F. de Filippi, Arch. p. la zool. i. (1862), p. 206; G. Hahn, Rev. Quest. Sci. Brussels (2), i. (1892), p. 178; H. Gadow, Nature, lxvii. (1903), p. 33 0. (G. A. B.)


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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also axolotl

German

Noun

Axolotl, m. (plural: Axolotl)

  1. axolotl

Simple English

Axolotl
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Caudata
Family: Ambystomatidae
Genus: Ambystoma
Species: A. mexicanum
Binomial name
Ambystoma mexicanum
(Shaw, 1789)

The Axolotl, Ambystoma mexicanum, is the best known of the Mexican mole salamanders: it belongs to the Tiger Salamander complex. The Axolotl shows neoteny: the larvae do not undergo metamorphosis, so the adults stay aquatic, with external gills.

Natural history

The species evolved in the lake underlying Mexico City. Axolotls are used a lot in scientific research because they can regenerate most body parts, are easy to breed, and have large embryos.

Axolotls are closely related to waterdogs, the larval stage of the closely related Tiger Salamanders Ambystoma tigrinum and Ambystoma mavortium. These are common in much of North America and also sometimes become neotenic. The mudpuppies, Necturus, are fully-aquatic salamanders which are not closely related to the axolotl but bear a superficial resemblance.

Wild axolotls are now near extinction [1] due to population growth in Mexico City, and the polluted waters of the lake. 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 its primary source of food.[2] The axolotl is currently on the IUCN Red List of threatened species.[3]

Genetics and devlopment

Compared with land-dwelling salamanders, the sexually mature adult Axolotl is a chimera (~ mixture) of larval and adult tissues. A mutation in hormone production slows the development of the non-sexual tissues compared to the gonads.[4] So, it ends up as an adult which looks like a larva, except that it is sexually mature. This is an example of heterochrony or neoteny.

References

  1. BBC - Earth News - Axolotl verges on wild extinction
  2. Weird creatures, with Nick Baker [Television series]. Dartmoor, England, U.K.: The Science Channel. Event occurs at 00:25.
  3. "Mexico City's 'water monster' nears extinction", David Koop, Yahoo News, November 2, 2008
  4. King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics, 7th ed. Oxford.p203


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