Axolotl
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| Axolotl | ||||||||||||||
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 leucistic specimen
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| Ambystoma mexicanum (Shaw, 1789) |
The Axolotl (or ajolote) (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. Axolotls are used extensively in scientific research due to their ability to regenerate most body parts, ease of breeding, and large embryos. They are kept as pets in the United States, Great Britain (under the spelling Axlotl), Australia, Japan (where they are sold under the name Wooper Rooper (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), widespread in much of North America, which also occasionally become neotenic, nor with mudpuppies (Necturus spp.), fully aquatic salamanders which are unrelated to the axolotl but which bear a superficial resemblance.
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[edit] Description
A sexually-mature adult axolotl, at age 18–24 months, ranges in length from 15–45 centimetres (6–18 in), although a size close to 23 centimetres (9 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 cloacas 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 mutants colours are leucistic (pale pink with black eyes) and albino (golden, tan or pale pink with pink eyes).
[edit] 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[1]. 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. 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. 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.
[edit] 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. Spontaneous metamorphosis has been known to occur very rarely, but attempts to do so artificially are best left to trained scientists. Artificial metamorphosis also dramatically shortens the axolotl's lifespan, if they survive 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.
[edit] 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. The surviving neotenes are thought to be the ancestors of the majority of pet and laboratory axolotls in existence.
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.
[edit] Feeding
The axolotl is carnivorous, consuming small prey such as worms, insects, and small fish in the wild. In captivity, they can be fed a variety of readily available foods, including trout and salmon pellets, frozen or live bloodworms, earthworms, and waxworms on occasion. Axolotls also enjoy beef liver and sometimes small prawns. 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 — even siblings. If not fed for even a sometimes short period, they can forget how to eat. Fish can also nibble an axolotl's gill stalks and appendages while it is asleep, leading to infection.
[edit] Captive care and husbandry
Axolotls live at temperatures of 14–20°C (54–68°F), preferably 17–18°C (62–64°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; the use of a cooling aggregate is strongly recommended. Temperatures above 22°C (72°F) are considered dangerous to their health[2]. Tapwater should be treated with a dechlorinator before the animal is introduced into it, as chlorine is harmful. Axolotls should be kept in a stable pH environment of between 6.5 to 8, with a slightly alkaline pH of 7.3 to 7.6 being the ideal. A single typical axolotl should be kept in a normal 10 U.S. gallon tank (44 litres) with around 150 mm (6 inches) of water. 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 accidentaly 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.
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.
[edit] References
- ^ Axolotls: The Fascinating Mexican Axolotl and the Tiger Salamander
- ^ Duhon, S. Short Guide to Axolotl Husbandry Abystoma Genetic Stock Center
[edit] See also
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