Axolotl

Description

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 to provide oxygen to their lungs.

Axolotls have four pigmentation genes which when mutated create different color variants. The normal wild type animal is brown/tan with gold speckles and an olive undertone. The four mutant colors are leucistic (pale pink with black eyes), albino (golden with gold eyes), axanthic (grey with black eyes) and melanoid (all black 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 that develops a black and white piebald appearance on reaching maturity. Because pet breeders frequently cross the variant colors, animals that are double recessive 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. Axolotls also have some limited ability to alter their color to provide better camouflage by changing the relative size and thickness of their melanophores.

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 artificially drained to avoid periodic flooding, 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), though it may fall to 6 to 7 °C in the winter, and perhaps lower.

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 its primary source of food.

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.

Neoteny

Axolotls exhibit neoteny, meaning 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.

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. In fact, in some high mountain lakes also live dwarf forms of salmonids, caused by deficiency of food and of iodine, in particular, which causes cretinism and dwarfism due to hypothyroidism, as it does in humans.

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. The adult form resembles a terrestrial plateau 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 Ambystoma velasci specimens 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 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.

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 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 axolotl is also considered an ideal animal model for the study of neural tube closure due to the similarities between human and axoltol neural plate and tube formation, which unlike the frog, 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.

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.

Captive care

The axolotl is a popular exotic pet like its relative, the tiger salamander (Ambystoma tigerinum). 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. Chlorine, commonly added to tapwater, is harmful to axolotls. A single typical axolotl typically requires a 40-litre (11-US-gallon) 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 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. If gravel (common in aquarium use) is used, it is recommended that it consist of smooth particles of a size small enough to pass through the digestive tract. 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 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.

In popular culture

Axolotl is one of a number of words, mostly of foreign origin, adopted by Mad Magazine as nonsense words for use as running gags; potrzebie and veeblefetzer are two others. These achieved some popularity with readers of the magazine; see for example the discussion of a poem (quoted in full) centering on the word.

The axolotl's real world ability to regrow limbs served as the inspiration for the Tleilaxu axlotl tanks in the fictional Dune universe created by Frank Herbert.

Celebrated Argentine writer Julio Cortázar included a short story entitled "Axolotl" in his 1956 collection Final del juego. The story concerns a man who becomes obsessed with the salamanders after viewing them in an aquarium in Paris.