Mexican Axolotls: Characteristics, reproduction, hábitats and more.

Learn through this article all about the Mexican Axolotls which are a very peculiar species endemic to the country of Mexico. They are usually called walker fish, but they aren´t related to this species and they are actually salamanders.

These creatures are related to the tiger salamanders. The name «Axolotl» comes from the Aztec language «Nahuatl». One of the most popular translations of the name connects the Axolotls with the god of deformations and death, Xolotl, while the most commonly accepted translation is «water-dog» (from «atl» for water, and «xolotl», that can also bad dog). The name «Axolotl» (pronounced ACK-suh-LAH-tuhl) comes from the Aztec language, «Nahuatl.»

What are the Axolotls and what’s the meaning of their scientific name.

Mexican axolotls are neotenic salamanders, which are related to tiger salamanders. They are characterized by being amphibious creatures that fail to develop a complete metamorphosis; in fact, they don´t develop lungs that allow them to move to land and for this reason they are aquatic species even during their adult stage. Ambystoma mexicanum is the scientific name of this species, which belongs to the Urodela order and the Ambystomatidae family.

Which are their Main Characteristics?

As for their size they reach up to 30 centimeters in length . Their most outstanding physical feature relies on their gills, which protrude from the back of their broad head and remain there during adulthood. Their legs are short and have four fingers on each of their front legs and five fingers on each of their hind legs. In nature, their coloration is dark, but an albino variety has been created in captivity. It has been raised in captivity (Utah Hogle Zoo 2003).

The Mexican axolotls can live up to 25 years, although the average species rarely exceeds the age of 15 years. Their heads are wide, and their eyes lack of eyelids. Their limbs are underdeveloped and have long, thin fingers. The males are identified by their swollen cloaca which is filled with papillae, while the females outstand by their wider bodies full of eggs.

Three pairs of external branchial stems originate behind their heads, which they use to move the water with oxygen. The outer gills are coated with filaments to increase the surface area for gas exchange.

They have four gill slits aligned with gill rakers that hide under the outer gills. Mexican axolotls have barely visible vestigial teeth, which develop during metamorphosis. Suctioning is their main feeding method, during which their rakes are intertwined to close the gill slits.

They use their external respiratory gills, although buccal pumping that absorbs air from the surface is also commonly employed by them to provide oxygen to their lungs. The Mexican Axolotls have a typical internal carnivorous anatomy, with the only exception of their teeth, which are stalked.  They employ them to grab their food.

They have a heart of three chambers, in contrast with that of four chambers of the mammalian, and like all amphibians, they are poikilothermous, i.e. their body temperature depends on their environment. As a curious fact we can mention that the Octopus has three hearts.  The Mexican Axolotls have four genes of pigmentation; and when they are mutated create different colors’ variations. Their normal color is brown with Golden flecks and a background color olive.

The four mutant colors are leucism (pale pink with black eyes), albino (golden with golden eyes), axanthic (gray with black eyes) and melanoid (all black without golden speckling or olive tone). In addition, there is great individual variability in the size, frequency and intensity of gold speckle and, at least, a variant that develops a black and white slate appearance upon reaching maturity.

Since pet breeders often cross colors, animals that are double recessive mutants are common in the pet trade, especially white / pink animals with pink eyes that are homozygous double mutants for the albino and leucistic trait.

Mexican axolotls also have a limited ability to alter their color, to have a better camouflage, by changing the relative size and thickness of their melanophores.

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Https://www.youtube.com/watch?v=Pa2OBdeFW7E

Neoteny in the Mexicans Axolotls

Axolotls develop neoteny, which means that they reach sexual maturity without experiencing metamorphosis. Many species within the axolotl genus are completely neotenic or have neotenic populations. In this Mexican species the metamorphic fault is caused by the lack of thyroid stimulating hormone, which is used to induce the thyroid to produce thyroxine in the transformation of salamanders.

The genes responsible for neoteny in laboratory animals may have been identified; however, they are not linked in wild populations, suggesting that artificial selection is the cause of complete neoteny in Mexican labolls and pets.

Neoteny has been observed in all salamander families where it seems to be a survival mechanism, especially in mountainous aquatic environments, with little food and, in particular, with a small amount of 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 large adult, which is terrestrial. If the salamander larvae gobble up a high amount of iodine, directly or not through cannibalism, they quickly undertake metamorphosis and transmute into larger terrestrial adults with higher dietary requirements. In fact, in some high mountain lakes live forms of dwarf salmonids that are caused by deficiencies in food, (particularly iodine), which causes cretinism and dwarfism due to hypothyroidism as it does in humans.

Unlike other neotenic salamanders, the Mexican axolotls can be induced to metamorphose with an iodine injection used in the production of thyroid hormones or with hormone thyroxine injections. The adult form resembles a terrestrial tiger salamander, but has several differences, such as the longer toes, which supports its status as a separate species .

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Habitat and Distribution

The axolotls are endemic to Mexico. They live in Lake Xochimilco, and on some occasion they also lived in Lake Chalco in central Mexico, but this body of water no longer exists, because it was drained artificially to prevent periodic flooding. The temperature of the water in Lake Xochimilco rarely increases above 20 ° C, although it may fall to 6 ° or 7 ° C during winter, and perhaps lower. The wild population has been under strong pressure due to the growth of Mexico City. As in the case of most of the neotenic species, they inhabit a high body of water surrounded by a risky terrestrial environment, which is believed to favor such condition. However, a terrestrial population of Mexican tiger salamanders occupies and reproduces in the habitat of the Mexican axolotl.

What do Mexican Axolotls Feed On?

Mexican axolotls are carnivorous and consume small prey such as worms, insects and small fish in the wild. These amphibians locate their food by means of their smelling sense. They feed by forcefully sucking the food from their stomach. Due to their lifestyle in a somewhat surprising way, they have few natural predators despite their relatively small size. Usually some birds and large fish are their predators along with the humans that capture these animals for use them as pets in artificial environments.

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Characteristic Behavior?

The Axolotls are characterized for being solitary and elusive creatures. No type of interaction between individuals is known, except in mating, when they come into contact by visual and chemical means. They are active throughout the day. In captivity they live up to 30 years, while in wildlife only three, sometimes up to six years. It is believed that currently, the difference is due to the contamination of their habitats.

How Mexican Axolotls Reproduce?

They reproduce in December, January and February, only once a year. The males can be distinguished by their cloaca (reproductive and excretory orifice) more elongated than that of the females. The mating takes place after a brief courtship, in which at the beginning, they join the region of the cloacas, and then perform a kind of slow dance, moving in circles. Then, the male moves away, while moving the back of his body and tail wavy, while the female follows him. Subsequently, the male vigorously shakes  his tail for about half a minute, in doing so releases a gelatinous mass with a cone-shaped sperm cover, called spermatophore, and continues to move forward.

The female slides on the spermatophore collecting it with her cloaca. Around 24 hours after the fertilization, the female releases between 600 and 1,500 fertilized eggs, which she deposits gradually, taking up to three days. They don´t build burrows, but they tend to lay eggs preferring vegetation with spongy and dense roots where they are more protected. The eggs hatch after 11 to 15 days, and the larvae can mature sexually by the end of the year, although this varies according to the individual.

Experiments with Mexican Axolotls

Different genetic studies are being carried out in several countries about this species, most of them focused on their neotenic characteristics and their unusual capacity for regeneration. For example, since 2009, the US Department of Defense has financed a group of scientists specialized in cell regeneration from the University of Tulane, in New Orleans, with 6.25 million dollars, to study the Mexican Axolotls, due to their  ability to regenerate their limbs, tails, jaw, skin, organs, and even parts of their brain. Similar studies are being carried out In Mexico, focused mainly on their taxonomy and systematics. Among the aspects that are being studied through genetic techniques are inbreeding, loss of genetic diversity, fragmentation of their population and genetic adaptation to captivity. However, other efforts are also made to understand the cellular process that takes the Mexican Axolotls to the aforementioned regenerative capacity.

The Mexican Axolotls in Culture

In the Nahuatl language , Axolotls  are known as «monster of water», but in another pre-Hispanic tradition it is said that their  name comes from the Aztec God of deformations and death called Xolotl, brother of Quetzalcoatl. The God Xolotl is also associated with the idea of ​​movement and life. On the other hand, the word Ambystoma, which is part of its scientific name, means «mouth in the form of a cup  (inverted) or flattened mouth», due to its large mouth. In 1863 the French general Elie Frédéric Forey sent to the Museum of Natural History of Paris, 34 live specimen of Mexican Axolotls  caught  in Xochimilco. The zoologist Auguste H. A. Duméril received, cared for and studied the axolotls making the first scientific studies and achieving their reproduction in captivity. He also sent specimens to several countries in Europe and eventually to other continents. Almost 150 years after their introduction to the old world, and because of the relative ease with which they can be bred in captivity, the Mexican Axolotls are now found in aquariums and laboratories around the world, and thousands of important studies have been carried out about them.

The research covers areas such as cell biology, anatomy, genetics and evolution, among others. As for Mexico there are written records about the Mexican Axolotls since 1648, when the work of Francisco Hernández was published  in 1587. In the mid-Sixteenth Century, Fray Bernandino de Sahagún in his great work “General History of Things in Nueva España” recounts the figure that the Axolotls represented for the natives before the Conquest.

Within the literature, the Mexican Axolotls have inspired many literary works. They have served as a muse to authors such as Julio Cortázar, an Argentine writer who in 1952 published the story «Axolotl», where he narrates, in some way, the transmutation of man, himself, into an Axolotl. Within the culinary culture and since pre-Hispanic times they have been used as food in a variety of dishes, including tamales and broths. A few years ago they were still easily found in the markets of Xochimilco, Toluca, Pátzcuaro and Zumpango. Perhaps because of their strange shape and characteristics, which the ancient inhabitants of the Valley of Mexico used to attribute mystical properties, they have been used for medicinal purposes in the treatment of asthma and bronchitis.. The fame of their healing properties still persists to this day, so it used to be easily found at the Sonora market in Mexico City and there are still ointments and tonics in which the main ingredient is the Mexican Axolotl.

Conservation Status

Mexican Axolotls are listed by CITES as an endangered species and by The IUCN as critically endangered, since their population is in constant decline. In 1998, 2003 and 2008, scientists made a count of Mexican axolotls, in which 6,000, 1,000 and 100 axolotls, respectively, per square kilometer were found in their habitat of Lake Xochimilco. A four-month search in 2013, however, found no surviving individuals in such area. Only a month later two specimens were detected in a channels´ network that started in Xochimilco, the city is currently working on the conservation of axolotls through the construction of shelters for them, as well as in the conservation of their habitats.

Their Main Threats

Since 2010, the wild Axolotls were near extinction due to the constant urbanization of Mexico City and the consequent pollution of its water. The introduction of invasive species such as tilapia and perch into their environments, their employ in scientific research due to their ability to regenerate limbs, as well as their use as food also attempts against them.  For these reasons the Mexican Axolotls are in danger of extinction

The Mexican Axolotls in Captivity

The Mexican Axolotl is a popular exotic pet as its relative, the Tiger Salamander. Like all poikilotherms organisms, lower temperatures further a slower metabolism and a poor appetite. As for their price, it is quite high since it is a species that is in danger of extinction and only sells on the black market. In this article we offer some tips for caring these creatures, and if you don’t have one, do not buy it.  Collaborate with the care of the wild species to prevent the extinction of the Mexican Axolotls.

The water´s quality is an important aspect for the health of this species.  They are indulgent animals, but their proper care in captivity only is possible under the proper water condition. We also must be cautious when considering what to put into in the water, whether they are ornaments or other tank mates. ( see our article  The best algae to decorate  aquarium for more information about it).

Tank mates

The young Axolotls, of less than 8 cm long must not remain together in a confined space. If they are to keep together, it is be advisable to use an aquarium that allows them plenty of space, since they tend to pinch their foot and gills, even more so when they are very small. Even amphibians of up to 15 or 16 cm can be cut to their tank’s fellow, occasionally inflicting severe damage.

This is particularly notable in the Axolotls from wild type. This behavior is often observed by scientists and amateurs alike and still not explained fully. Cannibalistic tendencies aren´t common in adult Axolotls. However, adults can eat Axolotls considerably smaller, so it is necessary to separate different generations until they reach adulthood. Finally, consider keeping the sexes separated, unless you are prepared to deal with the occasional batch of eggs.

Temperature and Cooling

The optimum temperature for Mexican axolotls is between approximately 16 ° C and 18 ° C. Lower temperatures lead to slower behavior, slower metabolism and decreased appetite. The axolotls don´t hibernate, so it is not useful to cool them below 10 ° C.

Some fans in temperate regions tend to keep axolotls in outdoor ponds, which can cause their death due to freezing. Whenever the winter is not particularly hard or long, axolotls can be very good in outdoor conditions. Temperatures above 24 ° C are very stressful for the amphibian, since they fasten the metabolism and, consequently increase the appetite. However, the stress that results from more than a day or two of exposure to such temperatures will quickly lead to illness and death. The first symptoms of heat stress in Mexican axolotls include the rejection of food and / or the development of pale patches of material similar to mucus on their skin.

If you have difficulty keeping the temperature of your salamander’s aquarium below 24 ° C, there are some options to consider. The easiest solution in the short term, is to move the aquarium to a cooler area of the house. Remember that  in all rooms, the temperature at ground level will be at least 1-2 ° C cooler than that on a shelf. In the summer, the same rule is valid for a house; the lower rooms of the house will be colder than the rooms on the upper floors. Therefore, moving the aquarium can easily allow you to reduce the temperature in your tank from 26 ° C to 23 ° C. Such a change in temperature could make the difference between the life and death of your Mexican axolotl.

If you plan to move the aquarium, be sure to remove most of the water first.  Just leave enough to cover the Mexican axolotl. Make sure there is a padded support at the destination such as a polystyrene board. In more extreme cases, and in places where high temperatures persist for more than a day or two at a time, you must take different measures. Some people use aquarium coolers. Another safe option to use for a week or two at a time is the «ice bottle.»

This is a 2-liter plastic water bottle used for carbonated soft drinks. The bottle fills between 80 and 90% of its capacity and then freezes. It is important to use a plastic bottle, a glass bottle filled with water will explode in the freezer.

Water flow

The water flow is usually caused by a filter or an air pump in an aquarium. The filter’s production can cause a significant flow and this is perhaps the most common cause of stress in the Mexican axolotl. Excess water flow, sooner or later, will lead to disease. A curly end is a sign of a stressed axolotl. Here are several tips to minimize the flow of concentrated water, if it has a built-in function to reduce the flow, use it. Use a spray bar, make one from a piece of pipe that has drilled holes, or buy a patented spray bar kit, the more, the better. Orient it against the glass to reduce the force of the water. Tilt the filter so that the water flow points to a glass side of the tank. Partially block the outlet of the filter with a piece of filter wool or a homemade device, such as a piece of filter tube. Be careful not to obstruct the outlet nozzle too much, as this can cause the motor and the impeller of your filter to wear out quickly.

Tank’s Acidity

A pH between 6.5 and 8 is quite acceptable, and the ideal is between 7.4 to 7.6. The pH can affect the toxicity of ammonia, if you have particularly acid or basic water, you can adjust the pH with the kits sold by your local aquarium retailer.

Hardness

The water’s degree of hardness can be considered as the amount of dissolved salts in the water. There are many types of salt besides those that people use in their food. Some commonly found salts include epsom salt (MgSO4), baking soda / bread (NaHCO3) and «low sodium» salt (KCl), which is commonly used by people as an alternative to ordinary salt (NaCl).

If you live in a hard water area, you have probably seen lime in your boiler or pipes. This is caused by the deposition of dissolved salts of calcium and magnesium in the boiler’s heating element and the inner surface of the pipes.

Soft water contains few dissolved salts, while hard water has significant amounts. There is usually a correlation between soft water and acidic pH, and hard water and alkaline pH. This is because these minerals affect the chemical balance in the water.

The Mexican axolotls prefer somewhat hard waters, and those that live in soft waters often suffer from temporary anemia: the animal becomes pale and its gills lose their coloration for a few minutes or hours. This is not a dangerous condition, but it can be prevented by supplementing the hardness of the water with added salts.