Axolotls (Ambystoma mexicanum) are caudata amphibians but are often mistakenly thought to be reptiles. They are classified in the family of the mole salamanders (genus Ambystoma). Both size and expected lifespan can vary greatly in some cases. As a rule, axolotls grow to between 15 and 30 centimetres (between 6 and 12 inches) and weigh between 50 and 300 grams (between 2 and 10 ounces). However, there are also specimens that grow up to 40 centimetres (16 inches).
Most axolotls live between ten and twenty years. Occasionally, however, some axolotls have been known to live up to 25 years. Axolotls need cool and oxygen-rich water. They are not solitary animals and should therefore be kept at least in pairs. Many owners decide to keep three animals. These should be about the same size!
Axolotls have external gills, and their lungs are not fully developed. Although they can switch between land and water for short periods, they should spend their lives in water as they are not adult moles but remain in the larval stage. The colder it is, the more often axolotls voluntarily go ashore.
Atl means water and Xolotl stands for monster but is also the name of an Aztec god. Axolotls probably got the name water monster because of their appearance, although nowadays many people find the little amphibians very cute and not monster-like.
Axolotls have their home in Mexico and are not too common in the wild. They lived in Lake Chalco, Lake Xochimilco and a few other bodies of water in Mexico. These are mainly stagnant waters and lakes.
The lakes mentioned are located in a volcanic basin near Mexico City. Today, they are largely dry, and the water bodies are only sparsely developed.
In 1805, Alexander von Humboldt, a German explorer, returned from an expedition in Mexico and brought back two axolotls. He wanted to study them more closely. Many years later, in 1863, a French research team brought a somewhat larger group of axolotls from Mexico to France. Over the years, the little animals were studied more and more intensively before they established themselves much later as popular pets.
The heads of axolotls are rounded and large, their limbs are relatively short. The eyes are wide apart. Behind the eyes they each have three gill branches that can be actively moved. Their tail is flat, has fin seams and is very long. Axolotls use their tail to control their swimming direction. Usually, the males have a slightly longer tail than the females.
What is special about axolotls is that although they grow steadily and also become reproductively mature, they always remain larvae. This peculiarity of axolotls is due to their limited thyroid function - hormones are produced in the thyroid gland of a living creature that are responsible, among other things, for the development of a larvae into an adult. Axolotls, however, have a malfunctioning thyroid gland, which is why they never become a 'proper' adult amphibian. Metamorphosis is therefore not possible for them. Through the external addition of artificial hormones, an axolotl can indeed develop into an adult caudate and then have a similar appearance to a tiger salamander, but this should be avoided. While an axolotl can live for ten to twenty years as a larva, it can only live for about five years if artificial hormones are administered. Life expectancy is therefore drastically shortened.
The permanent state in the larval stage is called neoteny. Although axolotls always remain larvae, they can of course reproduce.
Axolotls can regenerate their bodies very well and usually completely, as they are able to regrow their limbs, organs and even brain parts. How this is possible is still not fully understood. Research into the regenerative properties of axolotls is still ongoing. It is hoped that the research results and findings can also be applied in human medicine. However, some things have already been discovered. Axolotls have connective tissue cells that virtually reverse their development. The special body cells fibroblasts develop again into precursor cells that can form bones as well as skin and tendons. Thus, the regeneration of different types of connective tissue is possible. However, the adaptation to humans is proving difficult in research - humans also have fibroblasts. In the case of an injury, however, the human fibroblasts do not develop back into precursor cells, but they continue to develop into myofibroblasts - and these form the scar tissue in humans.
The genome of an axolotl is extremely complex. It is ten times as large as the genome of a human, as it has over thirty billion base pairs. Therefore, decoding this genome is not easy. However, it has already been researched that axolotls have some genes that are only found in them and other amphibian species. These genes are active in regenerating tissue.
The axolotl breathes in addition to atmospheric air thanks to a pair of rudimentary lungs, and, like all amphibians, it also breathes through the skin, which is devoid of scales and covered with a protective film, the mucus. It is a vital necessity, we will take care to always maintain an access to the air, notably in the bags of transport. Therefore, the skin is a particularly sensitive organ, exposed to diseases, to the effects of the degradation of the quality of the water etc... and its aspect is a good indicator of health.
The axolotl has four legs each carrying 4 (in front) or 5 clawed fingers (in the back). The sexual and excretory organs, not differentiated externally (the cloaca) are located between the back legs and the birth of the crest. The natural form is pigmented, grey speckled or mottled with black or more rarely brown-beige speckled with dark brown, with a lighter belly.
An adult Axolotl can measure anywhere from six to 18 inches. However, the usual size for this species is only nine inches. As for its weight, the males can weigh about 125 to 135 grams while the females can weigh about 170 to 180 grams – the females are usually bigger than the males.
The head of an Axolotl is wide – much like typical salamanders. Their eyes are lidless (unlike human eyes), so they can’t blink their eyes (this is common in different types of fish as well).
As for their limbs, they have four short legs with long, thin digits. Their digits are what you would call fingers and toes in humans.
The eyes of Axolotls are lidless – they can’t blink their eyes.
Note that the overall shape of the bodies of males and females are also quite different. The females have wider bodies than the males – as the females need space to carry their eggs when they get pregnant. This is also the reason why females are typically bigger and heavier than the males.
Remember their gills? They actually have external and internal gills. They have three pairs of external gills that are located behind their heads – so, there are three on the left side and three on the right side.
Underneath their external gills are gill slits. These gill slits connect with the internal gills of the Axolotl.
Males VS females
Male axolotls usually have a slightly longer tail than females. Males also always have a swollen cloacal region, even outside the breeding season. This is not the case with females. However, the gender can only be determined after about one year, or when they are about twenty centimetres big.
A male reaches reproductive maturity at about one to two years. Females usually become reproductively mature earlier and can reproduce after about one year.
As in many other species, the male performs a “mating dance” at breeding time to “seduce” the female. During the mating dance, the male has his long tail towered, wags it back and forth and also nudges the female to encourage her to mate.
The females usually produce around 80 to 700 eggs and lay them on water plants. This happens about every two months. The eggs are not always actually fertilised. When fertilised, a small axolotl develops inside the egg - this process is called paedogenesis. Usually, axolotls hatch after about 10 to 20 days. They then feed on the yolk for a few more days.
The babies are already born independently and can move around without any problems. After five weeks they measure about five to six centimetres.
As axolotls are not solitary animals, they should be kept at least in pairs.
Of course, if you have only males, egg-laying will never occur. Some owners prefer this.
Axolotls living together should be about the same size so that the larger axolotl does not - voluntarily or involuntarily - attack or try to swallow the smaller axolotl. Smaller axolotls are easily eaten by larger ones. As a worst-case scenario, the smaller axolotl may be even too big to eat and may get stuck in the mouth of the larger axolotl. Both animals can die from this! It is therefore very important that the animals are approximately the same size.
Types and colours
The axolotl types can be roughly divided into four categories:
1. Albinos
2. Coppers
3. Whites (Leucistic)
4. Wildlings (Melanoid)
Albinos are white or yellowish and can also be somewhat shiny or shimmery. Their eggs are white. Due to an enzyme defect, albinos do not have melanin, which would otherwise c a use a darker colouration. The enzyme that albinos do not have is tyrosinase (amino acid = tyrosine).
Coppers come in many colours, such as yellow, brown, white, copper. They can also shimmer a little. Their eggs are beige/light brown. In contrast to the albinos, they do have tyrosine or tyrosinase, but they do not produce melanin either, only phaeomelanin. Phaeomelanin is a pigment which, together with eumelanin (also a pigment), determines the colour of t he skin.
Wildlings (melanonids) are usually black and/or brown. Sometimes a yellowish colouration occurs. Their eggs are also dark.
The colour of the eggs does not (always) indicate what the larva will look like later. The colour of the eggs depends on the colour of the mother. However, the colouration of the emerging larva does not depend on the mother alone, but on both parents.
Unfortunately, in some countries axolotls have been dyed or given colour pigments so that they "shine" in numerous bright colours (red, pink, green etc.). It is not advisable to buy a dyed animal.
Like all amphibians, the "normal" axolotl has three kinds of cells responsible for the pigmentation of the skin and eyes, the chromatophores.
The melanophores carry pigments (melanin and others) responsible for black and brown colors The xanthophores carry the yellow color The iridiophores are pigmentary cells that reflect light .
There were already animals in the wild that showed a different color (or lack of color) from the usual wild type: pink axolotls were among the animals brought back to the Museum in 1863, which are largely (but not exclusively, as is sometimes said) the ancestors of our domestic axolotls. It is also known that the diversification of colors and coats is one of the first consequences of the domestication of a species.
The main genetic mutations determining the appearance of forms of different color are albinism, xanthism.
Albinism corresponds to the absence of black pigment, or to a very reduced production of them by the organism. In true albinism, the total absence of pigment, the eyes are pink. Albinism is genetically associated with many malformations (especially the absence of eyes) in some species, which makes the embryos not viable, and this i s probably also the case in the axolotl. Where possible, albino juveniles are more fragile and vulnerable than juveniles with wild-type coloration and suffer higher mortality rates at all stages of development. They are more easily spotted by predators and are less efficient at foraging due to their poor eyesight. They are also more susceptible to cellular dysfunction and more susceptible to disease. This is why a "marriage" likely to produce, in the same egg-laying, pigmented larvae and albinos in proportions predictable from the genetic probabilities usually results in an effective number of albinos lower than expected.
The pink axolotls common in captivity (which are in fact white, it is their red blood that makes them look pink) are not albinos in the strict sense, but partial albinos. The scientific term is leucistic. Their eyes are pigmented and they can show localized traces of pigmentation, usually on the gill racks and on the top of the head.
The full albino axolotl exists in captivity, however, it is the result of genetic manipulation in the laboratory. It is a hybrid, more exactly a "chimera". The American biologist Humphrey succeeded in the 1960s in transferring in vitro the gene of albinism taken from a female of Ambystoma tigrinum into the genome of an axolotl. The eggs developed into adult axolotl and their offspring proved to be stable in their characters and fertile, even after crossing with ordinary axolotls, although the breeding is more delicate. The neoteny is preserved. There are two forms of this type, the full albino and the "gold" axolotl. The first one is completely depigmented. The axolotl "gold" is completely devoid of melanophores, with red eyes. However, the xanthofores are present in attenuated form, as the melanophores were present in attenuated form in the leucistic axolotl and the animal has a more or less yellow coloration.
The leucistic mutation and albinism are called recessive. That is to say that their effects are manifested on the color of the animal - we say that the gene is expressed - only if the mutant gene is present TWICE on the allele formed by the two chromosomes (homozygous), therefore if it was transmitted by the father AND by the mother. When it is present only once (heterozygous), the animal is genetically carrier without affecting its appearance (phenotype). It can always be transmitted to its descendants. It follows that:
• Two mutant animals that are both pink, gold or albino only give mutant offspring: all pink, gold or albino, depending on the gene of the parents (homozygous mutants).
• Two animals without the mutant gene give only offspring resembling the parents (pigmented), not carriers (homozygous pigmented).
• Two parents of which one is a carrier of the gene and the other one is not, give only first generation offspring resembling the parents, pigmented, of which a part (50%) remain carriers of the gene (heterozygous mutants).
• Two parents who carry the gene without showing it give first generation offspring: for a quarter mutant (homozygous), for another quarter pigmented, non-carriers, and for half pigmented but carrying the gene (heterozygous mutants).
This is the simplest case because it involves only two cases, the presence or absence of mutation on a single gene, for a single color. Insofar as we can have several colors in play (melanophores and xanthophores), with variants of these, where some colorations involve several genes, and where a given genetic profile can lead to animals externally quite different from each other, for example, depending on the degree of pigmentation expressed in a leucistic axolotl, there are other forms of colorations. Hobbyists continue to select these and try to develop new ones, without genetic manipulation, by choosing the breeding stock in such a way as to strengthen this or that trait present in the parents.
In the end, we distinguish the following types, some of which are quite rare:
• Wild type (grey-black pigmented, melanophores, xanthophores and Iridiophores)
• Copper: Copper is a pigmented form with another color than the wild type, brown rather than gray, i.e. brown spots on a copper background; It is a rather rare color whose genetics is not completely elucidated. We do not know if it is a spontaneous color already present in the genetic heritage of the wild axolotl, a form that appeared spontaneously in captivity (mutation) and then retained by selection, or a hybridization. The reproduction of the "copper" is often laborious, all the eggs are not viable, clearly because of genetic problems. This may be due to hybrid inheritance or to excessive inbreeding as a result of inbreeding to fix the strain. It would be necessary to enlarge the gene pool by introducing wild type animals.
• Leucistics (melanophore inhibition). Animals that have visible melanophores, although inhibited overall, are said to be harlequin. A harlequin is genetically leucistic (homozygous for partial albinism), but it can carry the genes for other colors.
• Gold (inhibition of xanthophores) White albino: no pigment, the animal is both albino (without melanophores) and axanthic (without xanthophores)
• Melanic : pigmented animals but without iriodophores, which means for example that the eyes are without a golden circle around the pupil and that their coat tends to be more plain. They can exist in different colors and their gradations: Black, Grey, White, Gold, Albino or Copper.
Endangered Species
There have always been a few natural threats to the Axolotl’s survival, including predatory birds like herons.
More recently, however, Axolotls have begun to suffer from the introduction of large numbers of fish, like carp, into their lake habitat. These fish compete with Axolotls for food and also eat Axolotl eggs.
Herons are natural predators of the Axolotl. The Axolotl population has decreased as a result.
Axolotls are also popular as pets and as food. Roasted Axolotl is a festive treat in Mexico.
All these factors have had the effect of further shrinking Axolotl numbers in the wild. They are now considered a critically endangered species – a species likely to become extinct.
Their popularity as pets, however, means that despite the challenges they face in the wild, Axolotls continue to thrive. Even if they can no longer be found in their natural habitat one day, we will still be able to enjoy them in captivity for many years to come.
Jack Nelson
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