The axolotl (; from ; Ambystoma mexicanum) is a species of mole salamander. It is neotenic, reaching sexual maturity without undergoing metamorphosis, and the adults remain fully aquatic with obvious external gills. Axolotls may be difficult to distinguish from the larval stage of other neotenic adult mole salamanders, in particular the tiger salamander, or other species such as mudpuppies.
Axolotls originally inhabited a system of interconnected wetlands and lakes in the highlands of Mexico. They were known to inhabit the smaller lakes of Xochimilco and Chalco and are presumed to have inhabited the larger lakes of Texcoco and Zumpango. The desiccation of these lakes, initiated by the Aztecs and accelerated during the 20th century, has led to the destruction of much of the axolotl's natural habitat, an area now largely occupied by Mexico City. Wild axolotls have been driven to near extinction by the introduction of invasive species such as tilapia and carp; with a decreasing population of around 50 to 1,000 adult individuals, the species has been assessed as critically endangered by the International Union for Conservation of Nature (IUCN) and is listed under Appendix II of the Convention on International Trade in Endangered Species (CITES).
There currently exists a large captive population of axolotls, with specimens used extensively in scientific research because of their unusual ability to regenerate body parts, including limbs, gills and parts of their eyes and brains. The species is also used as a model organism. As aquarium technology has developed, axolotls have become a common exhibit in zoos and public aquariums, as well as an occasional pet in home aquariums. The axolotl is a popular subject in contemporary culture, inspiring a number of works and characters in the media.
Nomenclature
The term "axolotl" is a Nahuatl word which has been translated variably as "water slave", "water servant", "water sprite", "water player", "water monstrosity", "water twin", or "water dog". The word refers to Xolotl, the Aztec god who holds dominion over fire, lightning, the dead and the resurrected, dogs, games, grotesque or ugly beings, and twins (as he is the twin of Quetzalcōātl).
Some sources prefer the term "Mexican axolotl" to refer to this species unambiguously, as "axolotl" may be used for unmetamorphosed individuals of other Ambystoma species, although the word is most commonly used to refer to wild A. mexicanum and captive individuals.
Description
A sexually mature adult axolotl, at age 18–27 months, ranges in length from ; a size close to is most common, and any greater than is rare. Axolotls possess features typical of salamander larvae, including external gills and a caudal fin extending from behind the head to the vent.San Francisco Examiner (San Francisco, California) 7 August 1887, page 9, authored by Yda Addis Unlike most salamander species, axolotls retain their external gills when they mature into adulthood. This is a type of neoteny.
Axolotls have wide heads and lidless eyes. Their limbs are underdeveloped and possess long, thin digits. Three pairs of external gill stalks (rami) originate from behind the head and are used to move oxygenated water. These are lined with filaments (fimbriae) to increase the surface area for gas exchange. Four gill slits lined with gill rakers are hidden underneath the external gills, which prevent food from entering and allow particles to filter through. Males can be identified by their swollen cloacae lined with papillae, while females have noticeably wider bodies when gravid and full of eggs.
Axolotls have barely visible vestigial teeth; other salamanders only develop these during metamorphosis. Their primary method of feeding is by suction, during which their rakers interlock to close the gill slits. Axolotls use their external gills for respiration; buccal pumping (gulping air from the surface) may also be used to provide oxygen to their lungs. Buccal pumping can occur in a two-stroke manner, pumping air from the mouth to the lungs, or a four-stroke manner, reversing this pathway using compression forces.
The wild type animal (the "natural" form) is brown or tan with gold speckles and an olive undertone. They can subtly alter their color by changing the relative size and thickness of the melanophores, presumably for camouflage. Axolotls have four pigmentation genes; when mutated, they create different color variants. The four most common mutant colors are as follows:
Leucistic: pale pink body, black eyes Xanthic: grey body, black eyes Albinistic: pale pink or white body, red eyes Melanistic: black or dark blue body 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 leads to the development of a black and white piebald appearance upon reaching maturity. Pet breeders frequently cross the variant colors, and double homozygous 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 genes.
The 32 billion base pair long sequence of the axolotl's genome was published in 2018; the largest animal genome completed at the time, it revealed species-specific genetic pathways that may be responsible for limb regeneration. Although the axolotl genome is about ten times the size of the human genome, it encodes a similar number of proteins (23,251, compared with about 20,000 in the human genome). The size difference is mostly explained by a large fraction of repetitive sequences; these also contribute to increased median intron sizes (22,759 bp), which are 13, 16 and 25 times that observed in human (1,750 bp), mouse (1,469 bp) and Tibetan frog (906 bp), respectively.
Physiology Regeneration
The characteristic of the axolotl that attracts the most attention is its unusual healing ability; axolotls do not heal by scarring but are instead capable of tissue regeneration. Entire lost appendages, such as the limbs and tail, can regrow over a period of months. In certain cases, more vital structures can be regrown, such as tissues of the eye and heart. Axolotls can even restore parts of their central nervous system, including non-vital parts of their brains. They can readily accept transplants from other individuals—including eyes and parts of the brain—restoring these "alien organs" to full functionality. In special cases, axolotls have been able to repair a damaged limb while also generating an additional limb, resulting in an extra appendage that makes them attractive to pet owners as a novelty.
The three basic requirements for limb regeneration in an axolotl are: (1) the formation of a wound epithelium, (2) the presence of nerve signaling, and (3) the presence of cells from the different axes (lines of orientation) of the affected limb. An epidermis quickly forms to seal the site of the wound. Over the next few days, the cells of the wound epidermis divide and grow, forming a blastema, at which point the wound is ready to heal and undergo patterning to form the new limb. It is believed that during limb generation, axolotls have a special system to regulate their internal macrophage level and suppress inflammation, as scarring would prevent proper healing and regeneration. However, this belief has been questioned by other studies.
A fundamental growth structure credited for the axolotl's ability to regenerate whole limbs in early development is the apical ectodermal ridge (AER), with the apical ectodermal cap (AEC). The AER initiates the growth of appendages in embryos by signaling how they are shaped and extended. Ectodermal cells are needed to form limbs in most tetrapods, amphibians and humans, but unlike most other animals the AEC in the axolotl can send signals through growth hormones to activate blastema cells, which can rebuild whole amputated or damaged limbs or organs.
The axolotl has indeterminate growth, meaning it continues to grow throughout its life. Some consider this trait to be a direct contributor to the axolotl's regenerative ability, which declines with age but does not completely disappear. The ability to regenerate limbs is greatly diminished in metamorphosed individuals.
Neoteny
Most amphibians begin their lives as aquatic animals unable to live on dry land, often called tadpoles. To reach adulthood, they go through a process called metamorphosis, in which they lose their gills and start living on land. The axolotl is unusual in that it has a lack of the thyroid-stimulating hormone that triggers the thyroid to produce thyroxine, which is needed for metamorphosis to take place; instead, the axolotl keeps its gills and lives in water all its life, even after becoming an adult and able to reproduce. The trait of reaching sexual maturity without undergoing metamorphosis is called neoteny.
The genes responsible for neoteny in laboratory axolotls may have been identified; they are not linked to the genes of wild populations, suggesting artificial selection is the cause of complete neoteny in laboratory and pet axolotls. The genes responsible have been narrowed down to a small chromosomal region called met1, which contains several candidate genes.
Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations. Sirens, Necturus mudpuppies, and the troglobitic olm are other examples of neotenic salamanders, although unlike axolotls they cannot be induced to metamorphose by an injection of iodine or thyroxine hormone.
Metamorphosis
Over evolutionary time, the axolotl has lost the ability to naturally undergo metamorphosis. It has retained the capacity to undergo metamorphosis if provided with the necessary hormones through artificial administration. Under modern laboratory conditions, metamorphosis is reliably induced by administering thyroid hormones, including thyroxine, triiodo-L-thyronine, or thyroid-stimulating hormones.
Depending on the hormone used for induction, different outcomes may occur. Some hormones, such as triiodo-L-thyronine, can promote regenerative abilities while in some cases failing to produce complete metamorphosis. In contrast, thyroxine can inhibit regenerative abilities and accelerate metamorphosis.
After an axolotl undergoes hormonally induced metamorphosis and begins living on land, it experiences a number of physiological changes that help it adapt to terrestrial life. These include increased muscle tone in limbs, resorption of gills and fins into the body, the development of eyelids, and a reduction in the skin's permeability to water, allowing the axolotl to remain more effectively hydrated on land. The lungs of an axolotl, though present alongside gills after reaching non-metamorphosed adulthood, develop further during metamorphosis. Axolotls that complete metamorphosis are similar in appearance to the adult plateau tiger salamander, though axolotls have longer toes.
In the absence of induced metamorphosis, larval axolotls begin absorbing iodide into their thyroid glands at around 30 days post-fertilization. Larval axolotls do produce thyroid hormones from iodide, but the amount appears highly variable. In contrast, adult axolotls do not produce detectable levels of thyroid hormone unless metamorphosis is triggered.
Wild population