Axolotl

The axolotl ( ; from Classical Nahuatl: āxōlōtl [aːˈʃoːloːtɬ] ; 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 desicc...

Mexico

A sexually mature adult axolotl, at age 18–27 months, ranges in length from 15 to 45 cm (6 to 18 in); a size close to 23 cm (9 in) is most common, and any 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. 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.

Axolotls are native only to the Mexican Central Valley, and the population once extended through most of the lakes and wetlands in this region. The axolotl's natural habitat is now limited to Lake Xochimilco as a result of the expansion of Mexico City and is under pressure from the city's growth. The axolotl is on the IUCN Red List of threatened species. Surveys conducted in 1998, 2003, and 2008 found populations of 6,000, 1,000, and 100 axolotls per square kilometer, respectively, in Lake Xochimilco. A four-month-long search in 2013 found no surviving individuals in the wild, but one month later two were spotted in a network of canals leading from Xochimilco. Lake Xochimilco has poor water quality; tests have revealed a low nitrogen-to-phosphorus ratio and a high concentration of chlorophyll a, indicative of an oxygen-poor environment not well-suited to axolotls. This has been caused by the demands of local industries, such as aquaculture and agriculture, which maintain the lake's water levels through inputs of partially treated wastewater. Intensively used agricultural pesticides eventually enter the lake through runoff, and these contain chemical compounds that sharply increase the mortality rate in axolotl embryos and larvae. Of the surviving embryos and larvae, there is also an increase in morphological, behavioral, and activity abnormalities. The dramatic reduction in the native population has led to a significant loss of genetic diversity. This can be dangerous for the remaining population, as it causes increased inbreeding and reduced fitness and adaptive potential. Studies have found indicators of low interpopulation gene flow and higher rates of genetic drift. These are likely the result of multiple "bottleneck" incidents, where a sharp drop in the number of individuals in the population leads to decreased genetic diversity. The offspring produced after bottleneck events have a greater risk of poor fitness and are often less able to adapt. Several bottleneck events may even lead to extinction. Studies have also found high rates of relatedness indicative of inbreeding, which can cause an increase in the presence of deleterious, or harmful, mutations in genes. The detection of introgressed tiger salamander (A. tigrinum) DNA in the laboratory axolotl population raises concerns about the suitability of the captive population as an "ark" for potential reintroduction purposes. Another factor that threatens the population is the introduction of invasive fish species, such as the Nile tilapia and the common carp. These fish eat the axolotls' young and compete for their food. The presence of these species has changed axolotl behavior, causing them to be less active in an effort to avoid predation. This reduced activity greatly impacts the axolotl's foraging and mating opportunities. The fungus Batrachochytrium dendrobatidis has been detected in axolotls; this fungus causes the disease chytridiomycosis in amphibians and is a major concern for amphibian conservation worldwide. However, the axolotl displays resistance to both B. dendrobatidis and B. salamandrivorans, so chytridiomycosis is thought not to be a threat to this species.

The condition of the native axolotl population has improved little over the years. Many scientists are focusing their conservation efforts on the translocation of captive-bred individuals into new habitats or their reintroduction into Lake Xochimilco. Studies have shown that axolotls born in captivity and raised in a semi-natural environment are capable of surviving in the wild, catching prey and escaping predators with moderate success. These captive-bred individuals may be introduced into unpolluted bodies of water or returned to Lake Xochimilco; however, with the amount of pollution in the lake, the presence of invasive species and the region's continuing urbanization, the translocated axolotls might eventually have the same fate as the wild population. The Laboratorio de Restauracion Ecologica (transl. Laboratory of Ecological Restoration) of the National Autonomous University of Mexico has built up a population of 100 captive-bred axolotls, as of 2021. These are mostly used for research, but there are plans to establish a viable population in a semi-artificial wetland inside the university. A 2025 study confirmed the viability of releasing captive-bred axolotls into the wild, with recaptured individuals having gained weight since their release. However, this practice risks losing the axolotls through predation, as several of those released were preyed on by great egrets.

German naturalist and explorer Alexander von Humboldt noted in the 19th century that the Mexicans, having been vanquished by the Spanish Empire, lived "in great want, compelled to feed on roots of aquatic plants, insects and a problematical reptile called axolotl". In 1863, a shipment of 34 adult axolotls was sent from Mexico City to the Jardin des Plantes in Paris, from which thousands of specimens were captive-bred and distributed around Europe for scientific research. Unaware of their neoteny, French zoologist 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. In Prague, the Czech medical doctor Vilém Laufberger used thyroid hormone injections to induce an axolotl to grow into a terrestrial adult salamander. Unaware that it had already been carried out, Englishman Julian Huxley repeated the experiment using ground animal thyroid glands. Since then, experiments have often involved the injection of iodine or various thyroid hormones to induce metamorphosis.