Axolotl (Ambystoma mexicanum)

While wild axolotl are usually mottled brownish-green to dark black in color, other color phases and morphs have been produced in the laboratory. White, leucistic, and albino specimens are common in captivity.

The sexes can be easily distinguished in adult axolotls. Males can be identified by their enlarged cloaca, (similar to other urodeles,) while females have a smaller cloaca and round, plump bodies. Both sexes can reach lengths on average of 20 centimeters, (9 inches,) but can grow to more than 30 centimeters, (12 inches,) in length.

Because they are paedomorphic, neotenic aquatic salamanders that retain certain larval characteristics in the reproductive state, adult axolotls retain the feathery external gills and finned tails from their juvenile stages. These physical traits are suited to an aquatic lifestyle and are used for swimming.

Perhaps the most impressive aspect of the axolotl’s biology is its power of regeneration. Not only can it reproduce a new limb or tail when these have been lost by accident or through attack by a predator, the axolotl can also regenerate brain and heart cells, characteristics that have made it a focus of a considerable amount of research with a long-term application for human health.

Expected laboratory longevity of axolotls is 5-6 years, however, some have been known to live as long as 10-15 years. Most laboratory animals die shortly after metamorphosis, though.

SEXUAL DIMORPHISM

Females with Smaller Cloacas & Round, Plump Bodies

BODY LENGTH

22-31 cm. / 9-12 in.

BODY MASS

90-182 g. / 3-67 oz.

LIFESPAN

5-15 yr.

GENERATION LENGTH

2-10 yr.

No popular experimental animal is more misunderstood and none has been more diversely treated taxonomically than the axolotl, properly known at all stages of its life as Ambystoma mexicanum. The misconceptions made evident include application of the common name, the taxonomic status of the population to which the name axolotl is properly applied, the proper scientific and generic names for the axolotl, the proper name for the larval as opposed to the adult stage, the degree of neoteny occurring, and the cause for transformation.

Axolotls are paedomorphic or neotenic aquatic salamanders and are one of only five species of the Ambystoma genus which exhibits varying degrees of neoteny.

The closest relative of the axolotl is thought to be Ambystoma tigrinum, the tiger salamander. Indeed, the larvae of these species are visually very similar. Some even consider the axolotl to be a subspecies of the tiger salamander as viable offspring can be produced between the two species in the laboratory, though no hybrids have been discovered in the wild as of yet.

Larvae of other ambystomids, such as the larval stage of the tiger salamander, are often erroneously referred to as axolotls. The name axolotl should be used only when referring to Ambystoma mexicanum and not to any other ambystomid salamander.

Historically, the Mexican axolotl has been listed under more than 40 different names and spellings; all, except Ambystoma tigrinum, have been rejected by the International Commission on Zoological Nomenclature (ICZN).

KINGDOM

Animalia

PHYLUM

Chordata

CLASS

Amphibia

ORDER

Caudata

FAMILY

Ambystomatidae

GENUS

Ambystoma

SPECIES

mexicanum

SUBSPECIES

None

The axolotl is important in Aztec mythology. The word axolotl comes from the native Aztec language, or nahuatl, and roughly translates to: water slave, water servant, water sprite, water player, water monstrosity, water twin, or water dog. No one meaning can be considered correct to the exclusion of all others, any one of which is tenable and may have been an accepted connotation in living nahautl, in given contexts.

All of these names refer to the Aztec god, Xolotl, brother to Quetzacoatl. Xolotl was patron of the dead and ressurrected, (where he took the form of a dog,) and had exceptionally broad surveillance over games, twins, and monstrous and grotesque beings, including lifeforms with congenital deformities or other repulsive appearances. Aztec lore states that Xolotl, fearing his imminent sacrifice, threw himself into the water and transformed himself into an axolotl, among other things, to escape banishment from the earth, which would result in his death. As an axolotl, he was ultimately captured, killed, and fed to the sun and moon.

All of the forms Xolotl assumed were monstrous, ugly, or paired. The axolotl fits these themes with its supposed “ugly” features or through its twin relationship, in its aquatic form, to the terrestrial salamander, the two forms seemingly recognized as stages in the life cycle of a single species.

ALTERNATE

Mexican Axolotl, Mexican Salamander, Mexican Walking Fish, Walking Fish, Water Dog, Water Monstrosity, Water Player, Water Serpent, Water Slave, Water Sprite, Water Twin

GROUP

Harem

The axolotl is native only to central Mexico, on the southern edge of Mexico City, in canals and wetlands in the general vicinity of Xochimilco, including outside the Xochimilco city limits and around the Chalco wetland. Its area of ccupancy is less than 10 kilometers² and includes an ancient system of water channels made up of deepwater lakes, natural and artificial canals, and abundant aquatic vegetation.

The axolotl was originally found in Lakes Xochimilco and Chalco, and presumably in the connecting lakes Texcoco and Zumpango, but it has disappeared from most of its range. The vast wetland upon which Mexico City was founded, and which once provided a rich and productive habitat for the axolotl and other endemic fauna, is now reduced to a handful of small, isolated patches surrounded by development. Of these, the area called ‘Lake Xochimilco’ is the largest, covering just over two square kilometers. It, too, is no longer a lake, having been fundamentally altered by the development of the sophisticated ‘chinampas’ agricultural system, which started in pre-Aztec times. This consists of raised fields of mud and vegetation coralled by rectangular plantings of the water-loving willow, Salix bonplandiana, which has reduced most of the lake to a series of canals of varying widths, approximately 182 kilometers in total length.

The axolotl are not homogeneously distributed through their range and congregate in particular places. Records from close to downtown Mexico City in the Chapultepec Lake could refer to either this species or Ambystoma velasci, and require confirmation.

EXTANT

Mexico

The axolotl is native to the ancient system of water channels and lakes in the Mexican Central Valley.

The axolotl requires deep-water lakes, both natural and artificial canals, with abundant aquatic vegetation, and is sensitive to changes in water quality.

Structures such as plants are needed for the axolotl to lay eggs.

The axolotl is a paedomorphic species, living permanently in water, and does not undergo complete metamorphosis.

WETLANDS

Permanent Freshwater Lakes

ARTIFICIAL/AQUATIC & MARINE

Water Storage Areas, Canals & Drainage Channels, Ditches

Axolotls are solitary and may be active at any time of the day.

Axolotls communicate mainly via visual cues and chemical cues during mating. At other times of the year there is little to no intraspecific communication.

Axolotls can detect electrical fields and also use their vision and chemical cues to perceive their environment and discover prey.

Unlike typical amphibians, the axolotl does not change from an aquatic to an air-breathing lifeform through the result of metamorphosis. They, instead, undergo a process called paedogenesis becoming sexually mature in the aquatic larval form and living permanently in the water for up to 10-15 years. Metamorphosis can be artificially induced in axolotls via thyroid hormone injections. In the wild, axolotls rarely, if ever, metamorphose.

CIRCADIAN RHYTHM

Cathemeral

MOVEMENT PATTERN

Non-Migrant

Generally the top predator in their natural environment, axolotls will eat anything that they can catch, including mollusks, fish, and arthropods, as well as conspecifics.

Axolotls use vision and chemical cues to perceive their environment and discover invertebrate prey.

PREFERENCE

Generalist

STYLE

Forager

Axolotls generally breed in the wild from March to June.

The courtship behavior of the axolotl follows the general Ambystoma pattern. It first involves each animal nudging the other’s cloacal region, eventually leading to a waltz with both animals moving in a circle. Next, the male moves away while undulating the posterior part of his body and tail, (resembling a hula dance,) and the female follows. The male will deposit a spermatophore, a cone-shaped jelly mass with a sperm cap, by vigorously shaking his tail for about half a minute, and will then move forward one body length. The female then moves over the spermatophore, also shaking her tail, and picks up the spermatophore with her cloaca.

Axolotls communicate mainly via visual cues and chemical cues during mating.

Like many other amphibians, the axolotl lays its eggs in freshwater. Females deposit 100-300 eggs in the water and attach them to substrates. Structures, such as plants, are needed to lay the eggs. Up to 1,000 eggs can be laid every 3-6 months.

Axolotl eggs are surrounded by a protective jelly coat and are laid singly because they possess higher oxygen requirements, unlike frog eggs, which are laid in clumped masses. Axolotl eggs are often attached to substrates such as rocks or floating vegetation.

After 10-14 days the eggs hatch into larvae that are immediately independent. They obtain oxygen from the water using gills, develop four legs, and feed on small plants and animals.

It is thought to reach sexual maturity around 1.5 years in the next breeding season and, in captivity, lives until about 10-15 years in total; the generation length is estimated to be around 5.5 years.

BREEDING SEASON

March-June

BREEDING INTERVAL

3-6 Months

BROOD

###

PARENTAL INVESTMENT

None

INCUBATION

10-14 Days

BIRTHING SEASON

###

CLUTCH

100-300

INDEPENDENCE

Birth

SEXUAL MATURITY

1.5 Years

Generally the top predator in their natural environment, axolotls are important in structuring community dynamics.

Axolotls may be preyed on by large fish and conspecifics, such as Asian carp and African tilapia. These large fish have only recently been introduced into the lakes where axolotls are found, contributing to the demise of their populations.

Fishing is important to the local economy of the axolotl where approximately 80 fishermen reside. Non-native Asian carp and African tilapia have replaced the axolotl as the main catch, and as such, introduced fish have increased to high abundances and have impacted wild axolotls through competition and predation. A recent study collected 600 kilograms of tilapia in one small channel using a 100 meter net.

The axolotls are also being affected by disease, probably spread by invasive species, and as a result of poor water quality.

As well as being a popular pet, the axolotl is extensively used in medical research because it possesses remarkable biological traits that distinguish it from other vertebrates.

Despite its precarious status in the wild, throughout the world, the axolotl is a familiar sight in laboratories and aquaria and are one of the most widely used and studied laboratory animals. Axolotls are an important research animal renowned for their amazing regenerative capabilities and have been widely used in developmental studies of the regulation of gene expression, embryology, neurobiology, and regeneration. Because axolotls are polypoid and possess large cells, they are often used in histological studies. The axolotl’s impressive power of regeneration has made it a focus of a considerable amount of research with a long-term application for human health. Axolotls are excellent lab specimens as they are easy to raise and inexpensive to feed.

Nearly all modern laboratory axolotls can be traced back to 33 animals shipped from Xochimilco to Paris in 1864. Animals were originally collected in 1863 for the Natural History Museum in Paris, and many of today’s captive animals probably stem from these founders.

Even though there is a supply of captive-bred axolotls, wild animals are still being captured and sold illegally in local markets. This trade is very difficult to police. Wild harvest of axolotls is small, but not necessarily, in relation to population size. There is a local illegal trade in wild-caught axolotls for human consumption, medicinal uses, and pets.

It is assumed that all international pet trade is now in captive-bred animals. The harvesting is targeted at animals that are less than one year old.

The traditional consumption of the axolotl by local people is threatening the survival of the species. Occasionally taken as a food item, (substituted for fish,) axolotls are prepared by either roasting or boiling, and the tail is eaten with vinegar or cayenne pepper. Axolotls have been long recorded as edible, especially about Mexico City and patcuaro and remain today a dietary delicacy or staple in these areas.

There are no negative effects of axolotls on humans.

RESEARCH

National, International

PETS/DISPLAY ANIMALS, HORTICULTURE

National, International

MEDICINE

Local

FOOD

Local

Because known populations are few and far between, very little is known about the ecology and natural history of axolotls. There have been few ecological studies on wild populations.

The axolotl is possibly one of Latin America’s most threatened amphibians as the surviving wild population is very small. Although populations are difficult to assess, recent surveys covering almost all of its known distribution range have usually captured fewer than 100 individuals.

During 2002 and 2003, more than 1,800 net casts were made along Xochimilco canals covering 39,173m² and this resulted in a catch of only 42 specimens. In a study covering a span of six years, from 1998 to 2004, axolotl density had reduced from 0.006-org/ m2 to 0.001-org/ m2, although it is thought that this reduction could also be due to its own population dynamics. A recent scientific survey revealed no axolotls, although wild-caught animals are still found in the local market, which indicates that fishermen still know where to find them. There has not been a density study of the Chalco population, but evidence suggests that the population there is small and, furthermore, Chalco is a highly unstable system that runs the risk of disappearing in the near future.

MATURE INDIVIDUALS

50-1,000

FRAGMENTATION

Severely Fragmented

The desiccation and pollution of the canal system and lakes in Xochimilco and Chalco, as a result of urbanization, is threatening the survival of this species. Increased tourist activity is poorly regulated and adds further pollution.

The axolotls are also being affected by disease, probably spread by invasive species, and as a result of poor water quality. The species has tested positive to the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd), but is not thought to be susceptible to the disease. In addition, laboratory tests have shown that the species is resistant to the salamander chytrid fungus Batrachochytrium salamandrivorans (Bsal).

Although the water regime has changed in the last 10 years, and it is reported that pollution levels are decreasing, factors such as very high levels of bacterial contamination could still pose a serious threat.

RESIDENTIAL & COMMERCIAL DEVELOPMENT

Housing & Urban Areas, Tourism & Recreation Areas

BIOLOGICAL RESOURCE USE

Hunting & Trapping Terrestrial Animals

INVASIVE & OTHER PROBLEMATIC SPECIES, GENES, & DISEASES

Invasive Non-Native/Alien Species/Diseases

POLLUTION

Domestic & Urban Waste Water, Industrial & Military Effluents, Garbage & Solid Waste

The axolotl is protected under the category Pr (Special protection) by the Government of Mexico and is in process of being amended to a higher risk category. Mexican authorities are considering placing axolotl, (under the relevant Mexican legislation NOM-059-SEMARNAT-2001,) in a risk category that best fits the current risk status of the species. This would mean its transfer from “Species under special protection” (Pr) to “Endangered” (P), thereby conferring upon the species maximum protection under Mexican legislation.

Although this species is currently on CITES Appendix II, it is currently under the process of “Periodic Review of species included in CITES Appendices”. A proposal to upgrade the axolotl from CITES Appendix II, (controlled international trade,) to Appendix I, (species threatened with extinction and international trade permitted only in exceptional circumstances,) is currently under review by the Mexican authorities and the Animals Committee of CITES.

Building on its existing profile among the people of Lake Xochimilco, over the past three years a partnership of British and Mexican organizations has been developing a conservation program for the axolotl. The project was the brainchild of the late Dr. Virginia Graue of the Universidad Autónoma Metropolitana at Xochimilco (UAM-X), who contacted the Durrell Institute of Conservation and Ecology (DICE) in 1999 for assistance with development of the project.

As it was clear at that time that addressing the many threats that the axolotl faced would be impossible without the co-operation of local stakeholders, the project initially focused on embracing local people within the conservation planning process by developing the axolotl as a flagship species for nature tourism and conservation education within the region. Using UAM-X’s existing field station, CIBAC, on the shores of the lake as a base, and with funding from The Declining Amphibian Population Task Force (DAPTF) and the British Government’s Darwin Initiative programme (DI), the project partnership has run training workshops on amphibian biology and conservation for local students and conservation organizations, nature guiding for local boatmen, and souvenir production for unemployed artisans. In addition, the project has been engaged with ongoing biological research on population status and the assessment and impact of threats, workshop facilitation, and public education and awareness-raising.

As a result of the ease of availability of captive populations, there is considerable interest in restocking Lake Xochimilco with axolotls. This is seen as a way of preserving the species within what remains of the little-studied habitat that provided the unique conditions in which its remarkable biology first evolved, and where the key to understanding this biology may lie.

There are, however, several problems associated with such releases. At the very least, the threats to the species need to be neutralized, and potential disease and genetic problems addressed before captive animals are put back into the wild. The introduction of a disease or abnormal genes from a captive bred population could wipe out the remaining wild stock, so, any such introduction has to be conducted with extreme caution. Existing colonies also need to be analysed and compared to wild-caught specimens and Ambystoma tigrinum in order to determine their genetic status. Moreover, the population that does still exist, may well be so fragmented that each subpopulation has become genetically distinct.

The provision of disease-free, pure-bred axolotls and monitoring of the health status of captive and wild populations are key components of any program for the recovery and conservation of the wild population. Furthermore, even if a restocking program were to succeed, the question of whether a sustainable harvest could be made to meet local demands for the species is also likely to arise.