The name “axolotl” is thought to have originated from the Aztecs, derived from two words: atl, meaning “water”, and xolotl meaning “monster”. Axolotls do not develop adult characteristics but retain their gills, fins and other larval characteristics throughout their life. They live permanently in water, in the wetlands and canals associated with Lake Xochimilco and Lake Chalco, adjacent to Mexico City. Once eaten as a delicacy in Mexico City, they are now a protected species in Mexico and Critically Endangered in the wild.
Evolutionary Distinctiveness
Order: Caudata
Family: Ambystomatidae
The family Ambystomatidae or “the mole salamanders” is included within the four earliest or most primitive family lineages of the order “Caudata” (the salamanders), diverging from all other salamanders in the Early Cretaceous period over 140 million years ago, around five million years before the koala and dolphin lineages diverged from their common ancestor. The small number of species that represent the genus Ambystoma are highly evolutionarily distinct members of both the salamanders and the amphibians as a whole.The axolotl exhibits some highly unusual and distinct features, indicative of its evolutionary distinctiveness, including its rare “neotenous” life history, whereby the species never develops into an adult but instead retains its juvenile characteristics throughout life, essentially achieving reproductive maturity whilst still in its undeveloped larval form. There are a couple of theories for why neoteny (also referred to a paedomorphosis) develops in some mole salamanders. One idea is that the production or effectiveness of the hormone thyroxine is compromised, either by the species living in water bodies containing insufficient iodine (which is required in the manufacture of thyroxine by the body) or in water temperatures that are too cold for the thyroxine to be effective. This impacts upon the development of the species and sexually mature adults never develop adult characteristics but remain in the larval form. A second theory suggests that species evolving in pools surrounded by hostile terrestrial environments develop aquatic lives to obviate the need to exit the relative safety of their watery home. This is a common trait in species that inhabit high-elevation ponds.
Order: Caudata
Family: Ambystomatidae
The family Ambystomatidae or “the mole salamanders” is included within the four earliest or most primitive family lineages of the order “Caudata” (the salamanders), diverging from all other salamanders in the Early Cretaceous period over 140 million years ago, around five million years before the koala and dolphin lineages diverged from their common ancestor. The small number of species that represent the genus Ambystoma are highly evolutionarily distinct members of both the salamanders and the amphibians as a whole.The axolotl exhibits some highly unusual and distinct features, indicative of its evolutionary distinctiveness, including its rare “neotenous” life history, whereby the species never develops into an adult but instead retains its juvenile characteristics throughout life, essentially achieving reproductive maturity whilst still in its undeveloped larval form. There are a couple of theories for why neoteny (also referred to a paedomorphosis) develops in some mole salamanders. One idea is that the production or effectiveness of the hormone thyroxine is compromised, either by the species living in water bodies containing insufficient iodine (which is required in the manufacture of thyroxine by the body) or in water temperatures that are too cold for the thyroxine to be effective. This impacts upon the development of the species and sexually mature adults never develop adult characteristics but remain in the larval form. A second theory suggests that species evolving in pools surrounded by hostile terrestrial environments develop aquatic lives to obviate the need to exit the relative safety of their watery home. This is a common trait in species that inhabit high-elevation ponds.
Description
The axolotl is an Ambystomatid or mole salamander found only in the Laguna Alchichica in eastern Puebla, Mexico at an elevation of 2,290m above sea level. Mole salamanders are medium to large, stocky salamanders, usually measuring between 90 to 350mm from the tip of the nose to the end of the tail, which salamanders retain throughout their life. Males are often larger than females, owing to their longer tails. Ambystomatids generally exhibit both aquatic “neotenic” larval (or aquatic and permanently juvenile in form with external, feathery gills) and terrestrial “metamorphosed” (or ground-dwelling, fully developed adult in form with reduced gills) stages in their wild populations. Ambystomatids are often boldly patterned as adults, with well-developed costal grooves (successive vertical grooves along the sides of the body), especially the metamorphosing varieties. They have a rather flattened body with a wide, flattened head, a large mouth and smooth skin with many glands. The tail is roundish or laterally compressed, and, during the breeding season, males have a very swollen cloacal zone (the region around the reproductory and excretory opening in amphibians located underneath the base of the tail).Like all neotenic Ambystoma species, the axolotl retains its larval features into adulthood. A fully grown axolotl (at age 18–24 months) ranges in length from 150–450mm, although a size close to 230mm is typical and axolotls measuring more than 300mm nose to tail are rarely observed. The axolotl grows much larger than a normal larval or neotenic salamander species, and it reaches sexual maturity in this larval stage. Axolotls have distinctive fern-like gill structures that are uncovered or external – usually three stalks on each side of the head. They have tiny teeth which are used to grip food rather than to tear and chew it. In body colour, axolotls range from albino or white (the leucistic variety) to black, through greys, tans and browns. However, leucistic axolotls are rarely found in the wild.
Ecology
The most notable feature of the life history of the axolotl is that the species exhibits an extreme form of neoteny. Axolotls remain in their aquatic larval form throughout their entire lives, which means that when they reach sexual maturity at approximately one and a half years of age, they remain in all other respects a large larva. This would be akin to a tadpole being able to breed without ever turning into a frog. Neoteny involves the retention of “paedomorphic” features (referring to those features which pertain to the juvenile form) such as external gills that persist through life, a state described as “perennibranchiate”. The paedomorphic characteristics found in adult axolotls include the maintenance of external gills, fins, non-protruding eyes, no eyelids and an associated permanently aquatic lifestyle. Young axolotls feed on algae, but as adults their diet predominantly consists of aquatic insects.During reproduction in axolotls, the male releases sperm packets which are taken up by the female for internal fertilisation. Fertilised eggs are attached by the female to structures such as plants and hatching generally occurs after 2-3 weeks. In the wild, axolotls can live for ten to 12 years. The major predators of the axolotl are predatory birds such as herons.Axolotls famously have a fast regeneration rate which can allow them to regrow limbs and organs. In addition to respiring via their external fearthery gills, axolotls are able to breathe through their skin and also possess lungs.
The axolotl is an Ambystomatid or mole salamander found only in the Laguna Alchichica in eastern Puebla, Mexico at an elevation of 2,290m above sea level. Mole salamanders are medium to large, stocky salamanders, usually measuring between 90 to 350mm from the tip of the nose to the end of the tail, which salamanders retain throughout their life. Males are often larger than females, owing to their longer tails. Ambystomatids generally exhibit both aquatic “neotenic” larval (or aquatic and permanently juvenile in form with external, feathery gills) and terrestrial “metamorphosed” (or ground-dwelling, fully developed adult in form with reduced gills) stages in their wild populations. Ambystomatids are often boldly patterned as adults, with well-developed costal grooves (successive vertical grooves along the sides of the body), especially the metamorphosing varieties. They have a rather flattened body with a wide, flattened head, a large mouth and smooth skin with many glands. The tail is roundish or laterally compressed, and, during the breeding season, males have a very swollen cloacal zone (the region around the reproductory and excretory opening in amphibians located underneath the base of the tail).Like all neotenic Ambystoma species, the axolotl retains its larval features into adulthood. A fully grown axolotl (at age 18–24 months) ranges in length from 150–450mm, although a size close to 230mm is typical and axolotls measuring more than 300mm nose to tail are rarely observed. The axolotl grows much larger than a normal larval or neotenic salamander species, and it reaches sexual maturity in this larval stage. Axolotls have distinctive fern-like gill structures that are uncovered or external – usually three stalks on each side of the head. They have tiny teeth which are used to grip food rather than to tear and chew it. In body colour, axolotls range from albino or white (the leucistic variety) to black, through greys, tans and browns. However, leucistic axolotls are rarely found in the wild.
Ecology
The most notable feature of the life history of the axolotl is that the species exhibits an extreme form of neoteny. Axolotls remain in their aquatic larval form throughout their entire lives, which means that when they reach sexual maturity at approximately one and a half years of age, they remain in all other respects a large larva. This would be akin to a tadpole being able to breed without ever turning into a frog. Neoteny involves the retention of “paedomorphic” features (referring to those features which pertain to the juvenile form) such as external gills that persist through life, a state described as “perennibranchiate”. The paedomorphic characteristics found in adult axolotls include the maintenance of external gills, fins, non-protruding eyes, no eyelids and an associated permanently aquatic lifestyle. Young axolotls feed on algae, but as adults their diet predominantly consists of aquatic insects.During reproduction in axolotls, the male releases sperm packets which are taken up by the female for internal fertilisation. Fertilised eggs are attached by the female to structures such as plants and hatching generally occurs after 2-3 weeks. In the wild, axolotls can live for ten to 12 years. The major predators of the axolotl are predatory birds such as herons.Axolotls famously have a fast regeneration rate which can allow them to regrow limbs and organs. In addition to respiring via their external fearthery gills, axolotls are able to breathe through their skin and also possess lungs.
Habitat
The axolotl is native to the ancient system of water channels and lakes in Mexico City. This species requires deep-water lakes and water bodies (including both natural and artificial canals) with abundant aquatic vegetation. The axolotl depends upon vegetation and other suitable structures for the attachment of their eggs, following fertilisation. Lake Xochimilco is known for its “floating gardens,” or “chinampas,” which are strips of land between drainage channels where local people grow vegetables and flowers for the market. Axolotls may be found in these channels, as well as remaining lake areas.
The axolotl is native to the ancient system of water channels and lakes in Mexico City. This species requires deep-water lakes and water bodies (including both natural and artificial canals) with abundant aquatic vegetation. The axolotl depends upon vegetation and other suitable structures for the attachment of their eggs, following fertilisation. Lake Xochimilco is known for its “floating gardens,” or “chinampas,” which are strips of land between drainage channels where local people grow vegetables and flowers for the market. Axolotls may be found in these channels, as well as remaining lake areas.
Distribution
The axolotl is known only from central Mexico, on the southern edge of Mexico City, in canals and wetlands in the general vicinity of Xochimilco (including outside of Xochimilco proper, around the Chalco wetland). The axolotl lives only in wetlands and canals associated with Lake Xochimilco and Lake Chalco, both adjacent to Mexico City. Xochimilco and Chalco are part of a complex of five lakes, among which the Aztecs built Mexico City and around which the city has since expanded. Axolotls are not regularly distributed throughout their range but instead congregate in places where the habitat is still conducive for the survival and breeding of the axolotl.
Population Estimate
An accurate population estimate is currently unavailable for the species, although the surviving wild population of axolotls is known to be very small despite a large captive population. Although populations are difficult to assess, recent surveys covering almost all of its known distribution range have usually captured less than 100 individuals and during 2002-2003, following more than 1,800 net casts along Xochimilco canals covering 39,173m2, the resultant catch was just 42 individuals. A recent scientific survey in 2004 revealed no axolotls in the species’ range. However, wild-caught animals are still found in local markets, indicating that the local fishermen still know where to find them. There has not been a density study of the Chalco population, but evidence suggests that this population is small and, furthermore, Chalco is a highly unstable system which runs the risk of disappearing in the near future.
Population Trend
Exact population data are currently unavailable for the axolotl, although the population trend is assumed to be in decline in the IUCN Red List of Threatened Species based on notable declines in survey catches performed since 2002 along a 39,173m2 area of the Xochimilco canals.
Status
Listed as Critically Endangered in the IUCN Red List of Threatened Species because its area of occupancy is less than 10 km sq., its distribution is severely fragmented, there is a continuing decline in the extent and quality of its habitat and a sustained decline in the number of mature individuals.
Threats
Axolotl populations are suffering as a result of land drainage and the growth of Mexico City. Various efforts at flood control and sewage disposal starting in the seventeenth century have led to serious damage to the Xochimilco and Chalco lake complex. The digging of wells for the burgeoning population of Mexico City has also caused drying of the valley in which the lakes are located. The largest of these lakes, Texcoco, has been greatly diminished in size, while Lake Chalco has all but disappeared. Xochimilco has likewise suffered a decline in size and water quality. The major threat to the survival of the axolotl is therefore the desiccation, pollution and general degradation of the canal system and lakes in Xochimilco and Chalco, as a result of urbanization. The species is under pressure from traditional harvesting for consumption by local people and axolotls are also captured for medicinal purposes. The harvesting is targeted at animals that are less than one year old, and therefore generally before the individuals have had the opportunity to breed since axolotls reach sexual maturity at approximately one and a half years of age. Formerly, axolotls have also been captured for the international pet trade, although it is thought that no axolotls commercially available today are wild caught since doing so is strictly forbidden. The majority of axolotls currently available on the international market probably originated from captive or laboratory populations.Introduced predatory fish (such as tilapia and carp) have increased to high abundances – a recent study collected 600kg of tilapia in one small channel using a 100m net. These introduced fish species have had a negative impact upon axolotl populations through competition and predation. The species is also being adversely affected by poor water quality, arising from factors such as pollution and disease, probably spread via invasive species. Although the water regime has changed in the last ten 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 to axolotls in the wild.
Conservation Underway
Conservation action to protect axolotl populations in the wild is focusing on raising the profile of Lake Xochimilco through conservation education and a nature tourism initiative, coupled with work on habitat restoration and bioremediation. A species action plan is in draft. This species is protected under the category Pr (Special Protection) by the Government of Mexico and is in the process of being amended to a higher risk category. The axolotl is currently on Appendix II of CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora), restricting its international trade to protect this species from over-harvesting in the wild, where it has been listed since 1975. However, it is currently under the process of "Periodic Review of Species included in CITES Appendices".A Darwin project was recently completed focusing on the conservation of the axolotl, led by the Durrell Institute of Conservation and Ecology. This was designed to assist Mexico in the development of a sustainable development programme to conserve the axolotl and other endemic fauna and flora of Xochimilco through the promotion of nature tourism.
The axolotl is known only from central Mexico, on the southern edge of Mexico City, in canals and wetlands in the general vicinity of Xochimilco (including outside of Xochimilco proper, around the Chalco wetland). The axolotl lives only in wetlands and canals associated with Lake Xochimilco and Lake Chalco, both adjacent to Mexico City. Xochimilco and Chalco are part of a complex of five lakes, among which the Aztecs built Mexico City and around which the city has since expanded. Axolotls are not regularly distributed throughout their range but instead congregate in places where the habitat is still conducive for the survival and breeding of the axolotl.
Population Estimate
An accurate population estimate is currently unavailable for the species, although the surviving wild population of axolotls is known to be very small despite a large captive population. Although populations are difficult to assess, recent surveys covering almost all of its known distribution range have usually captured less than 100 individuals and during 2002-2003, following more than 1,800 net casts along Xochimilco canals covering 39,173m2, the resultant catch was just 42 individuals. A recent scientific survey in 2004 revealed no axolotls in the species’ range. However, wild-caught animals are still found in local markets, indicating that the local fishermen still know where to find them. There has not been a density study of the Chalco population, but evidence suggests that this population is small and, furthermore, Chalco is a highly unstable system which runs the risk of disappearing in the near future.
Population Trend
Exact population data are currently unavailable for the axolotl, although the population trend is assumed to be in decline in the IUCN Red List of Threatened Species based on notable declines in survey catches performed since 2002 along a 39,173m2 area of the Xochimilco canals.
Status
Listed as Critically Endangered in the IUCN Red List of Threatened Species because its area of occupancy is less than 10 km sq., its distribution is severely fragmented, there is a continuing decline in the extent and quality of its habitat and a sustained decline in the number of mature individuals.
Threats
Axolotl populations are suffering as a result of land drainage and the growth of Mexico City. Various efforts at flood control and sewage disposal starting in the seventeenth century have led to serious damage to the Xochimilco and Chalco lake complex. The digging of wells for the burgeoning population of Mexico City has also caused drying of the valley in which the lakes are located. The largest of these lakes, Texcoco, has been greatly diminished in size, while Lake Chalco has all but disappeared. Xochimilco has likewise suffered a decline in size and water quality. The major threat to the survival of the axolotl is therefore the desiccation, pollution and general degradation of the canal system and lakes in Xochimilco and Chalco, as a result of urbanization. The species is under pressure from traditional harvesting for consumption by local people and axolotls are also captured for medicinal purposes. The harvesting is targeted at animals that are less than one year old, and therefore generally before the individuals have had the opportunity to breed since axolotls reach sexual maturity at approximately one and a half years of age. Formerly, axolotls have also been captured for the international pet trade, although it is thought that no axolotls commercially available today are wild caught since doing so is strictly forbidden. The majority of axolotls currently available on the international market probably originated from captive or laboratory populations.Introduced predatory fish (such as tilapia and carp) have increased to high abundances – a recent study collected 600kg of tilapia in one small channel using a 100m net. These introduced fish species have had a negative impact upon axolotl populations through competition and predation. The species is also being adversely affected by poor water quality, arising from factors such as pollution and disease, probably spread via invasive species. Although the water regime has changed in the last ten 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 to axolotls in the wild.
Conservation Underway
Conservation action to protect axolotl populations in the wild is focusing on raising the profile of Lake Xochimilco through conservation education and a nature tourism initiative, coupled with work on habitat restoration and bioremediation. A species action plan is in draft. This species is protected under the category Pr (Special Protection) by the Government of Mexico and is in the process of being amended to a higher risk category. The axolotl is currently on Appendix II of CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora), restricting its international trade to protect this species from over-harvesting in the wild, where it has been listed since 1975. However, it is currently under the process of "Periodic Review of Species included in CITES Appendices".A Darwin project was recently completed focusing on the conservation of the axolotl, led by the Durrell Institute of Conservation and Ecology. This was designed to assist Mexico in the development of a sustainable development programme to conserve the axolotl and other endemic fauna and flora of Xochimilco through the promotion of nature tourism.
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