The Lake Patzcuaro salamander, locally known as achoque (Ambystoma dumerilii), is a paedomorphic species of salamander found exclusively in Lake Pátzcuaro, a high-altitude lake in the Mexican state of Michoacán. First described in 1870 by Alfredo Dugès, the species is named in honor of the French herpetologist Auguste Duméril. However, the salamander has been used as a food source and an ingredient in traditional medicines by the Purépecha people since the Pre-Columbian era. Ambystoma dumerilii are neotenic, meaning they retain their larval characteristics throughout their entire life. This results in adults that have long, heavily filamented external gills, gill slits lined with tooth-like gill rakers, and caudal fins. When stressed, Ambystoma dumerilii can undergo an incomplete metamorphosis, though this is process significantly decreases their lifespan and is often fatal.

Due to their similar morphology, taxonomy, and behavior, achoques are often compared to axolotls. While geographically isolated in the wild, achoques and axolotls are capable of mating in captivity. This, along with achoque's similarities to the tiger salamander, has led to the species being classified as a member of the larger Ambystoma tigrinum species group.

Ambystoma dumerilii are listed as critically endangered in the IUCN red list, and in Appendix II CITES due to pollution, overfishing, eutrophication, and invasive species. It is estimated that there are less than 100 individuals left in the wild, and that the species may go extinct in the wild within the next 20 to 30 years. Currently, there are 4 in situ colonies in Mexico and additional colonies abroad maintaining the species' population. The most notable of these is run by Sisters of the Dominican Order, at the , who currently maintain a colony of 300 members, the largest known population of the species in the world.

Taxonomy and evolution Ambystoma dumerilii is a species of mole salamander located in the suborder Salamandroidea, also known as advanced salamanders. The species was first described in 1870 by Alfredo Dugès, who described the species as Siredon dumerilii. The species epithet "dumerilii" is in honor of the French herpetologist Auguste Duméril. In 1889, Edward Drinker Cope reclassified the species as Amblystoma dumerili, with the classification being upheld in 1930 by Karl Lafrentz. Emmett Reid Dunn renamed the species Bathysiredon dumerilii in 1939, and Hobart Muir Smith employed the name Siredon dumerilii that same year. Between 1943 and 1948, Hobart M Smith, Walter Necker, and Edward Harrison Taylor returned the species to Bathysiredon dumerilii, but incorrectly claimed the species had gone extinct. Also, in 1948, Manuel Maldonado Koerdell described the subspecies Bathysiredon dumerilii queretarensis and Bathysiredon dumerilii dumerilii. A 2016 review of this classification concluded that the holotype specimen had been lost, making confirmation of Koerdell's discovery impossible. Previous assessments of the preserved specimen prior to it being lost concluded that the subspecies was not taxonomically valid.Flores-Villela, O., C. A. Ríos-Muñoz, G. E. Magaña-Cota, and N. L. Quezadas-Tapia. 2016. Alfredo Dugès' type specimens of amphibians and reptiles revisited. Zootaxa 4092: 33–54.

In 1958, Joseph Anton Tihen classified the species as Ambystoma (Bathysiredon) dumerilii. In 1976, Hobart M Smith, and Rozella B. Smith reclassified the species again as Ambystoma queretarense but this was rejected in 1992 by Southern Illinois University herpetologist Ronald A. Brandon. In 1984, H. Bradley Shaffer conducted breeding experiments and genetic distance reviews of the Ambystoma genus, identifying Ambystoma dumerilii as part of the larger Ambystoma tigrinum species group. In 2012, the species was lastly reclassified to its current name Ambystoma (Heterotriton) dumerilii by Alain Dubois and Jean Raffaëlli.

The following cladogram shows the phylogenetic position of Ambystoma dumerilii among select members of the genus Ambystoma, based upon genetic distance comparisons of 32 loci via gel electrophoresis:

Ambystoma dumerilii are colloquially referred to as the Lake Patzcuaro Salamander, Dumeril's salamander, and are locally called achoques. The vernacular name achoque is derived from the Purépecha word "achójki". The term is synonymous with the Nahuatl word "axolotl", or the Spanish "Ajolote", which is broadly applied to all aquatic forms of branchiate salamanders. The precise meaning of "achójki" is not known, but it might mean mud.

Evolution Ambystoma dumerilii evolved approximately 10 to 12 million years ago during the Neogene era. The Ambystoma genus diverged following the uplifting of the Trans-Mexican Volcanic Belt where the Mexican Plateau split from the Sierra Madre, isolating the ancestral species. From there, species along the Mexican Plateau split into 3 distinct lineages based upon the plateau's drainage patterns. During that same time, the Lerma River's drainage system was disrupted by volcanic activity, separating Lake Patzcuaro and Lake Texcoco, and isolating the ancestral species of Ambystoma dumerilii and axolotls into their respective environments. The maintenance of the Ambystoma tigrinum species group might be facilitated by changes in groundwater levels that periodically connect the geographically isolated members of this species group, preventing complete speciation.

Description

Ambystoma dumerilii are large, ectothermic salamanders; adult specimen have a snout to vent length of and a size range of to . Males and females are identical lengths. Some adults have been observed growing over in length; this places achoques as one of the largest species in their genus. The average adult weight is and females typically weigh more than males. The salamander's skin has a granular to smooth texture, with small dome-shaped protuberances along the head. Their dorsum can range from tan brown to green, sometimes with dark brown spots, while their undersides are a pale grey. Their toes, rostrum of the head, cloaca, and underside of the tail tend to have a darker coloring compared to the body. Rarely, the dorsum can be darker in males, and the fimbriae of the gills are most often red colored. Their dorsum color has been described by Chester Zoo curator Gerardo Garcia as "the color of dijon mustard" and the salamanders "resemble miniature versions of the flying dragon-dog Falkor in The Neverending Story".

Ambystoma dumerilii's head and mouth are flat and wide, and a caudal fin runs from the third gill arch to the end of the tail. This species also retains its gill rakers, and possess large, perennibranchiate (lifelong), hyperfilamentous gills that frame their head. The eyes are small and located near the snout. Their bodies are widest along the thorax and begin to taper following the hind legs into the tail. The legs are short and stocky, and their webbed toes end with four digits. Ambystoma dumerilii is the only member of their genus to retain webbed feet on all 4 limbs into adulthood. Morphologically, they are most similar to Anderson's salamander, with both species sharing a broad head and equal gill raker numbers. Achoques are also compared to the axolotl and Taylor's salamander since all three species share a similar geographical location, taxonomic ranking, and have broadly similar morphological features. Compared to most Ambystoma species, their heads and rostrum are particularly flat. Additionally, their gills have a larger number of fimbriae but fewer gill rakers.

Life cycle Mating occurs in winter and early spring. Unlike other species of tropical salamanders who continually produce gametes, Ambystoma dumerilii spermatogenesis follows a yearly cycle. Males initiate courtship by arching the base of their tails and swimming around the female; they lift their tails further to present their cloaca. If the female is receptive, she will begin to follow the male to a breeding site, where males release up to 16 spermatophores. Oviposition takes approximately 24 to 72 hours to complete, and individual eggs are attached to vegetation or rocks. Females can lay over 1000 eggs in a single clutch and eggs can measure up to in diameter. Ova begin maturation during the spring's rainy season and spawning occurs in response to increasing seasonal temperatures.

The life stages of Ambystoma dumerilii are largely defined by overall body length and the loss of some, but not all, larval features. Newly hatched salamanders measure approximately in length.They lack ectodermal projections (balancers) and gill fimbriae, are a brown colour, and have developing front limbs. When larvae reach a snout to vent length, they have all limbs developed, hyperfilamentous gills, and have begun developing dark pigmentation. They largely retain their current features into maturity. Stressors in the environment can induce morphological plasticity in larvae, which determines the length, width, and degree of tail development. When raised in captivity or under stressful conditions, specimen are generally smaller and have decreased swimming capabilities.

Sexual maturity is reached around 18 to 24 months of age, though some specimen can achieve this as early as 8 to 12 months. A snout to vent length of marks the beginning of sexual maturation while a snout to vent length of is considered sexually mature. Males tend to mature faster than females and can be identified by the swelling in their cloaca during breeding season. Females may be capable of reproducing earlier than this, but most of the resultant eggs would not be viable. The life expectancy of this species is unknown but captive specimen can survive up to 16 years.

Neoteny

This species is neotenic (also known as paedomorphism), meaning that they retain larval stage characteristics. This mainly includes their external, heavily vascularized gills, rather than losing them in adulthood like most salamander species. While many Ambystoma salamanders are capable of performing facilitative neoteny (entering metamorphosis only under favorable environmental conditions), Ambystoma dumerilii is only one of five Ambystoma salamanders that are found in a permanent, natural state of paedomorphism. As a result, while the species possesses functional, terrestrial lungs, most gas exchange occurs via their gills and skin.

Metamorphosis

A. dumerilii has not been observed metamorphosing in the wild, but can undergo metamorphosis in captivity. Approximately, one third of both wild caught and laboratory raised specimen undergo metamorphosis in a laboratory setting, and spontaneous metamorphoses often occur during mating season possibly due to hormonal factors. The metamorphosis is largely incomplete and can take up to 3 years. Incorrect husbandry practices are a source of stress in achoques in captivity and may be another driving factor in spontaneous metamorphosis. Metamorphosed specimen do not undergo periodic molting, but rather continuously shed the stratum corneum. Unlike normal amphibians post-transformation, the skin does not have Leydig nor mucosal cells.

Induction of metamorphosis via thyroxine shortens the lifespan of the salamander to 48 days from the start of metamorphosis, while non-induced spontaneous metamorphosis shortens lifespan to 5 months. These specimen will refuse to eat and require assisted feeding to survive. Hybrids between A. dumerilii and its relatives, often A. mexicanum (the axolotl) and A. tigrinum (the tiger salamander), are also prone to spontaneous metamorphosis but show a stronger tolerance to survival. This is because Ambystoma dumerilii lacks the genes necessary to carry out metamorphosis to completion, often leading to detrimental phenotypes, bodily asymmetry, and ultimately death.

Hybridization