Axolotl (Ambystoma mexicanum)
Natural history
Native to Lake Xochimilco, its canal systems, and a few neighboring waterways of Mexico City, the axolotl is a neotenic amphibian species closely related to the tiger salamander (Ambystoma tigrinum). Unlike its relative, the axolotl rarely matures to a terrestrial-adapted animal, instead maintaining many of its larval attributes into adulthood, such as external gills and a paddle-like fin lining the dorsum and tail.
Conservation status
Growth and development of Mexico City has led to a diminishing ecosystem suitable for this species to thrive. Coupled with its appearance in the pet trade and local cuisine, this species has been listed as critically endangered. The majority of specimens found in the pet trade are considered to be captive-raised stock as this species settles well in captivity and is known to breed successfully when given appropriate conditions.
Taxonomy
Class: Amphibia
Order: Caudata
Family: Ambystomatidae (salamanders)
Pronunciation
Just in case you were wondering…
Physical description
| Color | Natural colored specimens are a black to mottled brown color. Selective breeding in captivity has led to a few additional color variations in this species, including melanistic, leucistic, and albino animals. The introduction of transgenesis in this species has also led to a green fluorescent protein (GFP) line of animals, in which a specimen with this protein will produce a green glow over the entirety of its body when exposed to ultraviolet lighting. This change is much more prominent in leucistic and albino animals. | ||
| Size | Axolotls are considered large salamanders with some individuals reportedly reaching total lengths of 30-40 cm (12-15 in), although most specimens reach a total length of 23-25 cm (9-10 in). Females are larger than males and can weigh up to 300 grams. The average female will weigh 180 grams while males on average are 130 grams (Gresens 2004). | ||
| Sexual dimorphism | Mature males will have black nails and a cloacal bulge while mature females will not. Sexual maturity occurs at 12-18 months for females and 10 months for males. | ||
Diet
This species is carnivorous and favors a diet of mollusks, worms, insect larvae, crustaceans, and small fish. Some common food items in captivity include blood worms, blackworms, portions of earthworms (nighcrawlers)*, brine shrimp, tubifex worms, small feeder fish, and salmon pellets. Axolotls should be fed a varied and balanced diet.
Young animals should be offered food daily, while adults can be offered food two to three times per week. Obesity is a problem for this species, therefore attention must be given to the volume and frequency of feedings. Uneaten food should be removed from the aquarium to avoid pollution.
*CAUTION: There is a report describing stomach perforation and morbidity in two tiger salamanders (Ambystoma tigrinum) after consumption of a whole live nightcrawler (Lumbricus terrestris). It is therefore prudent to feed portions rather than whole earthworms to salamanders (Henson-Ramsey 2008).
Husbandry
| Water quality | pH | 7.4-7.6 preferred, 6.5-8.0 acceptable | |
| Chlorine/ chloramine | 0 ppm | ||
| Ammonia | 0 ppm | ||
| Nitrite | 0 ppm | ||
| Nitrate | <10 ppm preferred; prolonged, higher levels can lead to ophthalmic problems | ||
| Temperature | 15.6-17.8°C (60-64°F preferred),
<24°C (75°F) necessary |
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| Aquarium size | Juvenile: 38 L (10 gallons) Mature adult: 78+ L (20+ gallons) Larger aquariums maintain stable water quality parameters longer than smaller aquariums, so bigger is always better |
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| Water flow | Minimal, excessive flow can cause stress in this species | ||
| Filtration | Good mechanical filtration recommended to remove detritus introduced between water changes; use of a spray bar to dissipate current is recommended to prevent stressing animal(s). Visit the client education handout “Care of the Axolotl” for additional information. | ||
| Water changes | 30% of total water volume weekly | ||
| Lighting | Minimal, excessive light can cause stress on this species | ||
| Substrate | Fine aquarium sand: provides a textured surface for the animal to move along or rest while being less likely to cause impactions if accidentally ingested.
If stones are to be used, select large river stones and not pebbles as many axolotls will ingest smaller stones resulting in intestinal blockages. Visit the client education handout “Care of the Axolotl” for additional information. |
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| Social structure | Species-only aquarium; juveniles known for cannibalism; small fish may be consumed, large/aggressive fish may damage external gills; given plenty of space adults may be okay sharing an aquarium | ||
Lifespan
The average lifespan for this species is about 10-15 years, though some individuals have been reported to live up to 20 years. Sexual maturity occurs when the animals reach about 18 cm in total length. Males often achieve maturity sooner than females.
Anatomy/ physiology
| Respiratory |
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| Gastrointestinal | Short, simple carnivore tract. Prey items are prehended by a vacuum affect as the animal rapidly opens its mouth. The teeth are small with a blunt cone shape suited to gripping rather than shredding. | ||
| Cardiac | Three-chambered heart, capable of regeneration to a certain degree. Heart rate is strongly influenced by temperature. | ||
| Musculoskeletal | Appendage regeneration includes the regrowth of functional muscle and skeletal tissue. | ||
| Sexual dimorphism | Mature males will have a swollen appearance surrounding the cloaca while mature females will not. | ||
| Integument | Scaleless with minimal keratinization. Stratum corneum is shed and eaten regularly. Skin is highly vascular and produces significant amount of mucus. | ||
| Vision | Poor | ||
Restraint
For the purpose of a visual examination, the animal and some of its aquarium water should be placed in a small, clear, water-tight container that allows the individual to be viewed from any perspective.
Physical restraint should be avoided whenever possible to prevent damage to the delicate skin and exterior gills of this species, but when necessary a soft mesh net can be used briefly. If physical restraint is required (i.e. for diagnostic testing) the clinician should wear powderless gloves and handle the animal gently as not to disrupt the delicate skin. Low-lint cloths or Daylee® towels can be used to facilitate gentle restraint.
Chemical restraint has been successfully performed with the use of a common fish anesthetic, tricaine methanesulfonate (MS-222) and with the use of alfaxalone in a water bath (Burns 2019, Zullian 2015, McMillan 2011), or isoflurane bubbled into the water.
Venipuncture
Femoral vein
Ventral tail vein, lateral approach
Sedation/anesthesia is recommended to reduce patient stress during this procedure (see restraint above)
Drug administration routes
| Topical | Apply injectable medications to the dorsum and allow transdermal absorption. | ||
| Subcutaneous | Along the body | ||
| Intramuscular | Forelimbs, epaxial musculature | ||
| Intravenous | Femoral veins, branchial veins | ||
| Intracoelomic | Ventral and caudal to the kidneys (See Farkas 2015 for a diagram illustrating the dorsal location of the kidneys). | ||
Important medical conditions
The axolotl has been studied in laboratory settings for years because of its ability to regenerate damaged or lost tissue, including whole limbs, heart tissue, and portions of the central nervous system.
The use of exogenous thyroxine hormone is also known to cause terrestrial metamorphosis in this species.
Many diseases seen in captive axolotls are due to poor husbandry.
| Inappetance | Caused by bacterial/fungal infection, poor water quality/temperature and foreign body ingestion | ||
| Bloating | Caused by bacterial/fungal infection, poor water quality, gastrointestinal disease and liver failure | ||
| Obesity | Overfeeding coupled with slow metabolism and low energy demand puts axolotls at risk for obesity. | ||
| GI obstruction | Gastrointestinal (GI) obstructions caused by ingestion of gravel or other small aquarium decor can lead to serious health problems, including death. Smaller items may occasionally be regurgitated by the animal without intervention or they can be removed using lubricated forceps. Surgery may be required for lower GI impactions. | ||
| Hyperthermia | Water temperature exceeding 24°C (75°F) can lead to hyperthermia. Patients may present with poor to no appetite, ascites, and uncontrollable positive buoyancy.
Secondary bacterial infections are likely to occur in these cases. Aeromonas spp. and Pseudomonas spp. are common in septic cases. |
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| Neoplasia | Melanomas, fibropapillomas and cutaneous haemangiomas common Most are benign |
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| Bacterial infections | Due to poor water quality, overfeeding and stress Aeromonas spp. and Pseudomonas spp are common pathogens May lead to septicemia and death |
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| Fungal infections | Water mold infections are the most common fungal infection and appear as white cotton-like tufts on top of the skin. | ||
| Parasites | Axolotls can acquire enteric parasites from food items or other tank mates. Parasites of concern include Hexamita, Opalina, Trichodina, and Ichthyobodo. | ||
| Trauma | From tank mates or other species Males should not be housed together unless there is ample space to hide |
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| Toxicosis | Tetracyclines are contraindicated in this species (Loh 2015). Other products considered unsafe in axolotls include malachite green and copper-based treatments (Loh 2015). | ||
| Exophthalmia, corneal opacities: | Can develop with prolonged exposure to elevated nitrate (NO3) levels | ||
References and further reading
References
Clare JP. Axolotl Care Sheet. Nov 30, 2011. Reptiles Magazine. Available at https://www.reptilesmagazine.com/axolotl-care-sheet/. Accessed March 9, 2021.
Clare JP. Axolotls. Available at http://www.axolotl.org/. Caudata.org website. Accessed March 9, 2021.
Farkas JE, Monaghan JR. Housing and maintenance of Ambystoma mexicanum, the Mexican axolotl. Methods Mol Biol. 1290:27-46, 2015. doi: 10.1007/978-1-4939-2495-0_3. PMID: 25740475.
Gresens J. An introduction to the Mexican axolotl (Ambystoma mexicanum). Lab Anim (NY). 33(9):41-47, 2004. doi: 10.1038/laban1004-41. PMID: 15457201.
Henson-Ramsey H, Harms H, Stoskopf MK. Stomach perforation in tiger salamanders, (Ambystoma tigrinum), after nightcrawler (Lumbricus terrestvis) consumption. Journal of Herpetological Medicine and Surgery. 18 (3): 127–129, 2008. doi: doi: 10.5818/1529-9651.18.3-4.127.
Loh R. Common disease conditions in axolotls. Proc Annu Conf World Small Animal Veterinary Association World Congress; 2015.
McMillan MW, Leece EA. Immersion and branchial/transcutaneous irrigation anaesthesia with alfaxalone in a Mexican axolotl. Vet Anaesth Analg. 8(6):619-623, 2011. doi: 10.1111/j.1467-2995.2011.00660.x. PMID: 21988819.
Takami Y, Une Y. Blood clinical biochemistries and packed cell volumes for the Mexican axolotl (Ambystoma mexicanum). Journal of Herpetological Medicine and Surgery, 27 (3-4):104-110, 2017. doi: https://doi.org/10.5818/16-10-091.1
Talbot B. The axolotl consult. Proc Annu Conf Association of Avian Veterinarians Australasian Committee – Unusual Pets and Avian Veterinarians; 2018.
Zullian C, Dodelet-Devillers A, Roy S, Vachon P. Evaluation of the anesthetic effects of MS222 in the adult Mexican axolotl (Ambystoma mexicanum). Vet Med (Auckl). 7:1-7, 2016. doi: 10.2147/VMRR.S96761. PMID: 30050832; PMCID: PMC6055765.
Further reading
Baker BB, Meyer DN, Llaniguez JT, et al. Management of multiple protozoan ectoparasites in a research colony of axolotls (Ambystoma mexicanum). J Am Assoc Lab Anim Sci. 58(4):479-484, 2019. doi: 10.30802/AALAS-JAALAS-18-000111. Epub 2019 May 6. PMID: 31060645; PMCID: PMC6643089.
Burns PM, Langlois I, Dunn M. Endoscopic removal of a foreign body in a Mexican axolotl (Ambystoma mexicanum) with the use of MS222-induced immobilization. J Zoo Wildl Med. 50(1):282-286, 2019. doi: 10.1638/2012-0118. PMID: 31120693.
Del Valle JM, Eisthen HL. Treatment of chytridiomycosis in laboratory axolotls (Ambystoma mexicanum) and rough-skinned newts (Taricha granulosa). Comp Med. 69(3):204-211, 2019. doi: 10.30802/AALAS-CM-18-000090. Epub 2019 May 29. PMID: 31142399; PMCID: PMC6591673.
Del-Pozo J, Girling S, Pizzi R, Mancinelli E, Else RW. Severe necrotizing myocarditis caused by Serratia marcescens infection in an axolotl (Ambystoma mexicanum). J Comp Pathol. 144(4):334-338, 2011. doi: 10.1016/j.jcpa.2010.11.001. Epub 2010 Dec 17. PMID: 21168146.
Heinz-Taheny KM. Cardiovascular physiology and diseases of amphibians. Vet Clin North Am Exot Anim Pract. 12(1):39-50, v-vi, 2009. doi: 10.1016/j.cvex.2008.08.005. PMID: 19131029.
Latney LV, Klaphake E. Selected emerging diseases of amphibia. Vet Clin North Am Exot Anim Pract. 16(2):283-301, 2013. doi: 10.1016/j.cvex.2013.01.005. Epub 2013 Mar 15. PMID: 23642863.
Menger B, Vogt PM, Jacobsen ID, Allmeling C, Kuhbier JW, Mutschmann F, Reimers K. Resection of a large intra-abdominal tumor in the Mexican axolotl: a case report. Vet Surg. 39(2):232-233, 2010. doi: 10.1111/j.1532-950X.2009.00609.x. PMID: 20210972.
Takami Y, Une Y. Buoyancy disorders in pet axolotls Ambystoma mexicanum: three cases. Dis Aquat Organ. 127(2):157-162, 2018. doi: 10.3354/dao03187. PMID: 29384486.
Takami Y, Une Y. A retrospective study of diseases in Ambystoma mexicanum: a report of 97 cases. J Vet Med Sci. 79(6):1068-1071, 2017. doi: 10.1292/jvms.17-0066. Epub 2017 May 19. PMID: 28529268; PMCID: PMC5487785.
O'Shea R. Basic information sheet: axolotl. March 18, 2021. LafeberVet website. Available at https://lafeber.com/vet/basic-information-sheet-axolotl/