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This article takes a deep dive into a true oddity of nature, the Mexican Axolotl (Ambystoma mexicanum). First we will look into their history and biology, followed with how to care for them at home.
What is an Axolotl?
Axolotls are a fully aquatic salamander species within the class of amphibians. Axolotl is pronounced ACT-suh-LAH-tuhl. In the wild, they are 
found only in a small canal system in Xochimilco (pronounced SO-chee-MILL-koh) in Mexico City, Mexico. These canals are famous for being created by the Aztecs and for boat parties, unique enough to be recognised as a UNESCO World Heritage Site. Unfortunately for axolotls, due to urbanisation, pollution and the introduction of invasive fish species (as a food source), they are now listed as a critically endangered species. There are many more axolotls in captivity than there are left in the wild. This is due to their popularity in the pet trade, and for unique scientific research. Conservation efforts are underway to try and repopulate their native habitat. So far these are limited to small artificial waterways, disconnected to the main canals. Until the causes of their decimation in the wild are resolved, this is likely to be a slow process.
Unlike other species in the genus Ambystoma, the axolotl does not complete metamorphosis in to terrestrial adults (like traditional salamanders). They remain in water their entire lives, retaining their larval characteristics (a caudal tail fin and external feather gills). Remaining in a junior, larval-like state for life is called neoteny.
In the wild, axolotls have a dark green / brown colouration with reflective speckles (iridophores). Through selective breeding, there are now a variety of colour options in the domestic market. These include albino (white body with red eyes), leucistic (whitish body with black eyes), golden albino or melanistic (black).
Why are axolotls sometimes called waterdogs?
This question is rather rhetoric, as in the UK we rarely call axolotls waterdogs. However, in the USA the waterdog is a colloquial name for the larval stage of a similar species to the axolotl, the Tiger Salamander (Ambystoma tigrinum). Unlike axolotls, Tiger Salamanders do undergo metamorphosis into terrestrial adults.
When investigating the origin of the word axolotl, an interesting backstory unfolded. The word axolotl derives from the Aztec words for water ‘atl’ and dog ‘xolotl’. Xolotl was an Aztec god often depicted as a dog-headed god. Xolotl was a god of many things (lightening, monsters etc) but was generally portrayed in a negative light. He was the twin of Quetzalcoatl, one of the most important gods in the Aztec pantheon. Perhaps he was considered the evil twin?
The axolotl does not only bear the name of a god, it also plays a role in Xolotl's history. In Aztec mythology, all the gods agreed to sacrifice themselves in order for the newly created ‘Fifth Sun’ to move to help create humanity. Xolotl decided instead to hide and transform himself into three objects, one of these being the axolotl. His deception and trickery did not work, as he was eventually captured and killed.
So the word axolotl translates from Nahuatl (the Aztec language) to water-dog. Named for their mythological ties to the god Xolotl, who became an axolotl to evade capture. But it is their cousins, the Tiger Salamander, who carry the nickname waterdog in North America. Interestingly, it appears the nickname waterdog was coined from the salamanders vocal resemblance to a dogs bark, rather than their cousins mythological history!
What makes the axolotl special?
Axolotls have great scientific and cultural significance. For over a century and a half the axolotl has been studied for its regenerative qualities. The first axolotls exported for scientific research date back to 1863, where 34 animals were sent to Paris. Genetic studies have shown nearly all axolotls in the scientific and pet communities descent from these original 34!
Whilst a few species of amphibians can regenerate cells, the axolotl goes much further. They are able to regenerate whole limbs, spinal chord, skin and other organs, with no evidence of scarring. This make the axolotl a model species and one of high scientific importance for stem-cell research.
Axolotls have also been the subject of transplant rejection research. Studies have shown transplanted organs from another axolotl are not rejected by the host. Usually, the immune system of an individual will recognise the foreign object and mount an immune response. To prevent this, transplant patients usually have to take immunosuppressant drugs to slow this process.
Axolotl DNA has also recently been sequenced, with surprising results. Whilst the human genome contains 3 billion base pairs, axolotl DNA contains 32 billion base pairs. In fact it makes the axolotl genome the largest ever fully sequenced (at time of research, 2018)! It is in fact two species of fish that hold some interesting records in DNA research. The Marbled Lungfish (Protopterus aethiopicus) currently holds the record for the largest known animal genome, at 130 billion base pairs. It is not yet fully sequenced however.
The record for the smallest known genome in a vertebrate animal also belongs to a fish. The Green Spot Pufferfish (Dichotomyctere nigroviridis, formally Tetradon nigroviridis) has only 390 million base pairs of DNA. To put this into perspective, the pufferfish has 1/10th the DNA base pairs of the human. Meanwhile the lungfish has x40 times the DNA base pairs of the human. What that means remains to be figured out by genetic scientists!
The axolotl also has cultural significance in Mexico. Since the Mesoamericas days of the Aztecs axolotls have been associated with god-like importance. The recently announced updated 50 Mexican peso bank note will feature the face of an axolotl and the Xochimilco canal system. This is due to enter circulation in 2022.
Caring for axolotls at home
Axolotls can make a great pet, given the right care and attention. It’s important to research prior to purchase, as they do need a specific setup for long term care.
Temperature
Axolotls, although native to Mexico, are in fact a cold water animal. Their optimal temperature range is 16-18°C. Being a cold blooded animal, they 
cannot regulate their body temperature. They therefore rely on the water temperature to remain within this narrow range. If kept cooler, their metabolism slows and they cannot digest their food. If kept at temperatures too high, their metabolism becomes too fast. This increasing appetite and therefore waste production. Prolonged exposure to high temperatures (above 24°C) can cause stress and will shorten their lifespan.
It is safe then to assume axolotls do not need a heater in their aquarium when keep indoors in the UK. The bigger issue arrises in British summer, when temperatures can climb above that crucial 24°C threshold.
There are a few methods to keep axolotl aquariums cool in summer:
- Turning off the aquarium lights and keep the lids open.
- Purpose made, low-voltage aquarium fans that clip on to the side of the glass can decrease the water temperature by increasing evaporation on the water surface.
- Floating frozen bottles of water on the surface of the water will temporarily decrease water temperature (do not use ice cubes as once melted can affect the water quality).
- For the ultimate cooling (and constant water temperature without fluctuation) consider an aquarium chiller. Whilst these are a costly purchase, they provide a constant cool temperature without interference.
Aquarium size
Axolotls have the potential to reach an adult size of 30cm. In captivity, the average adult size is closer to 20cm, but this still presents quite a large animal to house long term. Axolotls do not possess a swimbladder organ like fish. This means they cannot fully utilise the complete area of an aquarium and spend most of their time on the bottom. Think carefully about the size and shape of the aquarium. It is better to have a tank that is long and wide, rather than tall. The minimum recommended size aquarium for an axolotl is at least 60cm x 30cm wide. Ideally, an aquarium larger than this would be better suited long term, especially if keeping multiple axolotls together. In the wild, axolotls are solitary animals, and so are happy being kept on their own. If multiple axolotls are to be kept together, enough space should be given for each one to have its own area away from others.
Filtration
Axolotls are messy animals, and sensitive to poor water quality. They therefore require excellent filtration to maintain pristine water conditions. A filter rated at x2 aquarium volume would be recommended, unless the water level is not filled to the top of the aquarium. Axolotls do no need particularly deep water, only requiring water as deep as their body length. If not filling the aquarium to its capacity, judge the vo
lume as a percentage of the stated aquarium size when filled. For example a 200 litre aquarium filled half way would only contain 100 litres (minus displacement from substrate and ornaments).
Like fish, axolotls excrete ammonia, predominantly through their gills. This makes them different to other aquatic species that are not fish, which generally excrete urea. To keep water quality in top condition it is recommended to use a filter with mechanical, chemical and biological mediums. Regular monitoring of water parameters using NT Labs Aquarium Lab Test Kit allows the owner to check for harmful substances including ammonia and nitrite so they do not reach dangerous levels.
Decor
Choosing the right decor is important for axolotl health. The substrate at the bottom of the aquarium should either be very fine, or too large for the axolotl to swallow. When feeding, axolotls inhale their food, creating a vacuum. This indirect technique often means unwanted items also get swallowed. Gravel that is to large to pass will cause digestive issues and lead to compaction inside the axolotl. Large pebbles aren’t generally recommended as food and detritus can become easily trapped and lead to water quality issues. Sand is recommended as it provides a good substate that can be passed if accidentally swallowed. Some axolotl keepers do not use a substrate, opting for a bare-bottomed tank. Whilst this is easier to keep clean, it provides the axolotl with little friction when walking on the base.
When choosing decor or ornaments, make sure they are smooth and not rough-edged, so as to not damage their delicate skin. It would also not be recommended to have any ornament which the axolotl can become trapped in. Whilst axolotls do get nearly all their oxygen requirements from the water using their external feather gills, they do also have lungs. Axolotls will sometimes be observed swimming to the surface for a gulp of air. This is perfectly normal behaviour on the odd occasion. If this is observed frequently, it may suggest low oxygen saturation in the water or poor water quality.
Companions
As mentioned before, axolotls are predominately solitary animals. They can be kept together with other axolotls, given enough space. It is not recommended to keep axolotls with any other species, including fish. Fish will more than likely end up as food for the axolotl, given their opportunistic predatory nature. Fish could also agitate the axolotl by biting at their external feather gills or sucking on their sensitive skin.
Feeding axolotls
Axolotls in the wild are opportunistic predators. They do not actively go hunting for their food, but react when something that resembles / smells of food presents itself. As mentioned previously, their hunting technique is vacuum inhalation. In the wild, their diet consists of worms, other invertebrates, crustaceans and fish. They are not particularly fussy, as long as it’s meaty! Being a cool water species, their metabolism is quite slow, and so a meal tends to take a few days to fully digest. This feeding regime should be recreated at home, only feeding the axolotl two to three times a week when an adult. When juvenile, they tend to need feeding more regularly to support their growing phase. Some keepers remove axolotls into separation boxes for feeding, to ensure they can easily find their food, and to minimise mess. There is an argument to be had for the negative stress unnecessarily handling an axolotl can have verses the positive gains from having less food waste in the aquarium. The axolotl will still digest and excrete in the aquarium so the gains on waste reduction are arguably negligible.
What to feed axolotls?
An axolotl should be offered a variety of foods to keep in optimal health, supported with a high quality staple diet. The recently launched NT Labs Pro-f Axolotl Pellet provides a daily diet for axolotls and other aquatic amphibians. It is available in a quick sinking pellet, in two sizes and varieties. The Axolotl Junior Pellet is manufactured in a smaller size for smaller mouths. It contains a higher protein content for the crucial growing phase in early life. The Axolotl Adult Pellet has a reduced protein content to reduce unnecessary waste output in adult axolotls. It does however have an increased vitamin A supplement level to maintain health and prevent disease in older animals. Vitamin A has shown to be important for vision, reproduction and supportive of the immune system in amphibians. The larger size of the Axolotl Adult Pellet makes it easier for adult axolotls to identify and swallow without unnecessary waste.
Axolotls will also appreciate the odd meaty treat. Earthworms, bloodworms and even lance fish will provide variety and recreate natural hunting behaviours. Just remember to remove uneaten food, especially meaty items, as they will quickly pollute the water.
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