AXOLOTL / MEXICAN WALKING FISH
Scientific name: Ambystoma mexicanum
Although also commonly known in Australia and New Zealand as a ‘Mexican walking fish’, in reality the axolotl (Ambystoma mexicanum) is not a fish at all, but a form of salamander - a type of amphibian. Axolotls are, however, native to Mexico, and their name is taken from the Aztec nahuatl, meaning ‘water dog’. Axolotls once constituted an important part of the Aztec diet and were commonly sold as a food item in local markets. They originate from two high altitude lakes which, due to the encroaching urbanisation of Mexico City, now exist only as a series of polluted canals. Sadly, the axolotl is close to extinction in the wild due to loss of habitat, pollution, and competition through the introduction of invasive and destructive species of fish, such as African tilapia and Asian carp, and the wild population is listed as being critically endangered.
Fortunately, in addition to its popularity in the worldwide pet trade, large numbers of axolotls are also bred for scientific research. They are often employed as a ‘model organism’ since they possess a large and easily manipulated embryo, the development of which is clearly visible. Considerable research has focussed on the axolotl’s remarkable healing abilities: it is capable of regenerating entire limbs as well as the tail and parts of the brain, eyes and heart! Axolotls are also unusual in that they exhibit neoteny, which means that unlike most other salamanders they do not undergo metamorphosis into a terrestrial adult but remain trapped in the larval form (similar to a tadpole), in which they have developed the ability to reproduce. Neoteny is more prevalent amongst larger salamanders from montane environments in which there may be little advantage to be gained by leaving the water. Axolotls are invariably neotenic, as this trait is now locked into their genetics. They may, however, be artificially induced to morph into the adult form through injections of iodine or thyroid hormones. This process is sometimes used in the aquarium industry, but morphed individuals can exhibit reduced vigour and longevity. Morphed adult axolotls closely resemble others in the tiger salamander (Ambystoma tigrinum) species complex. In their native habitat, axolotls live alongside the very similar Mexican tiger salamander (A. velasci) and it is likely that reports of spontaneously morphed axolotls may be confused with the latter.
Axolotls prefer somewhat alkaline, moderately hard water. Ideal parameters are as follows:
GH: 7-11o (or 125-200ppm) This helps prevent disease.
KH: 3-8o (or 50-140ppm)
Regular water changes will be required to restrict nitrate concentrations to a maximum of around 40ppm, and regular testing of nitrate levels will enable you to develop a feel for this process - start by changing about 25% of the water every week. Whenever you add tap water to your aquarium, a de-chlorinator must be used to remove harmful chlorine and chloramines. Remember that malachite green, a common ingredient of some aquarium medications, is highly toxic to amphibians. Similarly, you should avoid medications that contain heavy metals, such as copper.
Axolotls are carnivorous, and their natural diet includes insects, molluscs, worms and fish. In captivity they will take a variety of foods including pellets and earthworms. Feeder fish are not a good alternative due to axolotls’ susceptibility
to fish parasites. Frozen or live bloodworms are a good option for very small axolotls (which should preferably be kept in a bare-bottomed tank), while frozen fish and prawns are ideal for larger specimens, which can be fed individually.
The thawed food item can be waved in front of the axolotl using tweezers; uneaten food should not be left in the tank as this will unnecessarily foul the water. Larger axolotls will usually eat once every two or three days. Try to avoid foods that are high in oil and fat (like tubifex and white worms).
Axolotls breed in early spring, fully grown females can lay up to 1000 eggs usually on aquatic plants or on other submerged objects. The eggs hatch after 2-3 weeks, the larvae initially accept small foods such as Artemia nauplii, small Daphnia and Micro worms, eventually graduating to live black worm. The growth rate is usually irregular, quicker growing specimens need to be regularly sorted out to prevent cannibalism.
Axolotls become sexually mature at the age of about 18-24 months, and practice a primitive form of internal fertilisation, in which females pick up ‘packets’ of sperm in their cloaca. They will grow to a length of 25-30 centimetres, with a maximum life expectancy of about 15 years. The wild type, also described as ‘ólive’, varies in shades of brownish grey, with an overlay of shiny speckles. Part of the popularity of the species in captivity is no doubt derived from the variety of attractive colour forms now readily available, including jet-black melanistic specimens, yellow and white albinos, pinkish-white leucistic animals (with dark eyes), and the ‘çopper’ form, which is perhaps best described as a less extreme type of albinism. Some rare morphs command prices in excess of $1,000 overseas. Axolotl colour genetics is yet another fascinating aspect of the biology of these unusual creatures.
Click on link below for information on filtration
Too much stress is not good for anyone - or anything! Under appropriate conditions, axolotls are hardy, long-lived and easy to manage, however, there are two important causes of stress that need to be recognised. Firstly, axolotls do not like a strong water flow, and this is a common cause of stress-related disease. But a biological filter capable of detoxifying ammonia is an important addition to your tank; a common rule of thumb is that you should select a filter that turns over roughly five times the volume of water in your tank per hour. Here are some tips that will help you avoid a concentrated water flow from your filter output.
- If possible, use a spray bar (provided with most canister filters and some internal filters) to diffuse filter output.
- Aim the output nozzle at the glass, or at tank furnishings, or even upwards to reduce flow in the tank.
In captivity, an aquarium with minimum volume of around 130 litres is required due to the axolotl’s size and because it produces a large amount of waste. A light is not necessary, since axolotls are somewhat photophobic and highly sensitive to excess UVB radiation, and for this reason the provision of suitable hiding places are a good idea. Axolotls will quickly uproot plants and other decorations, but a bunch of elodea is also beneficial, as this tough water plant can be left to float on the surface (where axolotls like to sit in it) and will grow without additional lighting. A cover glass is sometimes recommended to prevent your axolotls from jumping out of the tank, but this is a very rare occurrence.
For many years, axolotl owners have been consistently cautioned against the use of aquarium gravel as a substrate since it may result in fatal gastrointestinal blockages if ingested by your pets. Larger stones (with a diameter of at least 3cm), sand and small rounded gravel (2mm or less) are considered to be safe alternatives. However, it should also be noted that researchers from the University of Manitoba found that axolotls in their large colony routinely and deliberately consumed gravel with no ill effect, and they could find no evidence of this practice proving fatal. On the contrary, experimentation indicated that the axolotls were using gravel particles as ‘gastroliths’, or rocks ingested intentionally by animals that lack a swim bladder to provide stability, or a weighted keel while swimming. Baby axolotls which had access to gastroliths swam faster and straighter than those that did not!
Not withstanding the debate concerning appropriate substrates, there is a general consensus that tankmates, with the exception of similarly sized axolotls, should be avoided. Axolotls are extremely sensitive to skin parasites carried by fish. Furthermore, fish will nip at the axolotl’s feathery gills, and in turn will probably end up in the axolotl’s stomach. Baby axolotls may be cannibalistic, and adults will also predate upon other specimens that are significantly smaller in size.
- Similarly, use tank furnishings to insulate your axolotls from strong flow.
The other major stressor of axolotls, especially in the Melbourne summer, is heat. In their natural environment, the water temperature may fall to 6oC (perhaps even lower), and rarely exceeds 20oC. Optimum temperature is about 16-18oC; if the water is much colder the axolotl’s metabolism will slow and its appetite will decrease, although this is unlikely to cause long term health problems. More importantly, prolonged (more than a day or two) exposure to temperatures more than around 24oC will cause heat stress. The first indication of this condition is again loss of appetite, together with the development of pale patches of mucus on the skin. Since heat stress is commonly fatal to axolotls, it is best avoided at all costs. Most homes are well insulated and fitted with air conditioning or other cooling devices, so external temperatures are not indicative of those experienced indoors, but choose an appropriate site for your axolotl tank where it is not subject to strong sunlight. Sometimes you will be away from home on hot days, with the air conditioning off. For such occasions, keep a plastic bottle filled with water in your freezer. Simply float the bottle in your axolotl tank to prevent the water becoming too warm. You can monitor the process with a thermometer; rapid fluctuations in overall temperature should also be avoided as they too can cause stress.
Cycling a New Tank.
For any new aquarium, there is a maturation period during which bacteria populates the filter, allowing it to process waste. It is essential that there are fish (or other aquatic animals) in the tank during this phase, since waste is a necessary prerequisite, however any aquarium occupants are also at risk of being exposed to a dangerous ammonia spike. Therefore, you should be careful to limit the number of fish in your tank, and get your water tested on a regular basis to pre-empt any issues that may develop. It may be prudent to cycle your tank with feeder goldfish, which can be swapped out for axolotl(s) once your filter has cycled. Seeding your tank with the regular addition of beneficial bacteria (which can be purchased in store) will dramatically accelerate the cycling process, typically to around four to six weeks.