The name Ambystoma mexicanum translated to English means “a mexican salamander with a cup shaped mouth”. Amby is translated to “a cup” and stoma is translated to “a mouth”. Mexicanum refers to the fact that the organism lives exclusively in Mexico.
The complete taxonomic classification of the Ambystoma mexicanum would be as follows:
Domain - Eukarya Kingdom - Animalia
Phylum - Chordata Class - Amphibia Order - Caudata Family - Ambystomatidae Genus - Ambystoma Species - Ambystoma mexicanum
We can place the A. mexicanum within the eukarya domain because it is a organism made up of cells with membrane bound DNA (nuclei) which defines the Eukarya domain (Trait 1, Figure 1). They fall under the Animalia phylum because they are multicellular, heterotrophic eukaryotic organisms (Trait 2, Figure 1).
For the phylum we categorized them within the chordata for several characteristics. By simply observing the axolotl we see that it has a level of organization that require true tissues (Trait 3, Figure 1) and that it has bilateral symmetry (Trait 4, Figure 1) which means it cannot be placed within the phylum Porifera or Cnidaria. A look at it’s developmental stage shows characteristics expressed by the A. mexicanum that define the chordata phylum, mainly the notochord, hollow dorsal nerve cord, pharyngeal slits, and a post anal tail (Trait 5, Figure 1). At around the twentieth stage of development (approximately four days from fertilization) the neural plates of the A. mexicanum embryo are fused along their lengths, forming the neural tube that is the precursor to the hollow dorsal nerve cord in the following stages; the notochord is developed alongside the neural tube (Schrekenberg 1974). The post-anal tail seen in juvenile and adult organisms begins development at the twenty-first stage while the pharyngeal slits begin to form around the twenty-fourth developmental stage, starting as a gill bulge, and eventually develop into the axolotls’ elaborate external gill structures (Schrekenberg 1974). These characteristics separate it from the Mollusca, Annelida, Platyhelminthes, Nematoda, and Arthropoda.
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Finally, we can categorize them within the Chordata because of A. mexicanum has an endoskeleton (Trait 6, Figure 1) which separates it from the Echinodermata which display exoskeletons.
Natural/Artificial Habitats and Geography:
As evidenced by its name, the Ambystoma mexicanum is found exclusively in Mexico, in lake Xochimilco near Mexico City, Mexico (Chaparro-Herrea 2013). This is the only location it is found naturally as it is highly endangered despite its relatively prolific numbers in captivity (laboratories and aquariums). The Xochimilco Lake is a freshwater lake more likened to a shallow system of slow moving canals. It once used to be a part of lake systems that took up the whole Mexico Valley, now it is one of very few scattered lakes, broken up by urban development. As it is, even this lake is beginning to dry up and soon may not exist anymore due to human actions. Lake Xochimilco relatively southern in latitude and therefore has stable temperatures with little daily fluctuations, excepting transitions between seasons. The Ambystoma mexicanum are a group of salamanders that do not go through adult metamorphosis and so spend the entirety of their lives within/below water.
The aquarium and laboratory habitats should, and usually do, closely mimic those of the axolotl’s natural habitat in terms of water flow, content, and temperature. A. mexicanum do best when their water temperature remains at a constant 18-20 degrees Celsius, much like their natural tropical home, this temperature is also the best for encouraging axolotls to breed (Khattak 2014). Temperatures below 18 degrees Celsius begin to slow the axolotl’s metabolism and may cause it to refuse food. Temperatures above this range begin to exert stress on the A. mexicanum and can eventually result in disease. The water flow within the tank should also be kept to a restricted minimum as high flow can result in irritation and damage of the axolotls sensitive tissues .
The axolotls are very sensitive organisms and can only handle certain levels of chemicals in their environment. Highly acidic waters are likely to damage the A. mexicanum delicate gills which can lead to other afflictions. Highly basic water is another danger to avoid. The ideal pH range of the water should be from 7.00 to 8.00 (Khattak 2014). Any deviation beyond this should be corrected through extra water changes or cleaning of the filter. As it is, water changes should occur once a week to prevent build up of the nitrogenous wastes that axolotls excrete in high amounts.
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Other chemicals to watch out for are chlorine, nitrite, nitrate, and ammonium. Chlorine is a gas that is highly toxic and reacts violently with respiratory tissue. Water must be dechlorinated before any Ambystoma mexicanum are placed within it otherwise massive gill tissue damage and death may occur. Ammonium, nitrite, and nitrate are toxic compounds built up, unfortunately, naturally in the wastes that the axolotls excrete. Though they are not extremely toxic at small levels it is in the best interests of the mexicanum to filter these out as efficiently and completely as possible. Nitrite levels should never go above zero mg per liter while nitrate should never max above 100 mg per liter (Khattak 2014). Ammonium can be dealt with through various biofilters that convert the compound into something or through aquatic plants which will naturally take it up.
Water hardness is the final characteristic of the A. mexicanum’s liquid environment that should be taken into consideration when setting up an artificial habitat via aquarium or laboratory. Axolotls grow, breed, and thrive best within the conditions of Holtfreter’s solution (Mendoza 2009) which consists of the following salts: 3.46 grams of NaCl, 0.05 grams of KCl, 0.10 grams of CaCl2, and 0.20 grams of NaHCO3 per litre of water (Malacinski 1989).
Though axolotls can live in bare tanks without trouble, the most comforting environment for them would be one where there are natural plants growing in some numbers and there are several places for the organism to hide. (Malacinski 1989).
Axolotl’s are carnivores with relatively large, cup-like mouth with a handful of rudimentary teeth more for gripping that tearing. In the wild, it will eat anything it can fit into its mouth from insects, worms, fish, to even other, smaller, salamanders. In lab cultures, they usually are fed and subsist on brine shrimp although some axolotl culture colonies fed their Ambystoma mexicanum salmon diet pellets. Both options are good staple foods for the axolotl and well balanced. Bloodworms, whiteworms, and Daphnia are also well-balanced staple foods that a hobbyist could provide to the Ambystoma mexicanum for full nutrition as long as they are retrieved from fishless waters so as to avoid parasites.
Axolotls are one of relatively few number of organisms that maintain a great ability to regenerate organs, limbs, and other essential tissues like the spinal cord and cardiac tissues after developing into juveniles (McCusker and Gardner 2011). That is, being ‘born’. The process of regeneration is a complex process that is still not completely understood by the scientific community, nonetheless, there have been proposed models such as the Accessory Limb Model. This model in particular emphasizes that regeneration is a step-wise process that begins with specific signalling from the injury that encourages the healing of the wound. That is, covering it with a new layer of epithelial or skin tissues. From here, the wound area may receive additional signalling from the nerves a blastema will form. From this, a new limb will eventually be grown to replace the amputated one. This process is dependent on signals from the wound epidermis, nerves and dermal fibroblasts of the limb (Satoh et al. 2008).
The blastema is a concentration of nearly pluripotent heterogenous cells with the ability to produce all the necessary cells to replace an amputated limb. The blastema functions as a sort of “limb bud” much like those in embryonic development and is the end result of the dedifferentiation of cells near the edge of the wound. Dedifferentiation is defined as the progress of mature cells returning to an embryonic undifferentiated state. It is thought that dedifferentiating cells (and regeneration overall) are aided by matrix-degrading enzymes, specifically that of 90-kDa (Yang and Bryant 1994). However, despite the fact that these cells are undifferentiated, it seems cells follow a sort of cell memory and only re-differentiate into certain types of cells during regeneration. The dermis cells within the blastema, which make up a large majority of the blastema, can transdifferentiate and become epidermis, connective tissue, cartilage, and tendon (Kragl et al. 2009). Meanwhile, muscle cells within the blastema seems to only become new muscle and cartilage appears to not re-differentiate into other types of tissues either (Kragl et al. 2009).
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Axolotl’s consistently retain the ability to regenerate their organs and limbs throughout their life, though it is noted that the older the organism the longer the regeneration takes to complete (McCusker and Gardiner 2011). However, no official studies have been made so there is no timeline for this degradation in regeneration speed or hypotheses for factors that might cause this decrease in speed.