Mexican axolotls have become an iconic figure for many aquarium enthusiasts due to their unique beauty, encouraging the breeding of this salamander.
The Mexican axolotl, Ambystoma mexicanum, is strongly associated with Mexican culture as it is linked to the deity Xólotl, the brother of the god Quetzalcóatl (Aguilar and Luría, 2016). It was called “axolotl” (water monster) by the ancient Mexicans.
Furthermore, the axolotl also served as a protein source for the inhabitants of the Valley of Mexico.
Today, the Mexican salamander is used as an animal model for tissue therapy research due to its regenerative capacity, allowing it to reconstruct various organs at any stage of its life (Antonio et al., 2021). Additionally, it is studied in the fields of development, electrophysiology, and regeneration (Randal et al., 2009).
Discover everything about the axolotl, an amphibian endemic to Mexico with incredible regeneration capabilities. Learn about its habitat, characteristics, conservation status, and why it is an important symbol of Mexican culture.
Characteristics of Axolotl
Axolotls, also known as Mexican walking fish, are a variety of salamander characterized by retaining their larval features and living exclusively in water. They do not undergo metamorphosis, except in cases of stress. The axolotl (Ambystoma mexicanum) is an endemic urodele amphibian found in Xochimilco, Mexico City. It is known for its peculiar appearance, maintaining larval characteristics in its adult stage, such as external gills and a dorsal fin.
Taxonomy of the Axolotl
Ávila et al. (2021) report that Mexico has 18 of the 33 species of salamanders in the Ambystoma genus, distributed from southern Canada to central Mexico, but most of them are in a high-risk category.
In this article, we will focus only on the “Xochimilco axolotl.”
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Amphibia
Order: Caudata
Family: Ambystomatidae
Genus: Ambystoma
Species or Scientific Name: Ambystoma mexicanum
Common Names: Axolotl, Mexican axolotl, Xochimilco axolotl, pink salamander.
The Mexican axolotl or Mexican walking fish, Ambystoma mexicanum, is a salamander; however, it differs from the rest of the salamanders in that it permanently lives in the water.
External Anatomy of Axolotls
The main characteristic of the axolotl is that it resembles a “large tadpole”; the amphibian has three large pairs of gills that look like “feathers,” located on the sides of its neck.
Also, Mexican salamanders have limbs with four fingers on the front limbs and five on the hind limbs.
The axolotl can reach a length of 30 cm; however, the average size is 15 cm. Additionally, the weight of this amphibian varies between 125 and 180 grams. The axolotl’s skin is smooth and covered in mucus. A more detailed description of the external and internal anatomy can be found in the article by Ramírez-Macal et al., (2022).
Basic Colors of Axolotls
The axolotl is typically black or brown with spots, although albino and white varieties are also common in its natural environment. However, experts report five common axolotl colors that have been derived from breeding:
- Pale pink with black eyes, sometimes with a belly tinted blue.
- Pale pink with dark spots.
- Pale pink-white with red eyes (Albino).
- Golden Albino.
- Dark black-blue without golden spots or olive green tone.
Where Does the Axolotl Live?
The axolotl inhabits cold and clean waters with abundant vegetation. It is found exclusively in Xochimilco, a system of canals and wetlands south of Mexico City.
What do Axolotls Eat?
Due to their peculiarities, you may be wondering: What do axolotls eat?
In their natural environment, the diet of axolotls, according to Zambrano et al. (2010), consists of organic matter, roots, seeds, plant leaves, algae, daphnia, rotifers, small crustaceans such as amphipods and isopods, insects, leeches, snails, and small fish; however, Mena and Servín (2014) emphasize that adult axolotls are strict carnivores.
On the other hand, axolotl larvae feed on small crustaceans such as cladocerans (Moina macrocopa, Macrothrix triserialis), ostracods (Heterocypris incongruens), and rotifers.
There are commercial foods available for juvenile and adult axolotls; however, if you find it difficult to obtain some of these products, you can use trout food supplemented with mosquito larvae, Artemia, water fleas, tubifex, earthworms, and guppy fry.
If you use a commercial diet and your axolotls do not consume it, Ocaranza et al. (2021) recommend using chicken oil as an attractant for balanced food for juvenile axolotls, while Montoya-Martínez et al. (2022) recommend the use of fish meal or shrimp meal. Manjarrez-Alcívar et al. (2022) report that artificial diets for axolotls with 45% protein provide the best results in terms of development and survival of the animals.
The feeding frequency of axolotls depends on the developmental stage. Mena and Servín (2014) recommend the following protocol:
- Larvae and juveniles: Once a day.
- Adults: Every three days.
Finally, you should train your Mexican axolotls to feed in the same place in their aquarium.
Axolotls in Danger of Extinction
Ambystoma mexicanum are in “critically endangered” status according to the 2019 International Union for Conservation of Nature Red List, due to pollution in Mexico City, habitat destruction, the introduction of more competitive species in Lake Xochimilco (Webb, 2023), and their consumption as food.
SEMARNAT (2018) reports that out of the 16 endemic species of axolotls, 15 species are listed within the NOM-059-SEMARNAT-2010 in some risk category. Additionally, SEMARNAT leads the “Action Program for the Conservation of Ambystoma spp Species.” Significant progress has been made in the conservation of the “Xochimilco Axolotl” to date; however, it still faces the risk of extinction.
Another threat to axolotls, like many aquatic species, is microplastics. Manríquez-Guzmán et al. (2023) reported that microplastics affect the feeding of Mexican axolotl larvae (A. mexicanum).
How Do Axolotls Reproduce?
The axolotl is a neotenic species, as it can reach sexual maturity while retaining its larval characteristics, in other words, it does not complete the metamorphosis process. The natural habitat’s breeding season is in February.
Figiel (2023) recommends that to optimize reproduction, male and female axolotls should be raised at different water temperatures; the researcher suggests a water temperature of 23°C for females and 19°C for males, aiming to achieve maximum gonadosomatic indices.
Mexican salamanders reach sexual maturity at one year of age, where differences between males and females can be observed. Male axolotls tend to be larger, and they can be distinguished by the shape of the cloaca.
In male axolotls, there is an increase in the size of the cloacal glands, which is noticeable to the naked eye (Mena and Servín, 2014). Ramírez et al. (2022) emphasize that the characteristic sexual dimorphism of the axolotl occurs in the peri-cloacal area, which exhibits hyperplasia in males compared to females.
Reproductive Courtship of Axolotls
Ambystoma mexicanum undergo internal and oviparous sexual reproduction. The male initiates reproduction by nudging the female then releases small gelatinous masses shaped like cones, called spermatophores, containing sperm. The female axolotl picks up one of the sperm packets and fertilizes her eggs internally.
If you are interested in commercially breeding Mexican axolotls, you can store the spermatophores for later use or transport them without much difficulty. Figiel (2020) concluded that it is possible to store A. mexicanum spermatophores at temperatures of 0, 3, and 6°C for 28 days. Additionally, Rivera et al. (2021) propose a protocol for the cryopreservation of axolotl spermatophores.
How do Axolotls Hatch?
Female axolotls lay between 100 to 600 eggs per clutch on any surface, but they prefer plants. Depending on the water temperature, the incubation time can range from 12 to 18 days. According to expert reports, at a water temperature of 20°C, the eggs develop and hatch in approximately 17 days.
A. mexicanum larvae begin feeding on the day after hatching. It is recommended to include hiding spots and shelters in the breeding aquarium to prevent cannibalism.
Axolotl care
To keep an axolotl in captivity, you can set up fish tanks or aquariums. Axolotls are not pets for beginners. If your interest is in breeding Ambystoma mexicanum and you lack the necessary experience, we suggest training with a freshwater tropical fish tank.
Once you have mastered aquarium care and maintenance, especially managing water quality, you can breed axolotls. This amphibian can live up to 15 years.
Breeding Parameters for Axolotls in Aquariums
Table 1 presents a list of water quality parameters that a fish tank must meet to provide optimal conditions for Ambystoma mexicanum breeding.
Table 1. Parameters for the breeding of the Mexican axolotl (Ambystoma mexicanum) in aquariums.
| Parameter | Ideal Value |
|---|---|
| pH | 6.5 – 8 |
| Water Volume | 40 – 50 liters per axolotl |
| General Hardness (GH) | 6 – 16 dh |
| Carbonate Hardness (KH) | 3 – 10 dh |
| Nitrate | < 25 mg/l |
| Nitrites (NO2-) | > 3 mg/l |
| Ammonia | 0 mg/l |
| CO2 Concentration | < 5 mg/l |
| Dissolved Oxygen | > 80% saturation |
| Temperature | 10 – 18°C. Maximum: 22°C |
| Plants | Java fern, Java moss |
For optimal axolotl breeding conditions, Robles et al. (2009) and Sanders (2017) recommend using the solution developed by the German researcher Johannes Holtfreter, which consists of:
- 1 liter of distilled water
- NaCl – non-iodized salt: 3.46 gr
- KCl – potassium chloride: 0.05 gr
- CaCl2 – calcium chloride: 0.1 gr
- NaHCO3 – sodium bicarbonate: 0.2 gr
- pH: 7.4
For the setup of axolotl tanks, experts recommend using substrates such as gravel and rocks; aquatic plants and hiding spots can also be added (Mena and Servín, 2014).
Ayala et al. (2019) studied the hourly distribution and movement patterns of axolotls in their natural environment and determined that these patterns were related to vegetation coverage. In this regard, vegetation serves as a refuge during daylight hours.
Compatibility of Axolotls with Other Species
Axolotls should be kept as the sole species in the tank, and when having more than one, the tank should be spacious.
If small fish are included, axolotls will likely consume them, and if the fish are too large, they might nibble on the amphibian’s gill fringes.
Mexican axolotls (Ambystoma mexicanum) should be kept with other axolotls of similar size to reduce the possibility of limb biting; with a significant size difference, axolotls may become cannibalistic.
Metamorphosis in Axolotls
Axolotls can occasionally undergo metamorphosis; however, it is a stressful process and not desired because it signifies death for the amphibian.
Scientific reports suggest that when Ambystoma mexicanum are subjected to abrupt temperature changes or water pollution, they may initiate the metamorphic process. Therefore, careful attention to water quality parameters in the aquarium is crucial.
Regeneration of Axolotls
The most remarkable feature of Ambystoma mexicanum is their regenerative ability; the Mexican salamander does not scar and can fully regenerate lost limbs over months.
The Mexican axolotl is one of the few tetrapod species capable of regenerating complete skeletal elements in injured adult limbs (McCusker et al., 2016), tail, and gills (Makanae et al., 2020).
These characteristics have made axolotls a model organism extensively used in research laboratories for tissue and organ regeneration, among other studies.
Tips for Buying a Mexican Axolotl
Mena and Servín (2014) provide a series of tips for a good selection when purchasing an axolotl:
- The coloration and texture of the axolotl’s skin should show continuity.
- There should be no injuries, crusts, spots, or bleeding areas on Mexican salamander specimens.
- The axolotl should have all four complete limbs, with four fingers on the front limbs and five fingers on the hind limbs.
- The axolotl’s tail should be complete, well-developed, and tapering to a pointed tip.
- The amphibian’s gills may vary in size, but they should be three well-implanted branches on both sides of the head, clean, and without the presence of cottony spots or crusts.
- Ask the seller about the age of the chosen specimen, its gender, what it is eating, and how often.
Diseases in Mexican Axolotls
Axolotls’ skin is prone to scratches; therefore, ensure that all aquarium decorations have no sharp edges, and remove anything that hinders the amphibian’s mobility.
Moreover, amphibians are highly susceptible to various skin diseases. The most common cause of skin injuries and blisters is poor water quality (Sanders, 2017).
An ill Ambystoma mexicanum will exhibit changes in behavior, abnormal swimming, tail arching, the presence of masses on the skin, color change, retained shedding, loss of appetite, or inconsistency in feces (Mena and Servín, 2014).
Chytridiomycosis
The Mexican axolotl has been heavily affected by the disease Chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd).
Rebollar (?) indicates that B. dendrobatidis infection causes lesions on the skin of the Mexican axolotl, disrupting its osmotic balance and gas transfer.
Michaels et al. (2018) recommend treating the infection with a liquid preparation (0.01% and 0.005%) of itraconazole (Itrafungol) for daily baths lasting between 7 to 15 minutes for seven days.
Saprolegnia
Saprolegnia spp. is a common pathogenic oomycete in aquatic environments. It is characterized by cottony growth on the skin and gills of affected axolotls, causing lethargy, anorexia, respiratory distress, weight loss, and even mortality (Mena and Servín, 2014).
Mena and Servín (2014) recommend treatment with the application of itraconazole in baths of 0.01% in a 0.6% saline solution.
Abdominal Air Accumulation Syndrome
According to Sanders (2017), axolotls are prone to accumulating air in the abdomen. The air leads to a distended abdomen and floating upside down. According to the expert, this occurs because their immature intestine adapts to a protein-rich diet.
Abdominal air accumulation syndrome will correct itself over time as the animal matures, but reducing the size of the food portion can provide an immediate solution.
Takami and Une (2018) also highlight that lung injuries are a significant cause of buoyancy disorders due to air infiltration into the body cavity of axolotls.
Hydrocoelom (Hydrocoel)
Hydrocoelom is defined as cellular distension with fluid and is a common disease in amphibians.
Clancy et al. (2015) report that female amphibians experience a hydrocoelom rate three times higher than males. Takami and Une (2017) reported that in a Japanese aquarium store, hydrocoelom was the most prevalent disease in Mexican salamanders.
Based on the research findings of Clancy et al. (2015), treatment of hydrocoelom in amphibians is recommended with enrofloxacin (10 mg/kg in 24 hours).
Conclusions
The Mexican axolotl is a species at risk; however, its unique beauty and cultural significance have spurred interest in its conservation and the development of breeding techniques.
Ambystoma mexicanum is the vertebrate model system with the greatest regenerative capacity (Caballero et al., 2018), making the axolotl a model in many research laboratories; protocols for its breeding have been developed.
Currently, axolotls are highly sought after by the ornamental industry; in this regard, axolotl aquaculture should be encouraged to meet the growing global demand for this amphibian.
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