AXOLOTL: CHARACTERISTICS, HABITAT, LIFE CYCLE, REPRODUCTION - BIOLOGY - 2024

The axolotl (Ambystoma mexicanum) is an endemic amphibian of the channels of Lake Xochimilco, located in the center of Mexico City. Although it belongs to the Ambystomatidae family, metamorphosis does not occur naturally in this species, so it remains in a larval state throughout its life.

Currently the axolotl is critically in danger of extinction, due to a notable decrease in the population in its natural habitat. It is a solitary animal that communicates by visual or chemical signals, almost exclusively during the mating season.

Source: Vassil, from Wikimedia Commons

However, it is not a fish, but a Neotenian salamander. Its neotenic character refers to the fact that it has the ability to reproduce, although as an adult it maintains the aquatic characteristics of its larval stage.

On very few occasions the axolotl can carry out the metamorphosis to the terrestrial phase. Artificially it could be induced in the laboratory, through injections of hormonal chemicals.

Naturally, the metamorphosis would occur only by hybridism or if the environmental conditions were adverse. However, these cases are very sporadic.

Mythological animal

The axolotl is an animal recognized as iconic of Mexico. In Aztec mythology, this animal is the aquatic invocation of the god Xólotl.

According to Aztec culture, to set the Fifth Sun in motion, all the gods had to be sacrificed. Xólotl hid himself, turning into a corn plant, when he saw himself discovered he hid, taking the shape of a mussel.

Again he was found by the executioner and had to flee to the water, where he transformed into an animal called an axolotl. He was finally caught and died. For this reason, the axolotl was one of the favorite delicacies of Aztec royalty.

Investigation

In today's scientific world, the axolotl is used as a model organism in various investigations. One reason is that this species is relatively easy to breed in captivity.

In addition, because the embryo is large and the egg is almost translucent, it allows visualizing development in its different stages. The ability to regenerate is a major draw to experiment in that area of ​​study.

Research is currently underway on heart defects. This is because in the axolotl there is a mutant gene that causes heart failure in embryos.

It is also a model in studies of the closure of the neural tube, since there is a great similarity between the neural plate of the axolotl and that of the human.

Regeneration

Humans and other vertebrate mammals are severely limited in their natural abilities to regenerate some parts of the body that they have lost.

In contrast, Ambystoma mexicanum does not heal its wounds by scarring, it does so by regenerating lost appendages or some vital structures, including specific areas of the brain. There are cases where the axolotl, in addition to repairing the injured limb, can regenerate an additional one.

Considering that several structures and systems of the axolotl have an anatomy similar to humans, managing the information regarding how the regeneration process occurs in this animal would yield important data for medicine.

However, these studies are limited by the difficulty of working at the molecular level with this species. The genomes are large, which has prevented it from being fully sequenced.

Currently this difficulty is being solved by working with the information contained in the mRNA. These data allow discovering the mechanisms that occur at the molecular level in regenerative biological processes.

General characteristics

Tail

This specimen has a tail that is characterized by being flattened laterally. It is of great length, its length being equal to half its body.

Spine

Axolotls have a skeleton that is not completely ossified. This can be seen in the branchial area, which is mainly made up of cartilage.

The spinal column is very poorly differentiated. However, the following regions can be distinguished: cervical, thoracic, caudal sacral, sacral and caudal.

In total it has 50 vertebrae, a figure that could vary considering that the tail can have between 30 and 35 vertebrae. They have rudimentary ribs that run along the body.

Skin

The skin is made up of the epidermis, dermis, cilia, papillae, and glandular tissue. Its function is to protect the animal from variations in the environment and to defend it from infections caused by some microbes.

In addition to this, it contributes to the regulation of body water levels and the elimination of waste substances. Unlike salamanders, axolotls do not shed their skin.

Axolotls have 4 genes related to skin pigmentation. When mutations occur, various shades are created that pigment the skin.

The natural coloration of the skin is characterized by a dark background, generally brownish green, with spots in olive, yellow, orange or cream tones. These are distributed dorsally, and a clear line can be formed on each side.

The four mutant tones are leucistic, in a light pink tone with black, albino eyes, where the skin and eyes are golden, axanthic, a gray body and black and melanoid eyes, a completely black skin, without specks.

In addition, this species has a limited capacity to alter the color of its skin, thus being able to camouflage itself in the environment in which it is found.

Fin

Ambystoma mexicanum has a tail fin that extends from the back of the head to its tail end, where it becomes a tail.

Head

Its head is wide and is separated from the trunk in the lower part. Their eyes are located on both sides of the head, they are small in size and they do not have eyelids. Their field of vision is not wide, therefore they depend on the senses of touch and smell to hunt.

In the mouth they have vestigial teeth, which are hardly visible. They also have a pair of nostrils, as they can breathe with their lungs.

Extremities

The axolotl has short and underdeveloped limbs. The front legs have 4 toes while the hind legs have 5.

Sewer

Males are easy to identify, since their cloaca is swollen, because it is full of papillae. Females do not have developed cloacal glands.

Gills

A particular characteristic of this species is its external gills, which it uses to breathe when in water. This organ consists of three pairs of stems that arise from the back of the head.

These branchial branches are covered with filaments, which increase the surface where gas exchange takes place.

Pulmonary sacs

These sacs have not developed as lungs. However, they are used to breathe on the few occasions when they surface for air.

Taxonomy

Animal Kingdom.

Subkingdom Bilateria.

Infra-kingdom Deuterostomy.

Chordate Phylum.

Vertebrate Subfilum.

Tetrapoda superclass.

Amphibian class.

Order Caudata.

Family Ambystomatidae

In most of the members of this family, terrestrial adults undergo metamorphosis. Their bodies and legs are elongated, while their head is short and rounded. They usually live under leaves or in burrows, returning to the pond to breed.

An exception to this is the Ambystoma mexicanum species, which maintain their larval state even as adults, since metamorphosis does not occur in them. Because of this, its life is spent mostly in water.

Genus Ambystoma

Species belonging to this genus usually have gills and reproduce in water, where they lay their eggs in visible groups. These are clear and float, so each stage of their development can be clearly observed.

The best known species are Ambystoma mexicanum and Ambystoma tigrinum.

Species Ambystoma mexicanum

Danger of extinction

The axolotl is currently categorized as a critically endangered specimen by the International Union for Conservation of Nature. The populations that inhabit freely are very few.

In 1998 there were about 6,000 specimens per square kilometer and by 2014 there were only 36 axolotls per km2.

Causes

There are several factors that are directly related to the decline in the population of this species. Among them are:

-The contamination and drying of the lakes and canals. This is the result of the modification that the environment has suffered, due to the creation of urban planning around these bodies of water. Another aspect that aggravates the situation is that large amounts of chemical substances are discharged into the waters, which alter the ecosystem.

-The capture of the axolotl to be used for medicinal and scientific purposes. In traditional medicine, axolotl syrups are made, used in the treatment of respiratory diseases.

In addition to this, its meat has a high nutritional level, which is why it is consumed locally and regionally. Also the axolotl is captured and sold as a pet.

-The introduction of exotic fish species such as carp and tilapia. These fish have increased their population, competing with the axolotl for food. In addition, these fish are natural predators of Ambystoma mexicanum.

-In a high percentage, young species constitute the population that is captured or predated. Consequently, the reproduction of the species is affected.

Conservation strategies

All actions revolve around the environmental control of Lake Xochimilco. These include the implementation of projects aimed at bioremediation and habitat restoration.

In 1989 the "Xochimilco Ecological Rescue Plan" was carried out, which includes a project for the conservation of this Mexican species.

In addition, some international governments, such as the United Kingdom, are supporting various projects, such as the "National Action Plan for the Management and Conservation of the Axolotl in Xochimilco."

Currently, a group of experts is proposing the creation of the «Refugio chinampa», on Lake Xochimilco. The intention is to eliminate the use of pesticides and chemical fertilizers in the fields near the lake. Added to this, it would be a refuge area for the axolotl.

Distribution and habitat

The axolotl is an endemic species that currently inhabits the channels of Lake Xochimilco, in Mexico. In times past it was also in Lake Chalco, which was artificially drained to prevent flooding. This resulted in the disappearance of the axolotl from that habitat.

Lake Xochimilco is located 2,220 meters above sea level. The current situation of this is the consequence of the management that for decades has had the soil that borders this important Mexican natural resource.

It has 207 kilometers of canals, in addition to eight small lakes and two seasonal wetlands. At the beginning of the 20th century, this system was fed by several springs, but today wastewater is drained into the lake, some treated and others not.

During the rainy season, which occurs between the months of June to October, rainfall also contributes to feeding this lake.

From the south to the north, this body of water has a water current that moves at 4m / h. To the south there are some natural springs and to the north is the place where the wastewater comes out.

Recent studies

Studies have been carried out to know the local distribution of the axolotl, taking into account its ecological niche. This is in order to identify suitable areas for the species and take them into account for its conservation.

The results of these investigations indicate that the location of Ambystoma mexicanum is limited to eleven sites in six isolated, reduced and dispersed areas. These are located mainly in those areas where the land is used for traditional agriculture.

Lifecycle

The life cycle in the vast majority of amphibians includes a stage in water and another on land. Between these phases, the animal undergoes a process of metamorphosis. However, Ambystoma mexicanum is the exception to this rule.

This is because the species is neotenic, so it does not metamorphose. Therefore, its entire life cycle takes place in water. The axolotl, within its development, goes through several stages. Some of these are:

Egg fertilization

Once fertilized, the egg reaches approximately 2 mm. During this stage the eggs are enveloped in a jelly-like secretion that contains sperm. In this phase the first cleavage groove and the animal pole appear.

Embryo

At 21 hours after being fertilized, it is already a blastula, having a smooth surface. When it is three days old, the embryo has an elongated shape. The neural folds are outlined, beginning to rise above the head area.

Beginning of the formation of organic structures

Between 3 and 4 days, in the embryo the neural folds at the level of the spinal area fuse. The optic vesicles are developing. A small swelling delimits the future region where the gills will be located. A depression appears in the ectoderm, which will become the primordium of the ear.

Appearance of the eyes and gills

When 10 days have elapsed, the gills are elongated and already have four pairs of filaments. The mouth is more clearly marked and the buds already protrude from the limbs.

Hatching

On day 12, the hatching process begins, where the larva makes convulsive movements, thus shedding the layer of gelatin that covered it.

Larvae

The young are considered larvae, from hatching to four months. They only have a head, gills and a body. The limbs will develop later.

In their first hours of life, the larvae of Ambystoma mexicanum feed on some remains of yolk, but very soon they will need microalgae, such as spirulina, to feed and continue to develop.

Youth and adult growth

When the axolotl is between 4 and 12 months old it is considered a youngster, generally it already measures about 5 centimeters. From 13 months the stage begins where it can reproduce, since it is sexually mature.

Reproduction

In axolotls, sexual maturity is reached around one year of age. Despite this, they maintain their larval stage. It is from that moment when the differences between males and females are most noticeable.

One of these characteristics is inflammation in the area of ​​the cloaca. In males the cloacal glands are inflamed, in addition these are usually thinner and with a longer tail than that of females.

The sexual activity of the axolotl is generally at night. To mate, males do not exhibit behaviors associated with courtship.

Fertilization

To start the fertilization process, the male axolotl goes to a rock or sand and secretes, through the cloacal opening, a gelatinous sac that contains the sperm. This granular envelope is known as a spermatophore. To fertilize them, the female approaches the sac and absorbs it through her cloaca.

In oviposition, the female lays between 100 and 600 eggs. The amplitude of the laying is variable, it could go from 40, which would correspond to a young female, to 1500, which would be placed by an adult female. This can happen in a single lay or with a few days in between.

The incubation time of these fertilized eggs will depend on the temperature of the environment where they are found. However, it is usually between 12 and 18 days.

The egg has three layers and its membrane is permeable. This characteristic could harm its development, because if the water where it is found contains toxic substances, the egg could absorb them.

After hatching, small axolotls may become easy prey for fish that share the same habitat.

Neotenia

Axolotls maintain a larval form throughout their lives. For this reason, they exhibit neoteny, which implies that they reach sexual maturity without undergoing a metamorphosis process.

This metamorphic failure is due to the degeneration of the thyroid gland, causing low levels of thyroxine. This hormone is directly related to this process of morphological change.

Neoteny has allowed the axolotl to survive in aquatic environments where little food may exist. This way of reproducing in the larval stage requires less quality and quantity of food, unlike if it were an adult and terrestrial animal.

Feeding

Axolotls are strict carnivorous animals. However, its diet may vary as it develops. In the first days of life as larvae, they feed on the remains of the yolk sac and microalgae. Then, around 11 days after hatching, the young will be able to eat insect larvae.

In its juvenile stage, this animal prefers small pieces of meat and worms. Once adults, the diet is much more varied, consisting of newly hatched fish, river lobsters, aquatic worms such as tubifex and adult fish, such as charal.

They also eat slugs, insects, frog tadpoles, snails, mosquito larvae, and worms.

Because they have poor vision, axolotls locate their prey using their sense of smell. They are also capable of detecting electric fields and some chemical signals, in this way they perceive the environment and discover the animals that they are going to eat.

The digestion

Ambystoma mexicanum has cartilaginous structures on both palates, which, due to their serrated shape, fulfill the function of teeth. In this particular case, they only use them to grab their prey, but not to chew or tear it.

The digestive tract is short and straightforward. To eat, this animal opens its mouth and absorbs the food, along with water, and swallows it whole. The oral cavity is separated from the esophagus by a sphincter, similar to the glottis.

The process of digestion begins in the esophagus, which secretes a kind of mucus that contains digestive enzymes. It also has cilia, which carry swallowed food through the esophagus to the stomach. This digestive organ is of the glandular type and has 3 zones: cardia, fundus and pylorus.

In the stomach the digestion of food continues. Then the food mass passes into the intestine, which in the axolotl is short.

Digestion is complemented by various organs, such as the liver and pancreas. The liver is large and functions as a storehouse for protein and fat. It also secretes bile fluids, which it pours into the initial part of the small intestine, helping in the digestion of fats.

The pancreas, located between the stomach and the intestine, produces pancreatic enzymes that participate in digestion. Bile fluids and pancreatic enzymes are secreted in the front part of the small intestine, where the absorption of nutrients takes place.

References

1. Wikipedia (2018). Axolotl. Recovered from en.wikipedia.org.
2. ITIS (2018). Ambystoma mexicanum. Recovered from itis.gov.
3. Majchrzak, A. (2004). Ambystoma mexicanum. Animal Diversity Web. Recovered from animaldiversity.org.
4. Horacio Mena González, Erika Servín Zamora (2014). Basic manual for the captive care of the Xochimilco axolotl (Ambystoma mexicanum). National Autonomous University of Mexico. Recovered from ibiologia.unam.mx.
5. Erika Servín Zamora (2011). Manual of maintenance in captivity and veterinary medicine applied to the xochimilco axolotl (Ambystoma mexicanum) in the Chapultepec zoo. Autonomous University of Mexico. Academy. Recovered from academia.edu.
6. Luis Zambrano, Paola Mosig Reidl, Jeanne McKay, Richard Griffiths, Brad Shaffer, Oscar Flores-Villela, Gabriela Parra-Olea, David Wake (2010). Ambystoma mexicanum. The IUCN Red List of Threatened Species. Recovered from iucnredlist.org.
7. Ministry of Environment and Natural Resources, Government of Mexico. (2018). Mexican axolotl, super gifted creature. Recovered from gob.mx.
8. Luis Zambrano, Elsa Valiente, M. Jake Vander Zanden (2010). Food web overlap among native axolotl (Ambystoma
9. mexicanum) and two exotic fishes: carp (Cyprinus carpio)
10. and tilapia (Oreochromis niloticus) in Xochimilco,
11. Mexico City. Springer science. Recovered from jakevzlab.net.
12. Victoria Contreras, Enrique Martínez-Meyer, Elsa Valiente, Luis Zambrano (2009). Recent decline and potential distribution in the last remnant area of ​​the microendemic Mexican axolotl (Ambystoma mexicanum). Science direct. Recovered from sciencedirect.com.
13. George M. Malacinski (2015). The Mexican Axolotl, Ambystoma mexicanum: Its Biology and Developmental Genetics, and Its Autonomous Cell-lethal Genes. Oxford academic. Recovered from academic.oup.com.
14. Hill, MA (2018). Embryology Axolotl Development. Embryology.med. Recovered from embryology.med.unsw.edu.au.
15. Larson, Allan (1996). Ambystomatidae. Mole Salamanders. The Tree of Life Web Project. Recovered from tolweb.org.
16. Haas BJ, Whited JL (2017). Advances in Decoding Axolotl Limb Regeneration. NCBI. Recovered from ncbi.nlm.nih.gov.