The Endangered Axolotl and Its Powers of Regeneration
A Critically Endangered Animal
The axolotl is an interesting and unusual amphibian that doesn't undergo metamorphosis. It stays in its larval form throughout its life, a phenomenon known as neoteny. The animal remains in its aquatic habitat and retains its external gills and its fins as it grows. Researchers have discovered that the axolotl has very impressive powers of regeneration. Studying these powers may help us to understand and even improve our far more limited ability to regenerate lost body parts. Sadly, the amphibian is critically endangered in the wild. It's doing well in captivity, though.
Axolotls are classified in the class Amphibia, the order Caudata (or Urodela), and the family Ambystomatidae (which contains the mole salamanders). Tiger and spotted salamanders also belong to the mole salamander family.
Axolotls in the Wild and in Captivity
The axolotl is also known as the Mexican salamander and the Mexican walking fish (though it's a salamander, not a fish). Its scientific name is Ambystoma mexicanum. It's found only in the canals and ponds of Lake Xochimilco in Mexico and exists in small numbers. It's also kept in zoos and as a pet. In addition, many animals are housed in laboratories where scientists are studying regeneration, other biological processes, and disease.
Using axolotls in regeneration experiments may not be pleasant to think about with respect to the well-being of the animals. Amputation of some kind must be performed in order to study regeneration. Captive members of the species could be very important in preventing the animal from becoming extinct, however.
Xolotl was an Aztec god. He is said to have entered the water and turned himself into an axolotl in order to escape his enemies.
My Introduction to the Animal
I first learned of the axolotl's existence in university. Although I was a biology major, I heard about the animal in a Latin American literature course. I've never forgotten the powerful story that I studied, which was simply entitled "Axolotl". It was written by Julio Cortázar and first published in 1952.
Cortázar's story describes a man who becomes fascinated by the axolotls in an aquarium located in a botanical garden, which he visits frequently. One animal in particular attracts him. The man eventually becomes the axolotl and looks at his previous self watching him from outside the tank. People still discuss whether the story should be interpreted as a fantasy, a description of a mental illness, or a statement about the nature of identity. I've provided a link to the story in the "References" section below.
Physical Appearance of an Axolotl
Mature axolotls are most often between nine and twelve inches long but may sometimes be shorter or longer. Although all axolotls belong to the same species, they have a variety of body and gill colours, which some pet owners greatly appreciate. Orange, yellow, pink, and albino forms seem to be popular in captive animals. The most common colours in the wild are a shade of grey brown or olive. Animals with these colours are often speckled. Their eyes have no lids and vary in colour.
The animals have a wide head and short legs that bear long and thin digits. There are four digits are each of the front feet and five on each of the back ones. Axolotls retain some characteristics of larval salamanders (or tadpoles) throughout their lives, including their fins and external gills. The gills are feathery and are located on three branches situated on each side of the head. The animals have a fin along their back and the upper and lower surface of their tail.
Metamorphosis is a normal part of the life cycle in most amphibians. The process involves a major change in body appearance and features as a larva changes into an adult. Adult salamanders generally lose their external gills and fins and breathe by lungs instead. Although axolotls don't undergo metamorphosis (at least under normal conditions), they have some adult features as well as larval ones. They have lungs, although these have a rudimentary structure. They also have mature reproductive organs, unlike the larvae of most salamanders.
Axolotls are genetically capable of undergoing metamorphosis and sometimes do when when environmental conditions are abnormal.
Daily Life and Reproduction
The axolotl is a solitary animal in the wild and is mainly active during the night. It's both a carnivore and a predator. It eats worms, aquatic insects, other invertebrates, and small fish. Its teeth are poorly developed. It rapidly sucks its prey into its mouth instead of grabbing it with its teeth. The salamander may occasionally swim to the surface of the water to take a gulp of air, which goes to its lungs. It also absorbs oxygen through its skin. It often flicks its gills to improve oxygenation.
Males and females find each other by detecting specific chemicals in the water and by sight if the animals are close enough. During courtship, the male performs a "dance" to attract a female. He also nudges her body, especially around her cloaca. She may respond by nudging the same place on the male's body. The male then deposits packets of sperm, or spermatophores, on rocks or underwater vegetation. The female picks the spermatophores up with her cloaca. Fertilization is internal.
The eggs are laid about twenty-four hours after the spermatophores are picked up. Several hundred eggs are deposited on the ground. They stick to their substrate via mucus. Two to three weeks after the eggs are laid, they hatch into juvenile animals. Axolotls can live for ten to fifteen years, at least in captivity.
The list of body parts that an axolotl can regenerate is amazing. The replacement process takes a few weeks to a few months, The parts that can be regenerated include:
- a foot
- a section of a limb
- the entire limb
- the testes
- up to one third of the heart ventricle (Unlike our four-chambered heart, the amphibian heart contains three chambers: two atria and one ventricle.)
- damaged sections of the spinal cord
- the front section of the brain (the telencephalon)
Regeneration in humans is very limited. When we're injured, our body generally heals the wound (sometimes with medical help) and then replaces lost material with scar tissue, which is nonfunctional. We do have some powers of regeneration, though. Minor wounds of the skin can be repaired with the correct tissue, the liver can be regenerated if enough of the organ remains, and the endometrium (the lining of the uterus) is shed and replaced each month during a women's menstrual cycle. We can't replace lost limbs or the tissue of most organs, however.
How Does Regeneration Occur?
Once an amputation of an axolotl limb occurs, the following sequence of events take place.
- First, bleeding from the wound is quickly stopped by a blood clot.
- Next, a layer of cells called the wound epidermis forms and covers the injured area.
- The wound epidermis and cells underneath it divide to form a structure called a blastema, which is cone shaped.
- The cells in the blastema become undifferentiated, or unspecialized, so that they resemble stem cells. A stem cell has the ability to divide repeatedly in order to form specialized cells.
- The cells in the blastema then divide and form specialized cells as required in order to recreate the missing part of the body.
Many details about the process are not yet known, but the fact that cells in the axolotl's body change into stem cells (or cells that closely resemble them) when necessary is very interesting. We have stem cells in our body. The ones in our red bone marrow make our blood cells, which is a vitally important function. In general, though, our stem cells seem to have limited ways to help us. This is one reason why scientists are studying regeneration in animals such as the axolotl with such interest. We seem to have the basic requirements for some significant regeneration, but the system is inactive in us.
In 2018, the axolotl's genome was sequenced. This may be helpful in understanding how the animal carries out regeneration.
The axolotl is threatened by urbanization, pollution, and the introduction of fish that eat the salamander's eggs and the juveniles. The animals were once a popular food for the local people, but their numbers are now too low for this use to be practical.
Another problem is that the axolotl's habitat is shrinking. The animal once existed in both Lake Xochimilco and Lake Chalco. The latter lake no longer exists because it was drained to stop flooding. The first one is actually part of Mexico City and exists as a series of canals that were once part of the original and larger lake.
At one point in 2014, no axolotls could be found in the wild. Later a few were found. Today researchers say that axolotls exist in the wild, but they also say that the animals are probably very rare.
The axolotl is a complete conservation paradox...because it’s probably the most widely distributed amphibian around the world in pet shops and labs, and yet it’s almost extinct in the wild.— Richard Griffiths, University of Kent, via Nature
Some conservationists are trying to help the axolotl in the wild, such as by stimulating the creation of urban parks containing canals where the animals live. They are also breeding the animals in captivity and then releasing them into protected areas in the canals and ponds in the Lake Xochimilco network to see how they do. At least one researcher has been tracking wild animals in an attempt to understand their lives better.
Some conservationists feel there is little point in releasing captive-bred axolotls into the canal system unless the current stresses are removed or at least reduced. They say that every time a major storm occurs in the area, water from local sewage treatment facilities overflows and reaches the canals, adding dangerous chemicals to the environment in which the salamanders live. Some of these chemicals can be absorbed by the animals' permeable skin. Agricultural runoff into the canals is also a problem, as is the existence of predators. Another area of concern is deciding exactly which captive animals should be released into the wild.
Saving a Species in Trouble (With Subtitles)
Lack of Genetic Diversity in Captivity
While it's true that many axolotls are living in captivity, this isn't an ideal situation. On the one hand, it's good that the species is unlikely to become extinct soon. On the other hand, since humans are controlling the animal's breeding in order to get desired characteristics, we are altering the nature of the animals.
The interesting colours of many pet axolotls are rarely found in the wild and inbreeding is a problem in lab animals. Laboratory animals with very similar characteristics are mating, which means that diversity in the offspring is being reduced. The ancestry of most animals in labs can be traced back to 34 axolotls collected from Mexico by a French expedition in 1863.
Another significant event in the ancestry of the laboratory animals was the addition of a few tiger salamander genes. Tiger salamanders are North American relatives of axolotls that sometimes exhibit neoteny. The reason why the genes were added is now obscure, but the altered animals have reproduced and been distributed to many labs.
Genetic diversity can give resistance to environmental stress. Some animals may have gene variants that enable them to withstand a stress that kills other animals, for example. Genetic similarity in lab animals does have one advantage, however. It increases the likelihood that results of the experiments in one lab can be reproduced in another one.
Genetic Diversity in the Wild
Unfortunately, genetic diversity is probably decreasing in the wild as well as in captivity because so few wild animals are available to mate. The loss of particular gene variants may be harmful for the wild animals and prevent us from making interesting discoveries in the future.
We really need to build up and maintain the wild population of axolotls as well as the captive ones. If we do this by releasing captive animals into the wild, we need to consider their genetic composition carefully. Hopefully, conservation efforts for the wild animals will be successful. It's uncertain whether they will be at the moment. It would be a shame to have only captive axolotls in existence.
References and Resources
- Facts about Ambystoma mexicanum from National Geographic
- Regeneration in axolotls from Business Insider (including an interview with Dr. James Godwin, who studies regeneration in the animals)
- What the axolotl can teach us about regrowing human limbs from Science in the News, Harvard University
- Ambystoma mexicanum status on the Red List of the IUCN (International Union for Conservation of Nature)
- The axolotl is racing towards extinction from the Nature science journal
- How to save the axolotl from the Smithsonian magazine
- Julio Cortázar's "Axolotl" story can be read at the Fullreads website.
Questions & Answers
© 2018 Linda Crampton