All About the Axolotl (Ambystoma mexicanum)

According to Aztec legend, the first axolotl (pronounced axo-LO-tuhl) was a god who changed his form in order to escape being sacrificed. The sneaky transformation from terrestrial salamander to a fully aquatic form did not save later generations from death. The Aztecs ate axolotls. Back when the animals were common, you could buy them as food in Mexican markets.

While the axolotl may not be a god, it's an amazing animal. Learn how to recognize an axolotl, why scientists are fascinated by them, and how to care for one as a pet.

Description

An axolotl is a type of salamander, which is an amphibian. Frogs, newts, and most salamanders undergo a metamorphosis to transition from life in the water to life on land. The axolotl is unusual in that it doesn't undergo a metamorphosis and develop lungs. Instead, axolotls hatch from eggs into a juvenile form that grows to become its adult form. Axolotls keep their gills and permanently reside in the water.

A mature axolotl (18 to 24 months in the wild) ranges in length from 15 to 45 centimeters (6 to 18 inches). An adult specimen weighs anywhere between 2 and 8 ounces. An axolotl resembles other salamander larvae, with lidless eyes, a wide head, frilled gills, long digits, and a long tail. A male has a swollen, papillae-lined cloaca, while a female has a wider body that is full of eggs. The salamanders have vestigial teeth. Gills are used for respiration, although the animals sometimes gulp surface air for supplemental oxygen.

Axolotls have four pigmentation genes, giving rise to a wide range of colors. The wild-type coloration is olive brown with gold speckles. Mutant colors include pale pink with black eyes, gold with gold eyes, gray with black eyes, and black. Axolotls can alter their melanophores to camouflage themselves, but only to a limited extent.

Scientists believe axolotls descended from salamanders that could live on land, but reverted to water because it offered a survival advantage.

Animals Confused With Axolotls

People confuse axolotls with other animals partly because the same common names may be applied to different species and partly because axolotls do resemble other animals.

Animals confused with axolotls include:

Waterdog: A waterdog is the name of the larval stage of the tiger salamander (Ambystoma tigrinum and A. mavotium). The tiger salamander and axolotl are related, but the axolotl never metamorphosizes into a terrestrial salamander. However, it's possible to force an axolotl to undergo metamorphosis. This animal looks like a tiger salamander, but the metamorphosis is unnatural and shortens the animals lifespan.

Mudpuppy: Like the axolotl, the mudpuppy (Necturus spp.) is a fully aquatic salamander. However, the two species are not closely related. Unlike the axolotl, the common mudpuppy (N. maculosus) is not endangered.

Habitat and Distribution

In the wild, axolotls only live in the Xochimilco lake complex, which is located near Mexico City. The salamanders may be found on the bottom of the lake and its canals.

Neoteny

The axolotl is a neotenic salamander, which means it doesn't mature into an air-breathing adult form. Neoteny is favored in cool, high-altitude environments because metamorphosis requires a huge energy expenditure. Axolotls can be induced to metamorphose by injection of iodine or thyroxine or by ingesting iodine-rich food.

Diet

Axolotls are carnivores. In the wild, they eat worms, insect larvae, crustaceans, small fish, and mollusks. The salamanders hunt by smell, snapping at prey and sucking it in like a vacuum cleaner.

Within the lake, axolotls had no real predators. Predatory birds were the biggest threat. Large fish were introduced into Lake Xochimilco, which ate the young salamanders.

Reproduction and Offspring

Much of what we know about axolotl reproduction comes from observing them in captivity. Captive axolotls become mature in their larval stage between 6 and 12 months of age. Females typically mature later than males.

The increasing temperature and light of spring signal the start of the axolotl breeding season. Males expel spermatophores into the water and try to lure a female over them. The female picks up the sperm packet with her cloaca, leading to internal fertilization. Females release between 400 and 1000 eggs during spawning. She lays each egg individually, attaching it to a plant or rock. A female may breed several times during a season.

The tail and gills of the larvae are visible within the egg. Hatching occurs after 2 to 3 weeks. Larger, earlier-hatching larvae eat smaller, younger ones.

Regeneration

The axolotl is a model genetic organism for regeneration. Salamanders and newts have the highest regenerative ability of any tetrapod (4-legged) vertebrates. The incredible healing ability extends well beyond replacing a lost tail or limbs. Axolotls can even replace some parts of their brains. In addition, they freely accept transplants (including eyes and brain portions) from other axolotls.

Conservation Status

Wild axolotls are headed to extinction. They are listed as critically endangered by IUCN. In 2013, no surviving axolotls were found in the Lake Xochimilco habitat, but then two individuals were found in the canals leading from the lake.

The decline of axolotls is due to multiple factors. Water pollution, urbanization (loss of habitat), and introduction of invasive species (tilapia and perch) may be more than the species can withstand.

Keeping an Axolotl in Captivity

However, the axolotl won't vanish! Axolotls are important research animals and fairly common exotic pets. They are uncommon at pet stores because they require a cool temperature, but may be obtained from hobbyists and scientific supply houses.

A single axolotl needs at least a 10-gallon aquarium, filled (no exposed land, like for a frog), and supplied with a lid (because axolotls jump). Axolotls cannot tolerate chlorine or chloramine, so tap water must be treated prior to use. A water filter is a necessity, but the salamanders cannot tolerate flowing water. They do not require light, so in an aquarium with plants, it's important to have large rocks or other hiding places. Pebbles, sand, or gravel (anything smaller than the axolotl's head) pose a risk because axolotls will ingest them and may die from gastrointestinal blockage. Axolotls need a year-round temperature in the low to mid-60s (Fahrenheit) and will die if exposed to prolonged temperature about 74 °F. They need an aquarium chiller to maintain the proper temperature range.

Feeding is the easy part of axolotl care. They will eat bloodworm cubes, earthworms, shrimp, and lean chicken or beef. While they will eat feeder fish, experts recommend avoiding them because salamanders are susceptible to parasites and diseases carried by fish.

Sources

•  Luis Zambrano; Paola Mosig Reidl; Jeanne McKay; Richard Griffiths; Brad Shaffer; Oscar Flores-Villela; Gabriela Parra-Olea; David Wake. "Ambystoma mexicanum". The IUCN Red List of Threatened Species, 2010. IUCN. 2010: e.T1095A3229615. doi:10.2305/IUCN.UK.2010-2.RLTS.T1095A3229615.en
• Malacinski, George M. "The Mexican Axolotl, Ambystoma mexicanum: Its Biology and Developmental Genetics, and Its Autonomous Cell-Lethal Genes". American Zoologist. Oxford University Press. 18: 195–206, Spring 1978.
• Pough, F. H. "Recommendations for the Care of Amphibians and Reptiles in Academic Institutions". Washington, D.C.: National Academy Press, 1992.