The axolotl (Ambystoma mexicanum) is a unique salamander native to the freshwater lakes and canals near Mexico City, particularly Lake Xochimilco. Unlike most amphibians that undergo a dramatic transformation from an aquatic larval stage to a terrestrial adult, the axolotl typically retains its juvenile features throughout its entire life. This distinct life cycle sets it apart, making its unusual development a subject of scientific interest and a popular pet.
From Egg to Larva
The axolotl life cycle begins with the egg stage in an aquatic environment. Female axolotls lay their eggs individually, attaching them to aquatic plants or other submerged surfaces. These eggs are encased in a jelly-like substance that protects them and ensures hydration.
After fertilization, axolotl eggs hatch within 10 to 14 days. The newly hatched larvae are small, possessing external gills, which appear as feathery structures, and a caudal fin resembling a tadpole. For the first one to two days, larvae absorb nutrients from their yolk sac. Once depleted, they become active feeders, relying on small live food items such as newly hatched brine shrimp or daphnia. Their feeding response is primarily triggered by movement.
The Neotenic Adult Stage
A defining characteristic of the axolotl is its neotenic adult stage, where it retains larval features into sexual maturity, bypassing metamorphosis. Adult axolotls continue to live entirely in water, possessing their distinctive feathery external gills and a tail fin. While they develop functional lungs, they primarily rely on their external gills and skin for respiration underwater. Their wide heads and lidless eyes are also features typical of their aquatic, larval form.
Neoteny in axolotls results from both genetic predisposition and environmental factors. Axolotls have a natural lack of thyroid-stimulating hormone, necessary for metamorphosis in most amphibians. This genetic trait prevents their transformation into a land-dwelling adult. Their native habitat, characterized by cool, stable freshwater lakes like Xochimilco, often lacks environmental triggers like sufficient iodine that might induce metamorphosis. This adaptation allows them to thrive in their aquatic environment, reaching reproductive maturity without the risks associated with a terrestrial existence.
Reproduction and Longevity
Axolotls reach sexual maturity around one year of age. Their breeding season in the wild occurs once a year, typically in spring. Reproduction involves an aquatic courtship ritual where the male nudges the female and performs a “hula dance” to create water currents. He then deposits small, cone-shaped packets of sperm, called spermatophores, onto the substrate. The female picks up these spermatophores with her cloaca, leading to internal fertilization.
After fertilization, a female axolotl can lay a large number of eggs, ranging from 100 to over 1,000, within a few hours to two days. She attaches each egg individually to aquatic plants or other submerged surfaces. The axolotl’s lifespan varies significantly between wild and captive environments. In their natural habitat, axolotls live for about 5 to 6 years. In captivity, with stable water conditions, proper nutrition, and protection from predators, they can live considerably longer, often reaching 10 to 15 years, with some individuals living up to 20 years or more.