The axolotl (Ambystoma mexicanum) stands out among amphibians for its unique life cycle. Unlike most amphibians that undergo a dramatic transformation from an aquatic larval stage to a terrestrial adult, the axolotl typically remains in its larval, aquatic form throughout its life. This phenomenon, known as neoteny, allows the axolotl to reach sexual maturity and reproduce while retaining juvenile characteristics such as external gills and a tail fin.
Understanding Axolotl Neoteny
Neoteny means axolotls retain their juvenile, aquatic features as adults. Their close relatives, like the tiger salamander, typically undergo metamorphosis, losing their fringed gills and caudal fin as they mature. Axolotls, however, do not naturally undergo this transformation in their native habitat.
This is largely due to a genetic predisposition that results in a lack of thyroid-stimulating hormone, which is needed to produce thyroxine, the hormone that initiates metamorphosis in other amphibians. While axolotls produce some thyroid hormone from iodide, the amount is often insufficient to trigger the full metamorphic process. Their aquatic lifestyle in stable environments, such as the lakes and canals of Xochimilco near Mexico City, also supports their neotenous state by removing environmental pressures that would otherwise encourage a terrestrial transition.
Triggers of Metamorphosis
While neoteny is the typical state for axolotls, certain factors can induce metamorphosis. The primary trigger involves thyroid hormones, specifically thyroxine, which can be administered to prompt the transformation.
Environmental stressors can also induce metamorphosis. Poor water quality, overcrowding, or significant changes in water parameters can stress the animal and potentially lead to hormonal shifts. The introduction of iodine into their environment, either through injection or in their diet, can also stimulate metamorphosis, as iodine is a component of thyroid hormones. In rare instances, a genetic predisposition can cause spontaneous metamorphosis, even without obvious external triggers.
Transformative Changes
When an axolotl undergoes metamorphosis, it experiences profound physical and behavioral changes. One noticeable physical transformation is the absorption of its feathery external gills and caudal fin, which are replaced by functional lungs for breathing air. The axolotl’s head also reshapes, becoming flatter, and eyelids develop, allowing them to blink.
Their skin texture changes, becoming thicker and less permeable to water, which is necessary for a terrestrial existence. The limbs strengthen and the webbed feet lose their webbing as the bones thicken, making them more suitable for walking on land. Behaviorally, metamorphosed axolotls shift from a fully aquatic lifestyle to a semi-terrestrial or terrestrial one, spending more time out of water. Their diet also changes, moving from aquatic invertebrates to more terrestrial prey.
Life and Care of a Metamorphosed Axolotl
Metamorphosis impacts an axolotl’s health and lifespan. Metamorphosed axolotls have a shorter lifespan, typically living one to five years after the transformation, compared to their neotenous counterparts who can live 10 to 15 years. They are also more susceptible to diseases and infections during and immediately after metamorphosis, as their immune system may be compromised.
Caring for a metamorphosed axolotl requires changes in husbandry. They need a terrestrial or semi-terrestrial habitat with both land and shallow water areas, allowing them to move between environments. The enclosure should include hiding places like rocks or plants on the land portion. Feeding routines also change, requiring a diet suitable for terrestrial salamanders, such as insects or small worms.