The axolotl, Ambystoma mexicanum, is a fascinating amphibian known for its unique appearance and aquatic existence. Native to the ancient lake complex of Xochimilco, located near Mexico City, this salamander typically spends its entire life submerged in water. Unlike many other amphibians, the axolotl retains its distinctive feathery external gills, which are prominent structures extending from its head. This characteristic, along with its aquatic lifestyle, sets it apart in the amphibian world.
Understanding Neoteny
The axolotl’s ability to remain in its larval, aquatic form while reaching sexual maturity is a biological phenomenon known as neoteny. This means they retain juvenile features like external gills and a tail fin, even after reaching sexual maturity. The primary reason axolotls typically remain neotenic relates to their thyroid hormone regulation. They often have lower levels of circulating thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), which are the key regulators of metamorphosis in amphibians.
This reduced thyroid hormone activity prevents the developmental changes that would lead to a terrestrial adult form. Environmental factors in their natural habitat, such as consistent cool temperatures and abundant food in deep water, also contribute to their neotenic state, which do not provide stressors that trigger metamorphosis.
Triggers for Metamorphosis
While axolotls typically remain neotenic, they can undergo metamorphosis under specific conditions. The most direct and common trigger is exposure to thyroid hormones, such as thyroxine, or to iodine, which is a precursor element necessary for thyroid hormone synthesis. Introducing these substances can stimulate metamorphosis.
Although less common, severe environmental stressors can also induce metamorphosis in axolotls. These include drastic changes in water quality, significant fluctuations in temperature, or the desiccation of their aquatic habitat. In captive settings, some instances of morphing may also be linked to genetic predisposition.
The Metamorphic Transformation
If an axolotl undergoes metamorphosis, it experiences a series of noticeable physical and physiological changes. One of the most apparent transformations is the gradual reduction and eventual absorption of its feathery external gills. Concurrently, the tail fin diminishes in size.
Internally, the axolotl develops more robust lungs and eyelids, adaptations for a terrestrial or semi-terrestrial existence. Its skin becomes thicker and less permeable to water, preparing it for life outside of water. The limbs strengthen to support the animal’s weight on land. This transformation is irreversible once initiated and can be a highly stressful period for the animal, sometimes resulting in health complications.
Life After Metamorphosis
Following metamorphosis, the animal becomes a terrestrial salamander. Caring for a morphed axolotl requires a significant shift in husbandry practices. These transformed individuals need an enclosure that provides both land-based and semi-aquatic areas, moving away from a fully aquatic setup.
Their dietary needs and feeding habits also change, adapting to a terrestrial diet. Morphed axolotls can be more susceptible to certain health issues, partly due to the physiological stress of the transformation and a potentially weakened immune system during the process. In many cases, morphing is not considered beneficial for the animal’s long-term well-being in a captive environment.