The axolotl, a unique amphibian native to Mexico, captivates observers with its perpetually aquatic form. Unlike most amphibians, it retains larval features throughout its life, reproducing while remaining fully aquatic. This raises questions about how such an animal could transform.
Understanding Neoteny
The axolotl typically exhibits neoteny, where it retains its larval characteristics into adulthood. This means that despite reaching sexual maturity, the axolotl does not undergo the metamorphosis commonly seen in other amphibians, such as frogs transforming from tadpoles. Instead, it maintains its feathery external gills and a tail fin, allowing it to remain fully aquatic.
This retention of juvenile traits is largely attributed to both genetic and environmental factors. Axolotls originate from stable, aquatic environments like Lake Xochimilco in Mexico, which provided little evolutionary pressure for them to develop a terrestrial life stage. A genetic mutation affects the production of Thyroid Stimulating Hormone (TSH), which is necessary for the thyroid gland to produce sufficient thyroxine, the hormone that triggers metamorphosis. This biological predisposition allows axolotls to thrive in their underwater habitat, as they can continue breathing underwater through their gills.
The Metamorphic Process
When an axolotl does undergo metamorphosis, it experiences a profound transformation involving both visible and internal biological changes. One of the most noticeable alterations is the regression and eventual disappearance of the external, feathery gills, which are replaced by developing lungs for air breathing. The dorsal fin, which runs along the length of its body, also recedes.
The axolotl’s skin undergoes significant changes, thickening and becoming less permeable to water to prevent dehydration in a terrestrial environment. The once smooth and translucent skin becomes more textured and may develop patterns or markings similar to other salamander species. Internally, the skeleton ossifies, becoming more bony and less cartilaginous. The jaw and teeth may change shape to adapt to a different diet, and the eyes also become more prominent and develop eyelids for vision in air rather than water.
Triggers for Transformation
The transformation of an axolotl into a terrestrial salamander is primarily triggered by the presence of specific hormones, particularly thyroxine. While axolotls naturally have low levels of thyroid hormones, increasing these levels can initiate metamorphosis. Thyroxine, regulated by the thyroid gland, acts as a signal for the body to undergo the developmental changes associated with a transition to land.
Environmental stressors can also play a role in inducing this transformation. Factors such as the drying up of water bodies, significant changes in water quality, or fluctuations in temperature can prompt metamorphosis. For instance, prolonged exposure to temperatures above 23°C (74°F) or poor water conditions with high levels of ammonia, nitrite, or nitrate can stress axolotls, potentially leading to metamorphosis. In laboratory settings, metamorphosis can be reliably induced by administering thyroid hormones, such as thyroxine (T4) or triiodothyronine (T3), either by adding them to the water or through injections.
Life After Transformation
After successful metamorphosis, the axolotl becomes a terrestrial salamander, fundamentally changing its way of life. The transformed animal moves to land, primarily breathing with its newly developed lungs. Its limbs become more muscular to support its body for terrestrial locomotion, and its tail becomes more rounded instead of flat.
The transformed salamander’s skin texture differs significantly, adapting to a drier environment, and its appearance may include new patterns or colors. Its diet shifts to include terrestrial insects and other small prey, reflecting its new habitat. This transformed state is generally irreversible, and while the animal is still genetically an axolotl, it functions as a distinct organism with a different lifestyle. However, induced metamorphosis can be stressful and may sometimes lead to a shorter lifespan for the animal.