The axolotl, a unique mole salamander sometimes called the Mexican walking fish, possesses a respiratory system unlike most amphibians. Although fully aquatic, it does have lungs, though they are rarely its primary method of breathing. Native only to the lake complex of Xochimilco near Mexico City, this amphibian utilizes four distinct methods for gas exchange to thrive in an environment where oxygen levels can fluctuate.
The Primary Role of External Gills
The most noticeable feature of the axolotl is the three pairs of delicate, feathery external gills, which fan out from either side of its head. These bright, bushy structures are the animal’s primary means of extracting oxygen directly from the water. The gills consist of stalks, known as rami, covered in fine projections called filaments.
These filaments greatly increase the surface area available for gas exchange, allowing oxygen to diffuse efficiently into the capillaries running just beneath the surface. The axolotl can actively move or “flick” its gills to increase the flow of water over the filaments, maximizing oxygen uptake. This constant movement sustains the flow of oxygenated water, a process known as branchial respiration. Additionally, small gill rakers are located underneath the external gills, functioning to filter out any particles or debris from the water.
The Truth About Axolotl Lungs
Despite being entirely aquatic, the axolotl possesses lungs, which are remnants of its ancestral terrestrial lineage. These lungs are rudimentary compared to those of land-dwelling amphibians. They exist as simple, sac-like structures within the body cavity, containing folded pockets surrounded by blood vessels for limited gas exchange.
The axolotl uses these lungs for supplementary respiration when the oxygen concentration in the water drops to a low level. In these circumstances, the animal rises to the surface and performs buccal pumping, gulping a small volume of air. This air inflates the lungs, providing a temporary boost of oxygen until water conditions improve. Frequent trips to the surface to gulp air are often an indicator that the axolotl is experiencing stress due to poor water quality.
Breathing Through the Skin and Mouth
Beyond the gills and lungs, the axolotl employs two other secondary methods of gas exchange to supplement its oxygen supply.
Cutaneous Respiration
Like many salamanders, the axolotl can respire directly through its skin in a process called cutaneous respiration. The skin is thin, moist, and highly permeable, with capillaries running close to the surface. This thin barrier allows oxygen from the water to diffuse passively into the bloodstream, while carbon dioxide is simultaneously expelled. Cutaneous respiration is especially useful when the animal is resting or when its gills alone cannot meet its metabolic needs.
Buccopharyngeal Respiration
A fourth method, buccopharyngeal respiration, involves the lining of the mouth and throat. The highly vascularized membrane in this area allows for gas exchange as water passes through. This provides an additional surface for oxygen absorption and carbon dioxide release.
Neoteny: The Key to Their Unique Respiration
The reason the axolotl retains its external gills and only develops rudimentary lungs is due to a biological phenomenon known as neoteny. Neoteny is the retention of juvenile or larval characteristics into adulthood, including the ability to reproduce. While most salamanders undergo metamorphosis, losing their gills and developing fully functional lungs to become terrestrial, the axolotl remains in its aquatic, larval form for its entire life.
This failure to metamorphose is attributed to a lack of the necessary thyroid-stimulating hormone or the inability of its tissues to respond to it effectively. The neotenic state ensures that the axolotl maintains its water-adapted features, such as the external gills and tail fin, while its lungs remain underdeveloped sacs. This unique evolutionary path has equipped the axolotl with a versatile, multi-faceted respiratory system suited for its specific aquatic habitat.