The axolotl (\(Ambystoma mexicanum\)) is a fully aquatic salamander native exclusively to the ancient lake complex of Xochimilco near Mexico City. This creature has captured worldwide attention due to its unusual appearance, which includes feathery external gills and a perpetual “smile.” Despite being frequently—and incorrectly—referred to as the “Mexican walking fish,” the axolotl is scientifically classified as an amphibian. Its biology and evolutionary lineage place it firmly within the Class Amphibia, not among the fish.
Why Axolotls Are Amphibians
The classification of the axolotl as an amphibian is based on fundamental biological traits it shares with frogs, toads, and other salamanders. It belongs to the Order Caudata, which encompasses all tailed amphibians. The axolotl possesses a skeletal structure typical of a tetrapod, featuring four limbs with distinct digits. This limb structure is a defining feature of amphibians, unlike the finned appendages of fish.
Amphibians are characterized by smooth, porous, glandular skin that lacks the protective scales found on most fish. Axolotl skin plays a role in cutaneous respiration by allowing gas exchange directly through the moist surface. The axolotl’s method of reproduction is also characteristic of amphibians; it lays soft, jelly-coated eggs in the water, which is distinct from the reproductive strategies of most fish species.
The Phenomenon of Permanent Youth
The primary reason the axolotl is often confused for a fish is neoteny, or paedomorphosis. Neoteny is the retention of juvenile or larval characteristics into the adult, sexually mature stage. This means the axolotl maintains the fully aquatic body form of a salamander larva throughout its entire life, never undergoing the metamorphosis that typically transitions an amphibian to a terrestrial adult.
The retention of feathery, external gills that fan out from the head is a consequence of this permanent youth; these are larval features lost by most salamanders upon reaching adulthood. The axolotl also keeps its dorsal fin, aiding its movement through the water. This failure to naturally metamorphose is linked to a lack of, or insensitivity to, the thyroid hormones required to trigger developmental changes. The axolotl remains an obligate water-dweller.
While the axolotl never fully transitions to a land-based form, it does reach sexual maturity and is capable of breeding, which defines biological adulthood. Metamorphosis can be chemically induced in a laboratory by administering thyroid hormones. The aquatic, neotenic form is the natural and most robust state for \(Ambystoma mexicanum\), allowing it to thrive in its cool, stable native habitat.
Distinguishing Axolotls From Fish
Comparing the axolotl to a true fish reveals distinct differences in anatomy and physiology that solidify its amphibian status. Fish use fins for locomotion and stabilization, whereas the axolotl possesses four fully developed limbs with functional digits. The axolotl uses these small legs to “walk” along the bottom of its habitat, a capability entirely absent in fish.
The respiratory system also differs significantly, even though both spend their lives in water and possess gills. Fish typically rely solely on gills for underwater breathing, drawing oxygenated water over internal gill arches. The axolotl, despite retaining its external gills, also develops rudimentary lungs and can surface to gulp air. It uses a combination of branchial (gill), pulmonary (lung), and cutaneous (skin) respiration.
The skin structure is different between the two groups. The skin of most bony fish is covered in scales for protection, while the axolotl’s skin is soft, glandular, and permeable, a hallmark of amphibians. Although the axolotl possesses a lateral line system like fish, its tetrapod limb structure and evolutionary lineage unmistakably place it in the amphibian family.