The axolotl (Ambystoma mexicanum) is a unique type of salamander known for its striking appearance and unusual biological characteristics. This amphibian, native to the lake complex of Xochimilco near Mexico City, is a popular choice for pet owners and a subject of intense scientific interest. While often referred to by the misleading name “Mexican walking fish,” the axolotl is a true tetrapod, meaning it possesses four distinct limbs.
The Definitive Answer: Counting the Limbs
The axolotl, like nearly all terrestrial vertebrates, has four limbs, which firmly places it in the tetrapod group. These limbs are relatively short and underdeveloped, a characteristic tied to its fully aquatic lifestyle. The limbs are primarily used for crawling along the lakebed or substrate rather than for sustained terrestrial walking.
A closer inspection of these four limbs reveals a specific and consistent anatomical structure, distinguishing the forelimbs from the hindlimbs. The two forelimbs, which correspond to the arms, typically possess four digits, or toes, each. Conversely, the two hindlimbs, corresponding to the legs, generally exhibit five digits apiece. This difference in toe count means the animal has a total of 18 digits across its four limbs, with a 4-digit/5-digit pattern on the front and back, respectively.
Neoteny and the Aquatic Lifestyle
The structure and function of the axolotl’s limbs are directly related to a biological phenomenon known as neoteny. Neoteny is the retention of juvenile, or larval, physical features into the adult, sexually mature stage. Most salamanders undergo a metamorphosis, shedding their external gills and developing stronger limbs and lungs for a terrestrial existence.
The axolotl, however, typically bypasses this transformation, remaining fully aquatic with its characteristic feathery external gills and a finned tail throughout its life. Their underdeveloped limbs are perfectly adapted for this permanent water-dwelling existence, as they rarely need to support their full body weight against gravity. This makes their short, paddle-like appendages highly effective for maneuvering and pushing off the bottom of their habitat. The aquatic environment of their native Mexican lakes is believed to have favored this neotenic state.
The Power of Regeneration
The most extraordinary feature associated with the axolotl’s limbs is their incredible ability to regrow them completely. Axolotls can regenerate complex structures, including lost limbs, parts of their spinal cord, and even portions of their brain, without forming scar tissue. This capability makes them a focus of regenerative medicine research, as scientists seek to understand how they accomplish this feat.
The regenerative process begins with the formation of a structure called the blastema at the site of the injury. The blastema is a mass of undifferentiated cells that essentially revert to an embryonic-like state, allowing them to rebuild the missing structure from scratch. This process is known as epimorphic regeneration.
The cells within the blastema possess a sophisticated molecular “GPS” system to determine which part of the limb needs to be rebuilt. A chemical gradient, controlled by the enzyme CYP26B1 and retinoic acid, effectively tells the regenerating cells if they are at the shoulder or the wrist. This molecular precision ensures the limb is regrown with the correct length and structure, replacing exactly what was lost, from the bone and muscle down to the proper number of digits.