The axolotl, a unique aquatic creature recognized for its “smiling” face and frilly external gills, has captured the public imagination. This amphibian (Ambystoma mexicanum) is native only to the complex of lakes near Mexico City. Its unusual appearance often leads people to question whether it belongs to the group of vertebrates or invertebrates.
Defining Vertebrates and Invertebrates
The fundamental distinction between the two largest groups in the animal kingdom lies in a single anatomical feature: the backbone. Vertebrates are defined as animals possessing a vertebral column, which is part of an internal skeleton, or endoskeleton. This internal structure supports the body and protects the central nervous system. Vertebrates include mammals, birds, reptiles, fish, and amphibians, all of which fall under the subphylum Vertebrata.
In contrast, invertebrates are animals that do not possess a vertebral column. This group encompasses the vast majority of animal species, including insects, mollusks, worms, and sponges. Many invertebrates rely on a rigid external covering, known as an exoskeleton, or simply have soft bodies for support.
The Axolotl’s Definitive Classification
Axolotls are definitively classified as vertebrates, possessing the internal skeletal structure required for this designation. They belong to the phylum Chordata and the class Amphibia, placing them firmly alongside frogs, toads, and other salamanders. As a type of mole salamander, the axolotl possesses a complex internal skeleton, even though it is largely cartilaginous.
The clearest evidence of their vertebrate status is the presence of a vertebral column composed of multiple individual vertebrae. This backbone extends from the skull through the tail, protecting the spinal cord and providing a framework for muscle attachment. Axolotls also have bones in their limbs, analogous to the humerus and femur found in other four-limbed animals.
Unique Traits That Cause Confusion
The confusion about the axolotl’s classification often stems from several unusual characteristics that are rare among vertebrates. The most prominent of these is neoteny, a phenomenon where the animal reaches sexual maturity while retaining its larval physical traits. This means the adult axolotl remains fully aquatic, complete with the feathery, external gills that protrude from its head.
Another trait that contributes to the misclassification is the axolotl’s extraordinary capacity for regeneration. They can regrow entire lost limbs, parts of the spinal cord, and even sections of the brain and heart without forming scar tissue. While this level of regeneration is uncommon in most vertebrates, it is a specialized biological function that does not alter the fundamental presence of their internal spinal column. The animal’s soft, smooth skin also contributes to an overall appearance that contrasts sharply with the exteriors of many familiar vertebrates.