Is an Axolotl a Fish? The Mexican Walking Fish Explained

What kind of animal is an axolotl?

The axolotl (Ambystoma mexicanum) belongs to the class Amphibia, the same class as frogs, toads and newts. Within the amphibians it is placed in the order Caudata (the salamanders and newts) and the family Ambystomatidae, the mole salamanders. Its closest relatives are the tiger salamanders of North America.

What makes the axolotl unusual is that it never completes metamorphosis. Most mole salamanders lose their gills and move onto land as adults, but the axolotl stays in its gilled, water-living larval form all its life. This is why it looks so much like a fish. Read more in why don’t axolotls turn into salamanders?.

Why is it called the Mexican walking fish?

The name is a popular nickname, not a scientific one. It describes what people see: an animal that lives only in water, swims with a tall tail fin and breathes with gills, yet walks slowly across the bottom of a lake or tank on four legs.

Many animals have misleading names. Starfish, jellyfish and cuttlefish are not fish either. The same goes for “walking fish”: real fish that move over land, such as mudskippers, use their fins, while the axolotl walks on true legs with toes.

How is an axolotl different from a fish?

At first glance an axolotl and a small fish may look similar, but their bodies are built very differently:

The legs are the most obvious clue. Fish do not have legs with fingers and toes. These evolved in the ancestors of four-limbed animals, or tetrapods, which include amphibians, reptiles, birds and mammals.

The axolotl also has a skeleton built on the tetrapod plan, with a pelvis and shoulder bones carrying its legs, and its heart has three chambers like other amphibians, while a typical fish heart has two.

Is an axolotl an amphibian if it never leaves the water?

Yes. Being an amphibian is about ancestry and body plan, not about where an animal lives. The word comes from Greek for “double life”, because most amphibians start life in water and spend part of their adult life on land, but plenty of amphibians stay in water for good.

Mudpuppies, sirens and the European olm are other salamanders that keep their gills and never leave the water. The axolotl is special because it can still turn into a land salamander if given thyroid hormone in a laboratory, which shows that it has kept the full amphibian toolkit.

For a broader look at how amphibians differ from reptiles, see reptiles vs amphibians. And to compare salamanders with their lookalikes, see lizard vs salamander.

Where does the name axolotl come from?

The word comes from Nahuatl, the language of the Aztecs, who lived around the lakes of the Valley of Mexico where axolotls are native. It is linked to Xolotl, an Aztec god associated with fire, lightning, death and transformation, who was often shown with a dog’s head. Translations vary, but “water dog”, “water monster” and “water servant” are common.

According to one legend, Xolotl tried to escape being sacrificed by changing into different forms, finally becoming an axolotl and hiding in the water. The story fits an animal that seems to be caught between two forms. Axolotls were also an everyday food for people around the lakes for centuries.

Are there other animals with confusing names like this?

Yes, and several are also amphibians. The mudpuppy and its relatives are sometimes called “waterdogs”, and the larvae of tiger salamanders are often sold as “axolotls” even though they belong to a different species. In the other direction, some true fish are named after other animals, such as the lungfish, which can breathe air, and the lizardfish, a fish with a lizard-like head. Names like these describe how an animal looks, not what it is.

Why does the axolotl’s identity matter?

Knowing that the axolotl is an amphibian helps explain both its abilities and its problems. Like other amphibians, it has thin, permeable skin that absorbs chemicals from the water, which makes it very sensitive to pollution. That sensitivity is one reason wild axolotls have almost disappeared from Mexico City’s canals and are now Critically Endangered.