Axolotl (Ambystoma mexicanum), artwork. This aquatic freshwater amphibian is a neotonic (larva-like) salamander. The larval form fails to undergo metamorphosis, so the adult form has the external gills (purple) of the larva and remains underwater rather than emerging onto land. The axolotl also has a caudal fin running from its head to its tail. It is a predator, feeding on worms, insects, and small fish. Axolotls are found in and around Mexico City, Mexico. Because of their characteristics, they are used extensively in biological research.