The Mexican walking fish isn't really a fish -- better known as the axolotl, the animal is actually a salamander that keeps larval characteristics in his adult form. Unlike most salamanders, the axolotl lives in the water for his entire life. His habitat, a couple of lake systems near Mexico City, is severely compromised, making the axolotl critically endangered.
Axolotls get their nickname of Mexican walking fish from the fact they live in the water and breathe through gills, but they also have legs. They retain their tadpole-like tails as they grow into maturity, which takes about a year. Most salamander species metamorphose into adults by growing legs and losing their gills and tails. Axolotls, however, grow in size, but their bodies don't change significantly from the time they hatch. They reach sexual maturity while still looking like large versions of their larval selves.
Mexican walking fish tend to be larger than many salamander species, topping out at about 12 inches, although most are closer to 9 inches long. This size gives them an advantage in their native habitat, making them the top of the natural food chain. They eat aquatic creatures such as fish, molluscs and arthropods. Their main natural predator is the heron. Unfortunately, humans introduced large fish species into the lakes and canals where the axolotls live, and these fish upset the ecosystem balance by eating the axolotls nearly into extinction.
Axolotls possess an amazing ability to regenerate. Unlike some lizards that can grow new tails when they lose theirs, axolotls can completely regenerate nearly any body part including legs and internal organs. Regeneration is usually seen in invertebrates such as earthworms and starfish, but it's rare in vertebrates. Scientists use captive axolotls for research to see how this regeneration capability might translate into medical use with people.
Although critically endangered from loss of habitat and introduced fish species, Mexican walking fish are relatively easy to keep as pets. They need large aquariums without fish, which tend to nip at their feathered gills. As carnivores, you should feed your axolotl every couple of days using worms, crickets or feeder fish, then remove any uneaten food before it can spoil in the water. These salamanders can live up to 15 years, but they don't tend to survive long if they completely metamorphose into a typical adult-looking salamander. This rarely happens, but lack of water or a water temperature that's too low can cause spontaneous metamorphosing.