Despite their precarious future in freshwater, axolotls (pronounced axo-LO-tuhls) have long flourished in aquariums. They have been bred successfully behind glass over the past century, raised as exotic pets or as laboratory specimens for scientists investigating their extraordinary ability to regrow a severed limb or tail.

The Mexican axolotl is an odd-looking salamander with a flat head and spiked feet, unusual because it often spends its entire life in the so-called larval stage, like a tadpole, without ever moving to land. “It grows and grows in the same shape, and has the capacity to reproduce,” said the biologist Armando Tovar Garza. “We don’t really know why it doesn’t change.”

Its gaze seems to captivate as its gills slowly beat. In Julio Cortázar’s short story “Axolotl,” the narrator is transfixed — “I stayed watching them for an hour and left, unable to think of anything else” — and experiences his own metamorphosis.

The Aztecs and their descendants consumed axolotls as part of their diet, and the amphibians are still stirred into a syrup as a folk remedy for respiratory ailments.

But in their only home, the canals of Xochimilco in the far south of the city, the axolotls’ decline has been precipitous. For every 60 of them counted in 1998, researchers could find only one a decade later, according to Luis Zambrano, another biologist at the National Autonomous University of Mexico.