The Eternal Larva of Xochimilco

An axolotl can regrow a piece of its own brain — no scar, no trace, as if the injury never happened. Not some patched-up substitute either, but working tissue, identical to the original. No other backboned animal on Earth can do that.

Biologically, it's the larva of the Mexican salamander, one that never quite finishes growing up. It reaches adulthood, it breeds, but it stays in its water-dwelling larval form for life. Scientists call this neoteny — and it's exactly why the axolotl has become the world's go-to lab animal for studying regeneration.

Nearly every axolotl living in captivity today — in tanks, in labs, in research centers around the world — descends from around thirty animals brought out of Mexico back in the twentieth century. Genetically, that's an extremely narrow starting pool. Labs have since bred a strain that glows green under ultraviolet light — a jellyfish gene spliced into its DNA — so researchers can literally watch regenerating tissue light up in real time.

In the wild, though — in the canals of Xochimilco, on the edge of Mexico City — the axolotl is running out of road. The water is polluted, the ancient Aztec system of floating garden-islands that once sustained this habitat has broken down, and the stretches of canal still fit to live in keep shrinking. Right now, conservationists and local communities are working to restore those canals — and what they manage will decide whether the axolotl stays a living creature or fades into something you only meet in an aquarium or a cartoon.

A creature that can rewind its own damage has ended up somewhere it may not be able to rewind its way out of.

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