Mexico City— It’s not yet noon on a Wednesday at Lake Xochimilco, a mosaic of ponds and canals to the south of this sprawling metropolis, but revelers on a brightly colored tourist boat have already broken out the beer and are whooping it up. On another boat, a mariachi band tunes up. Carlos Uriel Sumano Arias, paddling a flat-bottomed chalupa belonging to the National Autonomous University of Mexico (UNAM), slips into a quiet canal and lays up next to a chinampa, an artificial island for growing crops—a farming system invented by the Aztecs. “I’d like to show you an axolotl,” he says.
A darling of regenerative medicine, the axolotl (Ambystoma mexicanum) can regrow severed limbs, its eyes, its frilly external gills, and even brain tissue. About 1 million are in captivity in labs and aquariums worldwide. Cute to some, grotesque to others, the salamander has also made its mark in pop culture, appearing as a character in online games. In Mexico, its image graces the 50 peso note. The axolotl is omnipresent—and at the same time, vanishing. The only remaining wild population is making its last stand here, in canals isolated from a former habitat that has become too hostile to sustain it.
Now, this haven is also in jeopardy. Entrepreneurs have been buying up chinampas, converting some to soccer fields and building pavilions on others for torch-lit feasts. “This gentrification phenomenon is a huge threat,” as the businesses are less invested than the farmers in keeping the canals clean, says Luis Zambrano González, a UNAM biologist who has spent 20 years trying to stave off the axolotl’s extinction.
The animal’s best shot may be aggressive habitat restoration. Zambrano hopes to expand the number of canal refuges from the current 20, totaling 5 kilometers of waterways, to 200—enough, he says, to sustain a viable population. “A species is not a species if it’s not in its environment,” he argues. Others see salvation in breeding axolotls with closely related species, which could introduce genetic variation that might make wild or captive populations more resilient. “Other Ambystoma populations have recently swapped genes with the axolotl and are genetically similar,” says biologist David Weisrock of the University of Kentucky. “All hope is not lost.”
The 15 Ambystoma species found in central Mexico share a genetic heritage with the tiger salamander (A. tigrinum). Like most amphibians, it has gills and lives in the water as a larva before moving onto land as an air-breathing adult. But when axolotl ancestors colonized isolated lakes across the region, starting about 1 million years ago, ample food and few predators spurred a remarkable adaptation: Salamanders in some lakes began to spend their entire lives underwater, retaining juvenile features. “If there’s no pressure to get out … you can become sexually mature without having to leave the pond,” Weisrock says. The axolotl is one of four central Mexico species that rarely, if ever, metamorphose in the wild.
The Aztecs revered the axolotl, believing it to be the incarnation of Xolotl, a god of death and transformation. But after the Spanish founded Mexico City, they drained nearby lakes, shrinking axolotl habitat. The amphibian suffered another blow in the 1970s and ’80s, when authorities introduced carp and tilapia into Xochimilco as food for the area’s burgeoning population. Both species prey on axolotl eggs and young. Compounding the salamander’s woes, the shallow waters have grown warmer and more polluted. A 1998 survey tallied 6000 axolotls per square kilometer. But the latest census, completed in 2015, estimated just 36 per square kilometer.
By now the salamander appears to have vanished from most of Xochimilco. Over the past 2 years, Alejandro Maeda-Obregón, a Ph.D. student at University College London (UCL), analyzed DNA from the lake’s water, looking for signs of axolotls and other endangered species. Maeda-Obregón, working with UCL’s Julia Day and Elizabeth Clare at York University, has so far failed to detect mitochondrial axolotl sequences in Xochimilco’s touristed stretches. “That’s a collapsed ecosystem,” he says. They have now turned to the refuges, probing for axolotls known from past surveys to be living there.
The axolotl’s close kinship with other Ambystoma salamanders could allow biologists to reinvigorate the species by breeding it with relatives. Scientists are investigating the source of this unusual genetic similarity, says UNAM geneticist Gabriela Parra Olea. She suspects that the plateau tiger salamander (A. velasci) may have been “the promoter of genetic uniformity.” As a terrestrial adult, it may have crawled from lake to lake, mating with local salamanders.
Zambrano hopes such interventions won’t be necessary for Xochimilco’s axolotls. He and his team are working to create more havens, free of predatory fish, by closing off canals with nets or rock barriers. In one such refuge, the water is clear and cool. “Axolotls can do fine here,” Sumano says. Nearby, in the shade, are two water tanks. He leans over one and points out an axolotl larva, a few centimeters long, paddling along the edge. In the other swim mature axolotls that are 10 times bigger. The team is waiting on permits to release the captives.
Zambrano hopes to “turn our gentrification problem into an opportunity” by raising money from developers to create refuges. He recently launched an “axolotl adoption” campaign to restore habitats and support farmers who monitor the refuges. “The next 5 years are critical,” he says. For the salamander of the gods, time is running out.
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