It’s not even midday on a Wednesday at Lake Xochimilco, a mosaic of ponds and canals south of Mexico City, but revelers on a colorful tourist boat have already pulled out the beers. On another boat, a mariachi band strikes up music. Carlos Uriel Sumano Arias paddles his flat-bottomed boat, owned by the National Autonomous University of Mexico (UNAM), into a quiet canal and docks next to a chinampa, an artificial island for growing crops. This is a cultivation system invented by the Aztecs. “I’d like to show you an axolotl,” he says.
The Axolotl (Ambystoma mexicanum) is a favorite of regenerative medicine. It can regrow severed limbs, its eyes, its ruffled external gills, and even brain tissue. About a million specimens are kept in laboratories and aquariums worldwide.
Cute for some, grotesque for others. The salamander has also made a name for itself in pop culture, appearing as a character in online games. In Mexico, his picture adorns the 50 peso note. The axolotl is omnipresent – and at the same time threatened with extinction. The only remaining wild population fights here, in canals, isolated from their former habitat, which has become too hostile for them.
Now that sanctuary is in danger, too. Entrepreneurs have bought up chinampas, converting some into soccer fields and building pavilions for torch-lit parties on others. “This gentrification phenomenon is a huge threat” because companies are investing less than farmers in keeping the canals clean, says Luis Zambrano González, an Unam biologist who has been trying to prevent the axolotl’s extinction for 20 years.
Researchers suggest crossing the axolotl to make it more resilient
The animal’s best bet may be aggressive habitat restoration. Zambrano hopes to increase the number of canal sanctuaries from the current 20, spanning five kilometers of waterways, to 200 – enough to support a viable population, he says. “A species isn’t a species if it doesn’t live in its environment,” he argues. Others see salvation in crossing axolotls with closely related species, which could introduce genetic variations that make wild or captive populations more resilient. “Other ambystoma populations have recently swapped genes with the axolotl and are genetically similar,” says University of Kentucky biologist David Weisrock. “All hope is not lost yet.”
The 15 Ambystoma species found in central Mexico share a common genetic heritage with the tiger salamander (Ambystoma tigrine). Like most amphibians, this one has gills and lives in the water as a larva before going on land as an air-breathing adult. But when axolotl ancestors colonized isolated lakes in the region about a million years ago, abundant food and few predators led to a remarkable adaptation: in some lakes, salamanders began to spend their entire lives underwater, while retaining their juvenile characteristics . “If there’s no pressure to get out, you can get sexually mature without having to leave the pond,” says Weisrock. The axolotl is one of four species in central Mexico that rarely, if ever, transform in the wild.
The Aztecs worshiped the axolotl and believed it to be the incarnation of Xolotl, the god of death and transformation. But after the Spanish founded Mexico City, they drained nearby lakes, reducing the axolotl’s habitat. In the 1970s and 1980s, the amphibian suffered another blow when authorities introduced tilapia carp and cichlids to Xochimilco to feed the region’s growing population. Both species of fish feed on the eggs and young of the axolotl. A further complication for the salamander is that the shallow waters have become warmer and more polluted than before. A survey in 1998 counted 6000 axolotls per square kilometer. At the last count in 2015, there were only 36 per square kilometer.
“This is a collapsed ecosystem”
By now, the salamander seems to have disappeared from most of Xochimilco. For the past two years Alejandro Maeda-Obregón, a PhD student at University College London (UCL), has been analyzing DNA from the lake’s waters, looking for signs of axolotls and other threatened species. Maeda-Obregón, who works with UCL’s Julia Day and York University’s Elizabeth Clare, has so far been unable to find any clues in the tourist sections of Xochimilco. “It’s a collapsed ecosystem,” he says. The researchers have now turned to the sanctuaries where the axolotl is known to occur from previous research and searched for the animals there.
The axolotl’s close relationship with other Ambystoma salamanders could allow biologists to revive the species by breeding with relatives. According to Unam geneticist Gabriela Parra-Olea, scientists are currently investigating the cause of this unusual genetic similarity. She suspects that the plateau tiger salamander (Ambystoma velasci) may have been the “promoter of genetic uniformity”. As an adult land dweller, he may have crawled from sea to sea and mated with the local salamanders.
Zambrano hopes that breeding interventions will not be necessary for Xochimilco’s axolotls. He and his team are working to create more sanctuaries free from predatory fish by blocking off channels with nets or rock barriers. In one of these refuges, the water is clear and cool. “The axolotl can live well here,” says Sumano. Nearby, in the shade, are two water tanks. He bends over one and points to an axlotl larva a few inches long paddling along the edge. In the other tank, there are adult axolotls that are ten times larger. The team is awaiting approval to release the prisoners.
Zambrano hopes to “turn our gentrification problem into an opportunity” by raising money from developers to set up sanctuaries. He recently launched an axolotl sponsorship campaign to restore habitats and support farmers who monitor the sanctuaries. “The next five years are crucial,” he says. The salamander of the gods is running out of time.
This post is from Science Magazine Science. It is not an official translation of the Science-Editorial staff. In case of doubt, the English original, published by the AAAS, applies. German editing: wet