Pet axolotls have become hugely popular, and with so many on display in tanks, a few people have found themselves with ring-side seats to the transformation of rare axolotl morphs. This happens when captive axolotls spontaneously morph from an aquatic animal to a terrestrial one. While this is a lifestyle shift seen in many animals (tadpole to frog, for example), the motivation and emergence of rare axolotl morphs is a little more complex.

Axolotls are critically endangered in the wild, and the few animals that do exist are found exclusively in the canals of Mexico City – but they’ve spread worldwide as research and domestic animals. However, as axolotl expert Dr Catherine McCusker from the University of Massachusetts Boston told IFLScience, all captive axolotls were founded by just six individuals.

As we know from the many breed-specific health problems of domestic dogs, inbreeding carries the risk of increased rates of complications – something the growing colony of captive axolotls did not escape.

To try and increase their genetic diversity and make the animals healthier, captive axolotls were hybridized with tiger salamanders, a close relative. Where the animals differ is that axolotls are paedomorphic, meaning they retain some of their youthful traits into adulthood. This is because they don’t produce the necessary hormones that give their bodies the message to "grow up."

Tiger salamanders, on the other hand, change shape as they mature, shifting from their earlier aquatic morphologies to an air-breathing adult salamander.

By hybridizing the axolotl with the tiger salamander, humans created a critter with several options in their later life wardrobe. Some will remain aquatic axolotls, while others one day make the shift to become rare axolotl morphs – something that’s probably a bit confusing to the unresearched pet parent.

“The first most obvious change is that their gills begin to regress,” said McCusker, who studies axolotl regeneration. “Once that is completed, they can no longer breathe underwater, and must be provided with 'terrestrial' material to crawl around on."

"There are also many changes that happen to their skin to adapt to their new habitat. They lose the flipper and fin like skin on their tails.”

McCusker is currently studying limb regeneration using Mexican axolotls as a model. Rare morph axolotls lose some of their regeneration skills, but those who retain their paedomorphic state are masters of regrowing full limbs.

A blog posted to Imgur by Courtney Bailey (aka @SalamanderWithASign) details the transformation of an axolotl named Gollum. Bailey was aware of rare axolotl morphs, but never expected to witness the change first-hand.

“This is what Gollum looked like when I bought him. He was your classic axolotl. Webbed feet, fluffy gills, and a cute little smile. As with all axolotls, he was supposed to remain aquatic for his entire life," one post reads. "Gollum did not get that memo."

"One day I noticed that he was looking really ‘sick.’ I quarantined him from his tankmates and within a week, he looked like the above picture. No gills, no webbed feet, and he didn't want water anymore at all"

"This is incredibly, very, very rare.”

Since witnessing Gollum’s glow up, Bailey is determined to be an advocate for rare axolotl morphs, who are very difficult to care for owing to the lack of available information online.

“I've been talking to an expert on the matter who has successfully kept morphs for 10+ years. He says that with proper care, Gollum will likely live out a full life. The issue with most people is that they just don't know how to care for the animals properly [...] I want to share him with the world. I want to document how I care for him so that others can follow suit and expand the lifespan of their own morphs.”

You can find out more about Gollum’s transformation and tips for axolotl morph care here.