For starters, axolotls are opportunistic eaters, meaning they’ll eat the other fish in the tank. What’s more, the fish could also suck on the axolotl’s slime coat, affecting their health.

But urban sprawl, contaminated water and non-native fish that like to eat young axolotls have led to the salamander’s near-total collapse, according to population surveys. The axolotl is not the ...