In a more-extreme form of neoteny, development doesn't progress past the larval stage. One example is the Mexican salamander Ambystoma mexicanum, a species that's currently endangered because it lives in a single Mexican lake that's been heavily affected by human activities.
Salamanders typically progress from an aquatic larval stage to a terrestrial stage, but the Mexican salamander doesn't pass beyond the aquatic stage. The gonads mature, but the rest of the body keeps its larval form. It appears that this form originated through mutation in a thyroid hormone, and individuals can be made to change into a form more typical of adult salamanders if you give them hormone injections. A small mutation results in a dramatic shift in development — in the case of this particular salamander species, resulting in an adult that is entirely aquatic.
These salamanders have a couple of interesting features:
i They have enhanced regenerative abilities. They're able to manufacture replacement body parts to a degree much greater than that of salamanders that reach the terrestrial stage. In fact, if they're forced to metamorphose to the typical adult form via thyroid hormone injections, they lose this regenerative ability. It's the typical "Do I want great power or good looks?" dilemma. Unfortunately, these little critters don't get to decide for themselves. Why? Read on.
1 They're not uncommon pets in America. You'd be surprised at the number of parents who let their kids have creatures with fully mature gonads in a larval body that's able to regenerate larval parts as necessary.
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