Axolotl showing gills with a striped shell

Neoteny

Neoteny refers to animals that retains juvenile features even when they become adults. The most well-known example is the axolotl, a type of salamander that remains tadpole-like all its life, never losing its gills and never leaving the water to live on land. Neoteny is an important feature in evolution: human beings are neotenous primates and insects might be descended from a neotenous millipede-like ancestor.

Animals with this behaviour

Amphibians

BBC News about Neoteny