A variety of habitats
The two life phases of some species of Caudata (entirely aquatic and semi-terrestrial) have been used to exploit a greater variety of habitats. A Mexican salamander (Ambystoma mexicanum) regularly changes from an aquatic form to a land form. If there is a particularly wet season and/or the lake does not shrink greatly the larval stages are maintained and the larvae may become as big, or bigger than the land-living forms. A lack of iodine in the water may have prevented metamorphosis.
Another species of Caudata has reverted permanently to an aquatic life. It always breeds in a larval condition and its external gills develop into branching bushes on either side of the neck. Using a thyroid extract it can be induced to lose its external gills, develop lungs, and turn into an animal that resembles a burrowing salamander that lives in Florida. However, another species called the mud-puppyNecturus maculosus has reverted irrevocably to water-living and has external feathery gills and very reduced lungs. The Greater Siren (Siren lacertina) is more elongated, has lost its back legs altogether and also breathes using gills. The Three-toed Amphiuma (Amphiuma means tridactylum), from southern USA is extremely elongated with tiny legs that have no function and is known locally as a Congo eel. This tendency to retain larvae characteristics in adult forms is called paedomorphosis.