Neoteny is defined as the failure or delay of larva to metamorphose while becoming sexually mature. It is character of some amphibians.
The best example is axolotl larva of Amblystoma. It is aquatic. It has gills. It develops gonads. It lays eggs and attains large size. Amblystoma on the other hand, is terrestrial and without gills. It was considered the axtolotl as a separate genera in the beginning, with the administration of thyroxine, axtlotl lost its gills and develops lungs. It metamorphosed into the adult. Metamorphosis of axolotl can be induced by reducing the water level.
Proteus. and Necturus are permanently neotenous forms. They retained the larval features and reproduce sexually like a mature animal.
Kollman has distinguished two types of neoteny:
- Partial neoteny: Tadpoles of Hyla arborea, Rana escülenta during winter will show simple retardation of metamorphosis beyond the normal period.
- Total neoteny: It will retain its gills and becomes sexually mature.
Formerly full neotenic forms with external gills were considered as most primitive amphibians. But now it is believed that it is the result of secondary adaptive modifications in their larval stage.