are there any special salamander species usually don't undergo metamorphosis

Yes, there are a few unique salamander species that do not undergo metamorphosis and retain larval features into adulthood. These are typically referred to as neotenic or paedomorphic salamanders. Here are the main examples: 1. **Mexican Axolotl (Ambystoma mexicanum)**: - As mentioned earlier, axolotls maintain their gills and external morphological juvenile characteristics throughout their lives, living entirely in aquatic environments. 2. **Alpine Salamander (Salamandra atra complex)**: - Particular populations of alpine salamanders in Europe retain certain juvenile traits such as tails with fins even in adulthood, although they typically go through partial metamorphosis. 3. **Thorius Genus**: - Several species within this genus from Mexico and Guatemala may exhibit paedomorphic features, although information on this group is more limited compared to the axolotl. 4. **Some Cryptobranchidae Family Species**: - Members of this family, like the Chinese giant salamander (Andrias davidianus) and the Japanese giant salamander (Cryptobranchus japonicus), display metopomic metamorphosis, a different form that still retains gills for respiration into adulthood. This allows them to live subterraneously or submerge themselves in water without the need to breathe air frequently. These exceptions among salamanders exhibit neoteny, maintaining critical life-cycle stages which include the development of traits that facilitate efficient life in their specific environments. This adaptation can be an evolutionary result of factors such as habitat-specific pressures and resource availability. The preservation of larval traits in adults, whether it's gills or other physical attributes, provides survival advantages in specific ecological niches, where adults remain primarily or perpetually aquatic.