i think not all salamander species must undergo metamorphosis like frogs

You are correct. Unlike many frogs where metamorphosis is an obligate process from a tadpole (with gills and tail) to an adult frog (with lungs and limbs), salamanders display a more diverse range of life cycles:

1. Obligate Metamorphosis: In this process, salamanders develop from a larval stage with gills and a tail to a metamorphosed form without tails and with lungs. Most species within the family Salamandridae undergo this typical amphibian metamorphosis.

2. Pedomorphosis (Nueotes): Some salamander species do not undergo metamorphosis; they retain their juvenile features into their adult lives, which is a condition known as paedomorphosis or neoteny. This is observed in the Mexican axolotl (Ambystoma mexicanum).

3. Direct Development: Certain salamander species show direct development, meaning the young resemble the adults from birth, essentially skipping typical metamorphosis. For example, some plethodontid salamanders have embryos that develop external lung buds while still within the egg, enabling them to breathe air upon hatching. These juveniles are much like their adult counterparts in appearance and physiology.

4. Biphasic Metamorphosis: Some other species can either metamorphose into terrestrial forms or remain as aquatic adults. A notable example is the tiger salamander (Ambystoma tigrinum), which in some populations exhibits paedomorphic adult forms as well as a metamorphic form, leading to the existence of both aquatic and terrestrial variants of adult tiger salamanders.

In conclusion, the stages of salamander development are incredibly diverse, ranging from full metamorphosis similar to frogs, to direct development, biphasic metamorphosis, and paedomorphosis. These different life histories are reflective of the wide array of ecological niches salamanders occupy throughout the world.