do some neotenic salamanders have external gills

Yes, some neotenic salamanders, particularly those that are obligate aquatic species, retain their external gills as adults. These are gills that develop outside the body and are used for respiration in water. One of the most famous examples is the Axolotl (Ambystoma mexicanum), which is renowned for its distinctive, feathery external gills.

Adult neotenic salamanders with external gills typically do not undergo metamorphosis and maintain an aquatic lifestyle throughout their life. In addition to Axolotls, here are a few other examples of neotenic salamanders that retain their external gills:

1. Ambystoma opacum (Marmbled Salamander): Some populations of Marbled Salamanders retain their external gills in adulthood, although they may be less developed than those of Axolotls.
2. Dicamptodon spp. (Lungless Salamanders): While these salamanders have internal gills as larvae, some species retain gill-like structures into adulthood, although they are adapted for use in water.
3. Necturus spp. (Mudpuppies or Water Dogs): These salamanders have external gills as juveniles and some may retain reduced or vestigial gills into adulthood.

These salamanders use their external gills to extract oxygen directly from the water, which is a more efficient method of respiration than using lungs in an aquatic environment. The presence of external gills is an adaptation that allows these neotenic salamanders to thrive in their aquatic habitats without needing to transition to a terrestrial lifestyle as other salamanders do through metamorphosis.