Salamanders, a diverse group of amphibians, exhibit various respiratory strategies. Whether they have gills depends on the species and its developmental stage. Many possess gills at some point, while others do not, and some species lose them as they mature.
Salamander Gills: A Lifecycle Perspective
Most salamanders begin as aquatic larvae, nearly all possessing external gills. These feathery structures protrude from the sides of their heads, extracting dissolved oxygen from water. As larvae develop, many undergo metamorphosis, transitioning to a terrestrial or semi-aquatic lifestyle. During this process, external gills are typically reabsorbed as lungs become functional.
Not all salamanders follow this typical developmental pathway. Some species exhibit neoteny, or paedomorphosis, retaining larval characteristics, including external gills, into adulthood. The axolotl (Ambystoma mexicanum) is an obligate neotenic salamander, rarely metamorphosing and keeping its gills throughout its life. The mudpuppy (Necturus maculosus) remains fully aquatic, relying on its large, bushy external gills as an adult. Species like the tiger salamander (Ambystoma tigrinum) show facultative neoteny, where metamorphosis depends on environmental factors such as water availability and temperature.
The Mechanics of Gill Respiration
External gills, characteristic of many larval and neotenic salamanders, are specialized for aquatic respiration. These structures consist of a central stalk with numerous thinly walled filaments, giving them a feathery appearance. This branching increases surface area for gas exchange, where dissolved oxygen diffuses into the bloodstream and carbon dioxide moves into the water.
A dense network of capillaries lies beneath the thin membranes of the gill filaments, facilitating gas transfer. The red or maroon color of gills is due to this blood supply. Salamanders wave their gills to create water currents, increasing oxygenated water flow over surfaces and enhancing gas exchange.
Beyond Gills: Other Ways Salamanders Breathe
Beyond gills, salamanders employ other methods to obtain oxygen, particularly as they mature or adapt to terrestrial environments. All salamanders, even those with gills or lungs, utilize cutaneous respiration through their moist skin. Oxygen diffuses from the surrounding air or water across the thin, permeable skin into the underlying capillaries, while carbon dioxide is released. This method requires the skin to remain consistently moist, making salamanders dependent on damp habitats.
Lungless salamanders, belonging to the family Plethodontidae, rely solely on cutaneous respiration. This is the largest group of salamanders, and all adult members lack both lungs and gills, using their skin and mucous membranes lining their mouths and throats for gas exchange. They possess a dense network of blood vessels close to the skin surface.
Many terrestrial and some semi-aquatic salamanders develop lungs for air breathing after metamorphosis. Unlike mammals, salamanders lack a diaphragm, using a buccal pump mechanism to move air into their lungs. This involves rhythmic movements of the mouth floor: expanding the buccal cavity to draw air in, then compressing it to force air into the lungs. Some salamanders rely on lungs, while others use them supplementally, especially in low dissolved oxygen environments.