* Skin Breathing: Some fish, especially smaller ones, can absorb oxygen directly through their skin. This is particularly important in environments with low oxygen levels like mud or stagnant water.
* Lungs: Lungfish are a group of fish that have evolved lungs, allowing them to breathe air in addition to using their gills. This adaptation allows them to survive in oxygen-poor water or even out of water for short periods.
* Buccal Cavity: Some fish use their mouth cavity as a sort of "lung" to breathe air. They take in air through their mouth and then push it over their gills to absorb oxygen. This is common in species like the walking catfish and the African lungfish.
* Intestine: Some species, like the common carp, can absorb some oxygen through their intestines.
* Accessory Breathing Organs: Some fish have evolved specialized structures that help them breathe in low-oxygen environments. For example, climbing perch have modified gills that allow them to breathe air.
It's important to note that while these methods can supplement gills, they are not as efficient and cannot fully replace the function of gills in most fish.