Toads start out as tadpoles, fully aquatic, living in freshwater. They breathe through their gills by absorbing oxygen from the water. This is called cutaneous respiration. This continues until, in about 16 weeks, they have developed into frogs and now have lungs.
The highly permeable skin of amphibians is a major site of gas exchange in terrestrial, semiaquatic, and aquatic species. Cutaneous respiration accounts for some gas exchange in certain species of reptiles.
Frogs live in and out of bodies of water. They are semi-aquatic and semi-terrestrial creatures. They can breathe through their lungs out of the water and absorb the oxygen through their skin while submerged.
If there is plenty of oxygen underwater frogs can stay submerged for up to 4 months. They are cold-blooded ambush predators and don’t require a lot of energy or oxygen to stay alive. If a frog is underwater and running low on oxygen it must be able to exit or it will drown.
In 2015, scientists Hopkins and Brodie identified 124 frog species able to inhabit saltwater habitats. This is only 1.6% of all known frog species so most do live in freshwater.
Some frogs live on land most of the time only returning to the water to mate and lay eggs. These frogs breathe through their nostrils but if underground or hibernating they can absorb oxygen-rich moisture from the ground.
As mentioned, frogs are cold-blooded, and they need access to cool water or damp spaces to lower their temperatures when they get too hot. Frogs commonly find the nearest pond or moist area, or they bury themselves in damp soil during the day to remain hydrated while keeping cool.