How do fish breathe?
Fish breathe through gills. They take water in through the mouth and push it over the gills, where thin blood vessels absorb dissolved oxygen and release carbon dioxide.
Gills are made of many thin filaments covered in tiny folds, which give them a huge surface area. Blood inside the gills flows in the opposite direction to the water. This countercurrent system lets fish extract up to about 80 percent of the oxygen in the water.
Water holds much less oxygen than air, so fish need a steady flow. Most bony fish pump water using their mouths and the bony flaps covering their gills, called opercula. Some fast swimmers, such as tuna and certain sharks, rely on swimming with their mouths open.
Fish that breathe air
Some fish can also gulp air. Lungfish have lungs and can survive out of water for long periods in mud. Bettas and gouramis have a special labyrinth organ that lets them take air from the surface, so they can live in warm, oxygen-poor ponds.
Did You Know?
Fish can drown, in a sense: if the water lacks enough dissolved oxygen, for example in polluted or overheated water, they suffocate.