Life in Water: Vertebrates - Breathing main content.

Life in Water: Vertebrates - Breathing

Part of Hall of Ocean Life.

© Mark J. Thomas / Dembinsky Photo Associates

Fish are among the few vertebrates that can breathe underwater. Like humans, fish need oxygen to survive—but water contains far less oxygen than an equal volume of air. The oxygen that fish breathe is not the oxygen that is chemically combined with hydrogen to form water molecules, but oxygen from the air that is dissolved in the water. To extract the small amount of oxygen dissolved in seawater, fish must force large volumes of water over their gills.