The great white shark (Charcharodon carcharias) keeps water flowing through its gills by swimming with its mouth open. By staying in constant motion, it avoids having to pump water over its gills. © 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.

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Gills contain many filaments, each with thousands of tiny folds called lamellae, which greatly increase the surface area that comes into contact with water. Dissolved oxygen in seawater passes through thin membranes in the lamellae and enters the fish's blood, and carbon dioxide is eliminated. Inside the lamellae, blood flows in the opposite direction to the moving water—a counter-current system that makes gas exchange extremely efficient. About 75 percent of the oxygen passing through the gills is extracted, twice the percentage of oxygen that our lungs remove from a breath of air.

Other Ways of Breathing
The tarpon supplements its gill-breathing by using its gas bladder like a lung. A tarpon will actually drown if it cannot reach the surface periodically to gulp in air.

	COMMON MUDSKIPPER Periophthalmus kalolo

Whales and other ocean mammals lack gills. Instead, they come to the surface to breathe oxygen-rich air with their lungs.

Mudskippers climb out of the water for long periods by carrying a supply of water in a pouch around their gills. Like most fishes, they can also breathe oxygen through their skin, even underwater.