Fish need oxygen too, and they’ve got a clever way of getting it
Whether they live on land or underwater, animals need oxygen so that their cells can function properly. The lungs of land-dwelling mammals can’t cope with fluid and air is taken in directly, but fish are surrounded by water and so have evolved an alternative way of obtaining the oxygen their bodies need.
Fish have gills, which are feathery organs full of thousands of blood vessels (capillaries) located on either side of the body. As a fish swims, it takes water in through its mouth and then forces it out of the body through the gills. As it passes the walls of the gills, oxygen dissolved in the water transfers into the blood via the blood vessels, thereby supplying their body. The shape of the gills maximises the surface area that water can pass over, so that as much oxygen as possible enters the bloodstream.
Bony fish (which make up most of the living fish species) have a series of bones called the operculum that protects the gills and changes the pressure as a fish opens and closes its mouth to keep water flowing through the respiratory system. Other fish, like sharks and tuna, lack an operculum and so must swim constantly to keep water moving across their gills – if they stop moving then their cells will be starved of oxygen.