 The major means of respiration in teleosts , as in most other fish , is the transfer of gases over the surface of the gills as water is drawn in through the mouth and pumped out through the gills . Apart from the swim bladder , which contains a small amount of air , the body does not have oxygen reserves , and respiration needs to be continuous over the fish 's life . Some teleosts exploit habitats where the oxygen availability is low , such as stagnant water or wet mud ; they have developed accessory tissues and organs to support gas exchange in these habitats . 
