Not all fish species rely entirely on their gills. Some, especially when they are young, absorb a large proportion of the oxygen they need through their skin. In a few other species, the air bladder, which most modern fish use as a ballast organ to control their depth, is specialized as an accessory breathing organ or lung. These fish are obligate air breathers and will drown if they cannot breathe air, even in well-oxygenated water. The air bladder, not gills, is the structure that gave rise to the evolution of the lung and follows the same developmental pattern as the lungs of land vertebrates.
Fish are the most diverse vertebrate group and make up about half of all known vertebrate species.
More fish species are being discovered at a rate of 200 to 300 a year. All Rights Reserved . No images, graphics, scripts, or applets may be reproduced or used in any manner without permission from the copyright holders.