It has gills so it can breathe underwater. I hope this helps. In fact, I actually asked this question on Answers.com and when I couldn't find it, I was annoyed so I went on line to a different sight and found this simply good answer.
Fish Use Gills Instead of lungs. Gills Help fish to Breath. Fish Use Gills Instead of lungs. Gills Help fish to Breath.
All fishes are adapted to live in water because they dont have lungs they have gills which means that lungs work with air and gills work with water. Have you ever thought does a fish dies when taken out of the water it is because gills are the only way a fish breathes through. Gills work when a current of water slide over them so when the fish is taken out of the water, the gills stop working because the current of water didnt slid over it. They have wide eyes to see predators swimming around them.
fishes uses the oxygen resolved in water.
Lungs are what air breathing creatures use. That is how they evolved. Gills need to have water flowing over them so they work in water. So Lungs won't work in water and Gills won't work in air.
Its a simple closed circuit system. The blood flows through the fish body passing through the gills where the gills siphon out dissolved oxygen from the water. The oxygenated blood then flows through the fish expending the oxygen and picking up carbon dioxide and expelling it from the gills and picks up more oxygen.
Fish breathe using gills. Gas exchange occurs in a structure of the gills called the gill fillaments. Blood in the fillaments flows in the opposite direction of water, this is called a counter current system. Some fish will swim against water currents to increase the amount of oxygen they are getting. There are also some fish that, in addition to the gills, can gulp air and the oxygen diffuses in the fishes swim bladder.
Gills are designed to absorb oxygen only from water, and can only work if water is passing through them or they will collapse. When a fish is taken out of the water, the oxygen-absorbing filaments of the gills flatten out and the fish suffocates because it's gills are not made to absorb oxygen directly from air.The simple answer is that gills colapse when removed from water as they are adapted to water which is more dense than air.
There are two sea creatures commonly referred to as dolphins. One is the porpoise, which is a mammal and as such breathes air. Because they breathe air, they do not have gills.The other dolphin is a fish. Technical name: Coryphaena hippurus. It is also called Mahi-Mahi (Hawaiian) and Dorado. This is a true fish, with gills. It cannot breathe air so its gills do not work outside of water.
The fish hasgills that they use to filter from the water the oxygen they need to as you say "breath" ,so when you put them on land there gills are not working anymore by filtering the water for oxygen because there is no water it's as you know just air (oxygen) so there gills are still trying to work but there is no water to filter oxygen for them.
Fish gills are respiratory organs that allow fish to extract oxygen from water. They are made up of thin filaments that are highly vascularized, allowing for efficient exchange of gases. As water flows over the gill filaments, oxygen is absorbed and carbon dioxide is released.
to protect the gills
It has a pocket of air that it compreses to sink and float. The continuos water passing the gills is its form of Braeathing. Also magic.