Why Don’t Fish Float or Sink?

All animals in the world sink when put into water except fish. Why is that?

Fish are animal which have adapted to life in water. They are able to move in water, of course. Their body density is slightly smaller than the water in which they swim. Because of that, they are like the water itself and they do not sink.

But what about floating? To stop themselves from floating fish use one unique organ called swim bladder (also called air bladder or gas bladder). That is an organ full of air. The organ’s membrane is full of capillaries. The function of the air bladder is helping fish balance themselves. Fish balanced themselves in the water filling and emptying the air bladder. When fish float intentionally, the air from the bladder transfers into the blood and is being released from the gills. The opposite thing happens when fish sink into water. This happens constantly.  

However, some species of fish don’t have air bladder. These fish always have to swim or they would sink. Fish use the same method as airplanes in this case.