Question

A fish taken out of water cannot survive for a long time. Why?

Answer 1

Hint:The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The exchange of gases through gills occurs in water.

Complete Answer:
Fishes have gills as their respiratory organ. Some fishes have other organs too but gills are present in a majority of them. Some fishes have a hydrostatic organ, called the swim bladder, which lies in the body cavity just below the kidney and above the stomach. These bladders act as lungs for them. Such fishes are obligate air breathers and will drown if they cannot have access to the surface, even in well-oxygenated water. Fishes with this bladder can regulate the amount of gas in their bladder and can swim in depth. The swim bladder of fishes have the same developmental pattern as the lungs of land vertebrates. Lungfishes have these organs. These air bladders (swim bladders) are present in osteichthyes (bony fish). Osteichthyes have bony endoskeleton. They have four pairs of gills covered with operculum. Not all bony fishes use their air bladder as a respiratory organ. In modern fishes, this swim bladder serves as a purpose to maintain the buoyancy.
The respiratory organs of fish are gills which are richly supplied with blood capillaries. They lie at the sides of the mouth cavity. Gills can absorb the oxygen dissolved in water. Fish take in large amounts of water through their mouth and forcefully push it through their gills. When the water passes through their gills, the dissolved oxygen from the water passes through the walls of gills and blood vessels to enter the blood. $CO_2$ is released out through the gills. The oxygen is distributed throughout the body by heart. Most species of osteichthyes and all species of chondrichthyes use this mechanism. Chondrichthyes have a cartilaginous endoskeleton. They have gills without operculum. Example:-Scoliodon (Dog fish), Trygon (stingray) etc. Fish die when taken out of water because they cannot absorb oxygen from the atmosphere.

Note:There are other methods adopted by some fishes for breathing too, like some fishes breathe through skin, lung fishes are primitive fishes that have lungs for breathing.