If fish can use their gills to breathe oxygen in the water why can't they breathe out of the water?

Question

If fish can use their gills to breathe oxygen in the water why can't they breathe out of the water?

Answer 1

because the gills filter the oxygen from the water and outside of the water the gills do not have water to filter. the gills aren't actually a breathing mechanism.

Answer 2

In water, the gills are loose and seperate allowing a lot of water to flow around the individual strands. The more water that can flow around, the greater the efficiency and ease of the oxygen transfer.

Outside the water, the surface tension between the 'strands' or fibrils of the gills is so great that they clump together, reducing the total amount of gill tissue that is exposed for oxygen transfer to occur.

You can experiment with a mop. In water, it is loose with water flowing around each individual thread. when outside the water, the strands towards the center do not seperate like in the water. They stick together (high surface tension). Since only the outer surface of the gill is in contact with oxygen in the air, there is insufficient oxygen exchanged between the fish and the environment.

Compare also the specialization of cells. Your corneal cells affect your vision when drying of the front of the eye occurs. That is why the body keeps them moist with tears. Think of the cells in the gill strands as specialized to function best in a fluid environment. When drying occurs, they also lose their specialized function.

By deduction, these two reasons interplay to cause suffocation of the fish outside of their aquatic habitat.

Answer 3

They can, provided they can keep the gills wet, which most fishes cannot do. Eels, mud skippers, and walking catfish do a better job at this than most.