How many aquatic animals breathe air

That is why fish cannot breathe on land

It is actually strange that fish cannot breathe on land. The reason for this is surprising

Sure, they don't have lungs like humans do, but gills. But "oxygen can certainly be absorbed from the air through the gills, some fish do that too if necessary," says Jörn Geßner from the Leibniz Institute for Freshwater Ecology and Inland Fisheries. In principle, this would even be quite efficient for the fish, because the air we breathe has a much higher proportion of oxygen than water. Then why don't fish do that?

This has to do with the fact that fish in the water not only absorb oxygen with their gills, but also get rid of metabolic end products such as carbon dioxide and nitrogen-containing ammonia. The fish inhales and exhales with one gulp, refreshing its blood and removing residues from it at the same time.

“The eels migrate overland. . . "

In the dry air, on the other hand, the water-soluble ammonia can only leave the fish body via the gills to a very limited extent. So limited that the substance would poison the organism. But it usually doesn't get that far, because the wafer-thin membranes of the gill lamellae dry out so quickly in the air that the gas exchange of oxygen and carbon dioxide decreases rapidly. The fish suffocates.

And if someone objects: “The eels migrate overland. . . ”, Then he's right, but that's another story: Eels manage to supply themselves with oxygen through their skin for a few hours when it is damp and cool enough. Like mammals, they then excrete their nitrogen as urea - but via their gills and mucous membrane.