Respiration is a very precise process, especially in mammals where the lung must be dry and free from fluids to work properly. When we breathe, the tiny air sacks in our lungs absorb the oxygen from the air and carry it to the cells in our body. However, there are some living organisms that cannot breathe the same way that mammals do because they do not have lungs, such as fish. If fish have a respiratory system similar to that of mammals, a single breath underwater will fill its lungs with fluids, making them useless. Still, fish need to extract the dissolved oxygen from the water to breathe, so they depend on special organs known as "gills", which are feathery organs filled with blood vessels.
Fish breathe by bringing water into their mouths, then releasing it through gill passages. When water passes over the thin gill walls, the dissolved oxygen moves into the blood. To increase the efficiency of the oxygen intake, blood passes through the gills and is pumped into the opposite direction of water flow over these gills, then moves to the fish cells. Fish are not the only organisms that breathe through gills; there are other organisms that breathe the same way as fish do, such as frogs. Nevertheless, are there any fish that breathe in a way other than gills?
Most major fish groups breathe through gills; some groups breathe through the skin, but breathing through the skin is difficult in stagnant water with low oxygen. That is because gas exchange through the skin in water is restricted by the same physical conditions as on any other respiratory surface. Some animals other than fish breathe through the skin, such as the water salamander. However, are there any fish that breathe through lungs?
Whales and dolphins breathe through the lungs, but they are classified as mammals, not fish. Nonetheless, some fish breathe through the lungs, such as the Australian lungfish, which uses gills and lungs, and the African lungfish. Lungfish can float on the surface of the water, breathe, and survive when other fish cannot breathe because of the lack of oxygen in the water. Nevertheless, this kind of fish needs to compulsively float on the surface of the water on a regular basis to breathe; otherwise, it will die. An amusing example of lungfish is the Granddad fish, which lived more than 80 years at the Shedd Aquarium in Chicago, and was seen by more than 104 million visitors; unfortunately, it died a few years ago.
There are some types of fish that breathe in more than one way, and likewise in animals. Frogs, for example, breathe through gills, skin, and lungs. How does the dissolved oxygen needed for the aquatic organisms exist in rivers, seas, and oceans?
Seas and rivers are saturated with oxygen in many ways, including deployment, especially with the existence of wind, which moves the water and creates waves. That is in addition to algae and aquatic plants that carry out photosynthesis and produce oxygen, which dissolves in water and is consumed by fish and other aquatic organisms. Fish cannot survive for a long time if the dissolved oxygen is less than 5 mg/liter. Pollution, high temperatures, and dumping waste in water reduce the prevalence of dissolved oxygen in the water, which may result in the death of fish.
References
allyouneedisbiology.wordpress.com
petmd.com
qmul.ac.uk
sciencedirect.com (1)
sciencedirect.com (2)
sheddaquarium.org
wonderopolis.org