Respiration is based on a gas exchange between the external environment (air, or water for fish) and the cells of the organism.
Fish swallow water through their mouth and let it out through their gills. The gills are fragile and protected from the external environment by the operculum which is an external envelope able to open and close. To breathe, the fish alternates the closing of its mouth and its opercula in order to create a current of water to the gills.
The water swallowed by the fish passes through the different layers of the gills: the gill lamellae. It is at this level that the oxygen is captured and assimilated by the fish: the gill lamellae transport the oxygen-depleted blood, capture the oxygen present in the water and release the carbon dioxide instead.
Do fish release carbon dioxide?
Oxygen is not only present in the air, it is also present in the water! Like us, and like land animals, fish need to take in oxygen (O2). And like us, they release carbon dioxide (CO2) into the outside environment – in this case, water.
How does breathing work in fish underwater?
Breathing simply means taking in oxygen (found in the water or in the air) and sending it to our bloodstream so that all our cells can get it, otherwise they die. Our bones, our muscles, our skin, our heart, our brain, in short our whole body needs it constantly!
We humans breathe in air through our mouth and nose. This air then goes to the lungs, where the oxygen, one of the gases present in the air, passes into our blood (to be more precise, the oxygen passes into the blood at the level of the pulmonary alveoli).
Fish do not breathe outside the water. They must therefore use the oxygen present in the water! Their respiratory system is therefore quite different from ours. They do not have lungs, but gills. These organs are able to filter the oxygen dissolved in the water in order to pass it in their blood.
The gills are very red because they contain many small blood vessels to ensure a maximum of exchanges between the air and the blood. The sheets are made of a very large number of lamellae to optimize the exchange surface. The water will then circulate in the gills which will capture oxygen and send back carbon dioxide (a bit like us when we breathe, we inhale air to capture oxygen and then we blow out carbon dioxide).
Is the breathing of fish comparable to ours?
In the air we breathe there is about 21% of oxygen. So we have to breathe in a lot of air to have enough oxygen to live.
Fish have to make an even greater effort than we do because water is much denser and more viscous than air. In addition, there is less oxygen in water than in air. They must therefore swallow almost 500 liters of water to obtain 1 liter of oxygen. For your information, we need to take in 20 times less to get the same amount of oxygen.
How do fish breathe through their skin?
Many fish, such as trout and goldfish, take in oxygen through their skin, which is called skin breathing. They can absorb up to one third of the oxygen they need in this way.
The skin of fish is covered with a slimy substance called mucus. The mucus has a protective role: it prevents micro-organisms from penetrating the fish’s skin and causing diseases. In order not to damage this protective layer and not to interfere with the breathing, you should avoid touching the fish as much as possible.
The consequence of carbon dioxide for fish in the sea
While some 2.3 billion tons of man-made carbon dioxide dissolve in the world’s oceans every year, carbon dioxide concentrations in seawater will increase further by the end of the century. This poses a real threat to marine fish, whose auditory, olfactory and sensory abilities may be disrupted, preventing them from escaping predators or simply turning in the water.
The concentration of CO2 in seawater affects when it is too high a primordial receptor of the fish brain. A disturbance from which the babies suffer more than their predators, and which modifies the behavior of the fish and their sensory capacities.