How do fish get oxygen? 

In this blog post, we will answer the question “How do fish get oxygen?” and learn how the fish breathe underwater and how its physiology works, adapted to aquatic life.

Most living organisms on Earth breathe oxygen, either through pulmonary respiration or through other oxygen uptake systems.

In mammals, the respiratory system takes oxygen from the atmosphere and delivers it to the lungs so that it can be transferred to the blood and used by each cell of their body. This type of breathing is common in terrestrial animals, but not very common in aquatic animals. 

How do fish get oxygen?

Fish get their oxygen through processes called branchial breathing. Most fish species have respiration through gills, also called branchia. However, there are some fish that are able to breathe through lungs or with a branchia support system of air vesicles.

Branchial breathing occurs through the gills, which are richly vascularized structures. In the gills, the oxygen present in the water passes into the body and the carbon dioxide that is in the animal’s body passes to the water. This type of breathing occurs in most aquatic non-mammal animals, such as crustaceans, some molluscs and fish.

Before breathing, they need to have oxygen in the water

We know that the water has dissolved oxygen, so aquatic organisms need to have strategies to remove this oxygen from the water and be able to run their gas exchanges and body maintenance.

The drop in oxygen concentrations is among the most common causes of fish mortality, this happens when there are issues that affect the quality of the water. In frozen lakes, the oxygen level in the water drops drastically because of ice. Generally, dissolved oxygen values less than 2 mg/L are dangerous.

Deoxygenation increases the relative population of anaerobic organisms, particularly bacteria, resulting in fish death and other adverse effects. Such effects cause an ecological imbalance, increasing the concentration of anaerobic species at the expense of aerobic ones.

How do fish breathe in water?

The water passing by the gills goes on a unidirectional flow, in which the act of opening the mouth and the operculum causes a flow in only one direction. Entering through the mouth and getting out by the operculum. 

Most fish have skeletal musculature in the oral and opercular cavity, which maintains the frequent pumping of water into the gills, which maintains a regular O2 supplement. 

The gills of fish generally consist of four-gill bundles. These bundles contain rows of gill filaments, each filament has several lamellae, which are structures where gas exchange takes place.

Each branchial filament has two arteries, an afferent vessel, which carries blood to the end of the filament, and an efferent vessel, which ensures the return of blood. On the secondary lamellae, there is a connection between the efferent and afferent vessels. 

Keep swimming!

As water flows between these lamellae in one direction, the blood flows in the opposite direction, this type of flow is called counter current.

The fish need to swim to stimulate the counter-current process, the movement of the fish forces the water to pass through the gills, thus implementing respiration. On the other hand, some species of fish that have the habit of staying near the bottom, use to produce a water suction stimulus with the mouthparts, causing a current through the gills from the mouth.

In this way, when the blood is coming out of the coverslips, it encounters the water from where the oxygen has not been removed yet and, as the water passes through the coverslips, it encounters the blood with a lower oxygen pressure and therefore continues to release more oxygen. After passing completely through the gills, the water may have lost about 80 to 90% of its oxygen content.

In addition to the gills, many species of fish have structures capable of performing gas exchange such as gaseous vesicles, intestine, stomach, oesophagus, among other structures, these animals are called air-breathing fish. 

Most species of air-breathing fish inhabit aquatic environments where, at some point, the oxygen concentration is very low or in hypoxic environments, that is, places where the oxygen level is reduced constantly.

These fish will respond to a decrease in the oxygen concentration in the water by swimming to the surface to suck up air through their mouth, which results in an improved oxygen supply. 

Air-breathing can be optional or mandatory. If the environment is not hypoxic, the animal can get the oxygen necessary for its maintenance from the water, by just pumping the water through the gills.

When the environment is hypoxic, these species get a part of the oxygen necessary for their maintenance from the atmosphere (facultative air-breathing).

Mandatory air-breathing fish are those species that have to rise to the surface to breathe atmospheric air obligatory, these breath through the lungs.

Can fish drown?

If the fish have mandatory air-breathing is impeded from rising to the surface, it will drown.

There are few species of lung-breathing fish, in Australia, Africa, and South America. Some live in calm waters and lakes, where the lack of rain can provoke the total dryness of their habitat. These species hover until the next rainy season when they come out of their cocoons, buried in the mud. The Australian species inhabits lentic rivers and bodies of water, in which they also live during periods of drought.

How to take care of my fish’s breath?

The oxygenation of the fish tank occurs through the gas exchange between the aquarium water and the atmospheric air. 

There is a balance between the amount of gas left in the water after the fish breathing which is called the saturation point. It is very difficult to exceed this saturation point under natural conditions as it depends on high pressures to increase the amount of gas dissolved in the water. One factor that is very important in the amount of dissolved oxygen in the water is temperature, the higher the temperature, the lower the concentration of oxygen in the water.

How to identify that my fish are running out of oxygen?

Fish are likely to be the biggest consumers of oxygen in the tank and are also the fastest to suffer from the effects of lack of oxygen. 

They tend to reduce their metabolism when the oxygen rate is a little lower than ideal and this reduces their immunity and growth rate. When the oxygen rate is too low for the fish, they get very stressed. Fish tend to go near the surface and open their mouths, as near the surface the amount of oxygen in the water is greater than at the bottom of the tank.

How to oxygenate the tank water?

To solve problems such as lack of oxygenation in aquariums it is possible to use an oxygen pump. The air falls through the pump creating bubbles in the aquarium. The bubbles displace water, agitating it and sending carbon dioxide to the surface. For large aquariums, you may need such mechanisms, which are increasingly technological and safer for your fish.

For those who raise fish in small tanks without a pump, the water changes need to be more frequently, to renew the water in the aquarium. In addition, some products help to regulate the oxygen and other chemical compounds in the water.

Conclusion

This post responded and explained how the fish breathe underwater and how its breathing physiology works, adapted to aquatic life. We also saw how to make the water breathable for your fish.

Frequently Asked Questions (FAQs): How do fish get oxygen?

How long do fish go without breathing?

A fish with gill-breathing can stay up to five minutes without breathing.

Why do fish rise to the surface to breathe?

Gill-breathing fish rise to the surface when they need oxygen and cannot find it at the bottom of the tank. However, some fish may have facultative air-breathing or mandatory air-breathing. Thus, they go to the surface to capture atmospheric air.

Do fish from tanks die due to the lack of oxygen?

Yes. Tank fish need oxygen dissolved in the water, or available space on the surface to breathe if facultative air-breathing. However, they could die if this dissolved oxygen runs out, or could not reach the surface.

What happens when the fish is at the bottom of the aquarium?

Numerous reasons can lead fish to stay at the bottom of the aquarium. But the main causes are usually poor water quality, temperature, inadequate pH and chases from other fish.

What happens to fish when a lake freezes?

Fish that live in ponds can survive the winter when the water freezes because they can adapt to drastic climate conditions.

What are the bubbles pump in the aquarium for?

Air pumps are responsible for water oxygenation and also for generating a water flow. With this movement, CO2 is freed into the atmosphere and oxygen gets dissolved in the water for fish breathing.

References 

Hughes, G. M. (1966). The dimensions of fish gills in relation to their function. Journal of Experimental Biology, 45(1), 177-195.

Hughes, G. M., & Morgan, M. (1973). The structure of fish gills in relation to their respiratory function. Biological Reviews, 48(3), 419-475.

Graham, J. B. (Ed.). (1997). Air-breathing fishes: evolution, diversity, and adaptation. Elsevier.

Johansen, K. (1968). Air-breathing fishes. Scientific American, 219(4), 102-111.

Lefevre, S., Wang, T., Jensen, A., Cong, N. V., Huong, D. T. T., Phuong, N. T., & Bayley, M. (2014). Air‐breathing fishes in aquaculture. What can we learn from physiology?. Journal of Fish Biology, 84(3), 705-731.

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