Heather has taught high school and college science courses, and has a master's degree in geography-climatology.
What are Bony Fish?
Everyone knows you can't breathe underwater - if you could, you'd be a fish! Yet both humans and fish breathe oxygen. How do fish manage it? Well, today we'll talk about how a large group of fish can extract oxygen from the water.
If you had tuna for lunch or salmon for dinner, or ever had a pet goldfish, you have encountered a member of the class Osteichthyes. Greek for bone fish, Osteichthyes includes all bony fishes, and given the diversity of animals found in the world's oceans, it should come as no surprise that it is the largest of all vertebrate classes. There are nearly 28,000 members of Osteichthyes currently found on Earth, and numerous extinct species found in fossil form.
Let's explore how these fishes breathe.
What Does a Respiratory System Do?
All living organisms have body systems, which are specific organs or groups of organs designed to do a specific job. In the case of the respiratory system, its job is to allow for respiration, or gas exchange, between the inside of the animal's body and the outside world.
In humans, this exchanges takes the form of breathing in oxygen and breathing out carbon dioxide. Fish too breathe in oxygen and breathe out carbon dioxide. But how?
The Osteichthyes Respiratory System
Unlike humans, who live on land among oxygen-rich air, fish live in saltwater seas or bodies of freshwater. With the exception of some who are able to breathe at the surface, the majority of fish get the oxygen they need from the water they live in. Water contains dissolved molecules of oxygen that the fish are able to use for respiration, just as humans are able to use the oxygen in the air.
Fish have a very specific organ in order to extract oxygen from the water, as well as expel carbon dioxide back into it. This organ is called the gills. When fish take in water through their mouths, the outer cover of their gills, called an operculum, closes, to help keep that water in their bodies.
The swallowed water passes into their gill area, where it comes in contact with the gill filaments. Gill filaments are folds in the gill tissue that help to increase the surface area of the gills, allowing more of the gill structure to come in contact with the water. The more of the gills that can touch the water, the greater the odds of extracting oxygen.
Inside the filaments are capillaries, a type of blood vessel, which allow the absorbed oxygen to move directly into the fish's blood stream. To exhale the carbon dioxide found in their bodies, fish simply open their operculum back up and the carbon dioxide molecules are expelled back into the water.
Once the oxygen is inside the body, it can travel around in the blood stream exactly as it would in a human. So in many ways, you have far more in common with your pet goldfish than you might have imagined!
How Do Fish Breathe at the Surface?
If you have ever had a pet betta (sometimes called the Siamese fighting fish), you probably have seen it come to the surface to breathe. While it is true that they have gills and are able to follow the respiration process we just read about, it is also possible for some species of bony fish to breathe surface air.
Bettas are part of an order of fish known as anabatoids, which have a special organ called the 'labyrinth' for processing surface air.
There is also a prehistoric subclass of fish known as lungfish, which have over the centuries developed a primitive lung structure that allows them to not only breathe air but also become slightly amphibious and survive out of water for periods of time.
The class Osteichthyes is the largest class of vertebrates, with around 28,000 members. All members of the class are bony fishes, residing in both fresh and saltwater conditions. Regardless of the environment they live in, all bony fishes have a respiratory system.
Their respiratory system is used for exchanging gases (carbon dioxide and oxygen), just as it is in humans. But instead of lungs, bony fishes have an organ called gills. Water is swallowed by the fish, passed over the gills, and then interacts with the gill filaments, which help the fish to extract dissolved oxygen from the water. After extracting oxygen, the operculum opens back up to allow waste products like carbon dioxide to be removed.
Despite the fact that their respiration takes place in water, the Osteichthyes respiratory system is very similar to the respiratory system of humans. There are also a few varieties of bony fish, notably anabatoids and lungfish that have special adaptations that allow them to breathe surface air.
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