How Do Fish Breathe Underwater?

Fish have a unique and fascinating way to breathe underwater; discover how these amazing creatures survive in their underwater world.

Aug 13, 2023By Lauren Rey
how do fish breathe underwater

Uniquely adapted for aquatic life, fish have a fascinating way to breathe underwater that involves specialized organs that evolved hundreds of millions of years ago. While fish have adapted to life underwater since long before dinosaurs even roamed the earth, most people aren’t aware of how the extraordinary process works. Here’s a deep dive into how fish breathe underwater!

The Fascinating World of Fish

school of fish
Photo Credit: Getty Images/Canva

Fish are one of the most fascinating creatures. There are approximately 34,000 species found in waters around the world, in both salt and freshwater. Fish are also one of the few groups of animals found in every ocean and on every continent. Even the icy waters of Antarctica have thriving populations of fish.

It is believed that fish were among the first groups of vertebrates to appear on Earth over 530 million years ago. Since that time, fish have undergone an extraordinary evolutionary process, uniquely adapting each species to its individual underwater environment. One of the most fascinating adaptations of fish is their unique ability to breathe underwater.

The Fish Respiratory System

fish breathing
Photo Credit: Getty Images/Canva

One of the most fascinating things about fish is not only that they breathe underwater, but how they breathe. Unlike land animals that have lungs, fish have developed a unique respiratory system that enables them to intake oxygen from the water. This includes a process of drawing in water, extracting the dissolved oxygen, then releasing carbon dioxide back into the water. Similar to how humans and other land animals breathe in oxygen from the atmosphere and release carbon dioxide as they exhale.

The adaptation that allows fish to breathe underwater and complete this extraordinary respiratory process is due to specialized organs known as gills.

All About Gills

koi fish
Photo Credit: Welcomia/Canva

Instead of lungs, fish have gills. These specialized organs are how fish became uniquely adapted to extract oxygen from the water. Fish gills are located on the sides of their heads underneath a protective covering known as the operculum that opens and closes to allow water to pass.

When fish open their mouths, water flows over the gills and comes into contact with the gill filaments, which contain thousands of branchlike structures called lamellae. These lamellae hold a network of capillaries or tiny blood vessels. As the water passes through, it diffuses the oxygen and delivers it into the bloodstream. Carbon dioxide is then pushed out into the surrounding water. This remarkable process allows fish to breathe underwater.

Fish are not the only aquatic animals with gills. Crustaceans such as crabs and lobsters also have gills, as do starfish, squid, mollusks, and others. Sharks, which are in the family of cartilaginous fish, also respirate via gills. Aquatic salamanders like axolotls that live in freshwater also have gills.

Other Types of Breathing Adaptations

betta fish
Photo Credit: Getty Images/Canva

Like everywhere in nature, certain species have developed additional adaptations to aid in their survival. To supplement gill respiration, certain species of fish also possess a secondary respiratory organ known as a labyrinth. This organ allows the fish to more efficiently process oxygen taken in from the mouth at the surface of the water in low-oxygen conditions.

This group of fish, known as labyrinth fish or gouramis, are typically found in shallow freshwaters like ponds, streams, and rice paddies. One of the most well-known examples of a labyrinth fish is the betta fish which is native to Asia but commonly found in aquariums across the world.

Another amazingly adapted creature, the mudskipper, can spend up to a few days out of water by switching to cutaneous respiration, or absorbing oxygen through the skin. Once back in the water, the mudskipper will once again use its gills to respirate.

Dropping Water Oxygen Levels

fish school
Photo Credit: Getty Images/Canva

Sadly, just as air quality in certain areas can be affected by pollution and ecological disasters, so can the oxygen levels in the water. When oxygen levels in the water are low, fish will congregate at the surface to try to get as much oxygen as possible. If they can’t find enough oxygen, they will head to other waters.

When oxygen levels are continuously low or become increasingly polluted or filled with algal blooms that consume oxygen and contribute to hypoxia, these areas turn into dead zones. These zones become uninhabitable for fish and other aquatic life. One of the most well-known and frequently studied dead zones in the United States is along the Gulf Coast. It is believed that pollutants carried by the outflow of the Mississippi River are at least partially responsible for the reduced oxygen in this area.

Scientists have warned that these dropping oxygen levels can lead to abnormal fish migration patterns that can affect entire ocean ecosystems. Certain fish species will face an uncertain future while others take over and thrive in high numbers not commonly seen in concentrated areas. This can all lead to a decrease in biodiversity and have ripple effects throughout the entire ocean food chain.

Lauren Rey
By Lauren Rey

A lover of all animals, Lauren’s background is in the veterinary world, but she is now a content writer on travel, wildlife, and all things pets! She’s based in Florida, but when not writing, she’s usually plotting out a new road trip route with her partner-in-crime. Pickles is a mixed-breed rescue dog that loves hiking, road trips, and Starbucks just as much as her mom does!