How do gills work & how do fish breathe underwater?

The marine world is truly fascinating, and there are always new facts to discover. Knowledge is important, and gaining more of it will be helpful. Fish and humans are different in many ways, and that includes the respiratory system as well. 

While the process is simple to understand in humans since we do it constantly, fishes use gills for breathing underwater. While they spend all their lives under the water, they still need oxygen to survive, just like we do. 

But instead of breathing pure air, they extract oxygen from the water. Gills are intended for this purpose, and fish have developed them through millions of years of evolution. In this article, we will talk about how gills work and how fish breathe underwater. Continue reading if you want to expand your oceanographic knowledge. 


Fish breathe underwater with the help of gills. Gills work by pulling in water from the ocean and extracting its oxygen content through the capillaries.

Here are various topics we’ll cover:

  • Whales
  • Crabs
  • Dolphins
  • Turtles

Do fish need air to live?

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We know that humans cannot live without oxygen, but you might be surprised to find out that fish also need it. Every species in the world needs oxygen, and this rule also applies to plants. But while humans get the oxygen from the air, fish will have to gather it from the water by using their gills.

The fish’s breathing is seriously affected if the water contains low oxygen levels. The entire aquatic life will have a problem without oxygen, which is necessary for all life. If you have fish in your aquarium at home, make sure to create ideal conditions for them.

In nature, fish will not struggle to find oxygen. Where fauna and flora thrive underwater, it means there is a high oxygen concentration in that area. 

How do gills work?

The gills are organs located on the side of their heads. They have several blood vessels called capillaries, and they are very small in size. It is easy to understand the breathing process if you think about it logically. When the fish opens its mouth, water will run over these gills, and blood will pick up the oxygen through the capillaries.

Oxygenated blood will move through the body to deliver oxygen, which is the same in humans. While they are commonly found on fish, gills are not exclusive to them. You can also find some mollusks and crustaceans that have gills.

They work in the same way, and the purpose of oxygenating the blood is the ultimate goal. To breathe properly, the creatures under the water need to run a lot of water over their gills. 

Gills consist of 3 main parts:

  • Filaments
  • Rakers
  • Arches
Type of GillsDescriptionExamples of Animals with These Gills
External gillsGills that are located outside of the body and exposed to the surrounding water or air.Larval amphibians, some aquatic insects
Internal gillsGills are located inside the body and covered by a protective structure.Fish, crustaceans, mollusks

Can fish breathe out of water?

Because their respiratory system allows them to live under the water their entire life, fish do not have to come to the surface frequently to get the oxygen they need. While they also breathe oxygen just like humans do, they extract it from the water they are swimming in with the help of gills.

Some species can also breathe on land, where they take oxygen from the air. But most fish will suffocate and die when they are out of the water as their gills collapse. Their blood vessels are no longer exposed to oxygen in the air, so they should not spend a long time outside the water.


Unlike other species, whales do not have gills, but they still have a complex respiratory system. Whales are born with blowholes, and they cannot breathe underwater. They have to come to the surface often, or they risk drowning.

Because whales are rather marine mammals than fish, they have lungs and nostrils. It means they cannot extract oxygen from the water, and they breathe by inhaling air through their blowhole when they reach the water’s surface.

We are all familiar with how this process is done. The videos showing whales blowing water through their blowhole shows how they are exhaling. Their trachea and esophagus are disconnected, so they cannot breathe through their mouth. 


Because they are not similar to fish or whales, it is important to know how crabs actually breathe. You can see them both in water and on land, and their respiratory system has evolved to adapt to both environments. Crabs have gills that are located under their body near the first pair of legs. 

Under the water, they can extract oxygen from the water with these gills. The process is similar to the one in fish, where blood vessels capture oxygen and pass it into the bloodstream. 

When they are walking on land, crabs can seal their gills and keep them moist, so they don’t suffer the same fate fish do when they are outside the water. While some types of crabs can stay on the water for a long time, others cannot survive on land. 


Because they are mammals, dolphins cannot breathe through their gills. They are more similar to whales than fish, as they have lungs and a more complex respiratory system. To stay alive and avoid drowning, dolphins must make frequent trips to the water’s surface.

The blowhole at the top of their head is like a nose, and because of its position, the dolphin has no struggle in surfacing for air. There are 75 species of dolphins, and not a single one of them features gills. Even though they live in the ocean, these creatures breathe similarly to how humans do.

A dolphin will have no problem holding its breath between 8 to 10 minutes but must resurface frequently. If they cannot get enough air again, they risk drowning. 

Animals with gillsMisconceived animals without gills
LampreysSea lions


Turtles do not feature gills on their body because they are considered to be reptiles. And just like other reptiles, turtles have lungs and a respiratory system different from those commonly found in fish. Their lungs are located under the carapace to protect them as it is an important system in their body.

Turtles breathe by making movements of their muscles, and you can often see them rocking their shoulders when they are on land. This movement helps them breathe effectively simply by changing the pressure inside the lungs.

When they are under the water, sea turtles can hold their breath for up to one hour. However, they only dive for 5 minutes and resurface to breathe for a few seconds regularly. 

AnimalPosition of Gills
FishOn the sides of the head, protected by a bony plate called an operculum
Amphibians (larval stage)Along the sides of the body
Crustaceans (e.g. lobsters, crabs)On the sides of the body, protected by a carapace
Mollusks (e.g. clams, oysters, snails)On the sides of the body, protected by a gill chamber
Echinoderms (e.g. sea urchins, starfish)On the surface of the body, in the form of tiny tube feet
Tunicates (e.g. sea squirts)In an internal sac called a pharynx
HagfishesOn the roof of the mouth
LampreysBehind the eye and on the sides of the head
Sharks and raysIn between the gill slits located on the sides of the body
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