Respiration Facts: Animals That Breathe Through Gills

Written by Sarah Psaradelis
Published: December 1, 2022
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Some animals don’t breathe through their lungs as we humans do. Instead, they breathe through an organ known as the gills. Many animals respire through their gills to live and breathe in an aquatic environment. Without gills, many species of aquatic animals would not be able to survive.

Even though there are aquatic animals that don’t breathe through a set of gills, a large percentage do. The way gills work is quite fascinating, and learning more about how animals can respire through gills can give us a better understanding of our aquatic friends.

What Are Gills and How Do They Work?

A shoal (school) of glue/silver striped bass swimming in an aquarium with a background that is very blue on the left side of the frame, and almost black on the right.

The gills’ main function is to supply the animal with oxygen underwater.


The gills’ main function is to supply the animal with oxygen underwater. Animals that live underwater do not breathe the same way animals on land do because of the different environments. Gills allow the animal to breathe underwater without drowning, and it works quite differently than lungs, even though both have the same purpose.

Gills are the animal’s respiratory organ, and it allows for different gases to be exchanged through the animal’s body. Gills look like a pair of flaps on fish, but they can appear differently in other types of animals. A thin epithelial layer covers the gills for protection, and gills contain comb-like filaments known as lamellae, which help to increase the surface area of the gills.

There are usually more sets of filaments in gills because gill-breathing animals need to work harder to take in enough dissolved oxygen from the water.

So, how do gills work?

Respiration Through Gills

Gills work by taking in oxygen from the water that enters the animal’s body, usually through the mouth. The water is then processed, and oxygen is absorbed into the animal’s bloodstream through absorption surfaces (lamellae).

The gills then filter the oxygen from the water into the animals’ tissues and blood for cellular respiration. Once the gills have filtered the oxygen, the carbon dioxide is expelled from the gills, which will open and release it into the water. This is a continuous process for the animal.

In animals such as fish, respiration happens when they open their mouth to draw in water. The water then flows through the mouth and over the gills network and then out behind the opercular valve that is located behind the gills.

Which Animals Have Gills and Which Don’t?

Are Blue Whales Endangered - A Blue Whale Near the Surface

Whales do not have gills; they breathe oxygen from the surface of the water.

©Andrew Sutton/

Interestingly, even if an animal lives underwater, it doesn’t always mean that they use gills to breathe. There are complex animals that use other methods to survive underwater. However, most aquatic life uses gills to breathe in the water.

Let’s look at some examples…

Aquatic Animals With Gills

  • Fish
  • Amphibians
  • Certain insects
  • Mollusks
  • Crustaceans

Many animals rely on gills for respiration. The gills will be lined with surfaces that absorb the oxygen in the water once it passes through an opening of the animal, and then excess waste from the respiratory process will be expelled into the water.

Aquatic Animals Without Gills

  • Whales
  • Mudskippers
  • Dolphins

There are also some animals that do not breathe through gills, even though they live underwater. Whales do not breathe through gills, even though they live in water. Instead, they come to the surface of the water to breathe oxygen.

Some fish can also breathe through their skin, which is seen in some eels that use a buccal cavity to breathe air. Some species of catfish can also absorb air right into their digestive tracts to get a direct source of oxygen.

Dolphins, just like whales, also lack a pair of gills. Whales and dolphins, both mammals, breathe oxygen from the surface of the water.

Animals That Can Breathe on Both Land and Underwater

  • Frogs
  • Toads
  • Newts
  • Salamanders
  • Hermit crabs
  • Water scorpions
  • Mosquito larvae

These animals can breathe on land and in the water through complex respiratory systems. Certain species of frogs can breathe through their skin. Tadpoles will have gills that they lose as they mature and turn into lungs to help them breathe on land.

With crabs, they use their gills located below their carapace to take in oxygen from either the moisture in the air or from water. They draw in water over their scaphognathite appendage, and then the water is released from their gills. Crabs have articulating plates that allow them to breathe on land. These plates seal in the water in their gills which are kept moist. The mixture of airflow and moisture on the crabs’ gills causes bubbles to release from the crab’s mouth, thus allowing them to breathe on land too.

Fascinating Facts About Gill Respiration in Animals


Gills have a high surface area so that enough oxygen can be taken in.


  1. Gills can extract the dissolved oxygen in the water and excrete carbon dioxide and respiratory waste into the water as the animal opens and closes the gill flaps and mouth.
  2. Animals can either have gills that are located on the outside of the body, such as those seen in amphibians, or they can be located inside the body, which can be seen in fish.
  3. The filaments in the gills have more mass than that of the lungs in a terrestrial animal.
  4. Unlike humans who breathe air in and out, gills are only a one-way passage. The water flows into the mouth of fish, where the dissolved oxygen from the water enters their bloodstream, and then the respiratory waste is released from the gills.
  5. Not all gill-breathing animals have the same set of gills. Some animals have different types of gills that have adapted to suit their survival needs.
  6. The concentration of oxygen in the air is around 200,000 parts per million, whereas the gas diffusion in water is only at a ratio of 4:8 parts per million, which means animals with gills need to take in much more water to get the same amount of oxygen as we would by breathing in air.
  7. Gills are more like a ventilation system, as oxygen is only extracted once it passes over the gills.
  8. Gills have a high surface area so that enough oxygen can be taken in.

Gill vs. Lung Respiration

Respiratory organ of aquatic animals.Respiratory organ of terrestrial animals.
Enables the animal to breathe dissolved oxygen from the water.Allows animals to breathe oxygen from the air.
Consists of a row of filaments that water flows through.Two sacs connected to breathing tubes.
Found on either the inside or outside of an animal’s body.Found only on the inside of an animal’s body.
The gases diffuse between the capillaries and the water.The gases diffuse between the inhaled air and the capillaries.

Gills can be compared to lungs because they have the same purpose – to secrete oxygen and excrete carbon dioxide. This allows the animal with lungs or gills to breathe for them to live.

The main difference between these two types of respiration is that gills allow the animal to breathe underwater, while lungs allow the animal to breathe air.


The vast differences in many animals’ respiratory systems are interesting, but gills seem to be the most complex. From animals that use gills to live on land and underwater, as seen in crabs, to the simpler gill function in fish, gills have many functions that can benefit the animals and allow them to survive in environments we, as humans, cannot.

The photo featured at the top of this post is © Pavaphon Supanantananont/

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About the Author

Sarah is a writer at A-Z Animals primarily covering aquatic pets, rodents, tarantulas, and reptiles. Sarah has over 3 years' experience on writing and researching various animal topics. She is currently working towards furthering her studies in the animal field. A resident of South Africa, Sarah enjoys writing alongside her pets and almost always has her rats perched on her shoulders.

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