The air-breathing lungs of mammals, including humans, must be dry and empty of fluids to work properly. When we take a breath, tiny air sacs in our lungs pull oxygen out of the air and carry it to our bodies’ cells.

The lungs of mammals would not work very well for a fish, because one breath underwater would fill them with fluid and make them useless. Nonetheless, fish need oxygen to breathe, too. In order to remove oxygen from the water, they rely on special organs called “gills.”

Gills are feathery organs full of blood vessels. A fish breathes by taking water into its mouth and forcing it out through the gill passages. As water passes over the thin walls of the gills, dissolved oxygen moves into the blood and travels to the fish’s cells.

If fish can breathe underwater, then why do some fish, like dolphins and whales, swim to the surface of the ocean? Because dolphins and whales aren’t fish at all! They are mammals, just like humans.

Dolphins and whales are similar to humans in many ways: They give birth to live babies instead of laying eggs, are warm-blooded and have lungs for breathing air. When a whale or dolphin surfaces, it breathes air through its nose (commonly called a “blowhole”) on the top of its head.

Fish facts:

  • There are more species of fish than all the species of amphibians, reptiles, birds and mammals combined.
  • Fish have been on the earth for more than 450 million years.
  • The largest fish is the great whale shark, which can reach 50 feet in length.


22 Join the Discussion

1 Star2 Stars3 Stars4 Stars5 Stars  (85 votes, avg. 4.04 out of 5)
    • That’s really AWESOME that you’ve visited the aquarium featured in the video for this Wonder, Rithik! Thanks for sharing that with your friends in Wonderopolis today! :-)

    • We’re glad to know that this Wonder piqued your interest, Devin! Thanks for joining us as we Wonder about how fish use their gills! :)

  1. Thanks for recommending me to read this wonder!!!!! I really enjoyed it!!!! I thought that the video was really cool because you got to see all the different types of fish in the water!!!! GREAT WONDER!!!!:) :) :)

    • You’ve got it, happy girl! We’re also very happy to know that you learned about a variety of fish with us! We hope your day is under-the-sea terrific! :)

  2. I love this post! I have a pet beta fish and I always wondered why only the special animal fish can breathe underwater! Thanks for the post! I think you should make a post about why beta fish fight other fish!

    • HOORAY, thanks for telling us how much you enjoyed this fishy Wonder, Esmaeil! We appreciate your comment and look forward to WONDERing with you again soon! :)

  3. Does it hurt the fish when is breathes? The way you described it, it sounds like it does. “A fish breathes by taking water into its mouth and forcing it out through the gill passages.” The FORCING IT OUT PART. otherwise it is SO AMAZING!!!!!!!!!!!!!!!!!!!

    • Great question, Anastasia! The fish are not in pain, but their gills work together with their breathing in order to prevent the fish from choking! Pretty cool! :)

  4. I didn’t really find any of this useful. Maybe try adding how fish contribute to the water, carbon, oxygen, and nitrogen cycle.

  5. Thank you.You actually gave me all the answers I needed for my science homework ‘how do fishes breathe?’ And also you answered all of the questions my science teacher didn’t answer.

    • Wow, that’s terrific, Wonder Friend! We’re really glad you stopped by Wonderopolis today! Thanks for WONDERing with us! :)

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Have you ever wondered…

  • How do fish breathe underwater?
  • Do all sea creatures have gills?
  • Why do dolphins and whales swim to the surface of the ocean?

Wonder Gallery

Try It Out

Feeling fishy? You can create your very own fish friend using recycled objects from around your home.

Start by using an empty dish soap bottle as a body. Ask your parents to help you cut out the body of the fish using these instructions.

Once you’ve got your fish bottle body, get creative! Challenge yourself to use other recyclable materials to give your fish some flair.

With just a little paint, an old cereal box can become colorful fins. Old bottle caps make great fish eyes! With more than 25,000 species of fish in the sea to inspire your design, anything goes.

When you’re finished, email a picture to or send us a picture of your creation to Wonderopolis, so we can see what sorts of recycled materials you used to build your aquatic masterpiece:

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Who Sees The Seas As Important? Visit National Geographic Xpeditions to learn more about the careers of oceanographers, marine biologists and many others who are devoting their lives to exploring the oceans’ ecosystems and preserving the health and inhabitants of these amazing places.


Wonder Categories/Tags



amphibian  gill  lung  mammal  oxygen  reptile 

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