How do we know?
↻ One of the pages with a moving picture
Scientists looked closely at gills.
- A fish gulps water in through its mouth.
- The water flows over its gills. Each gill has thin, feathery parts full of tiny blood vessels.
- Oxygen mixed into the water slips into the blood. Blood and water flow in opposite directions, so lots gets in.
- The used water flows back out under the bony gill cover.
Scientists looking closely at gills found that each one has thin filaments, and each filament has thousands of tiny folds called lamellae. That’s where the oxygen gets in.
A fish takes in 75% of the oxygen in the water passing over its gills. That’s twice as much as our lungs take from a breath of air.
Not every fish does it the same way. Crocodile icefish in Antarctic waters have blood with no red blood cells at all. The giant arapaima in the Amazon has tiny gills and gulps air with a part that works like a lung.
Wow number
75% of the oxygen in the water passing over a fish’s gills gets taken in. That’s twice as much as our lungs take from a breath of air!
Myth or fact?
Fish breathe by splitting water into oxygen and hydrogen. Myth. They use oxygen gas mixed into the water.
Whales don’t have gills. Fact. They swim up to breathe air with lungs.
Think about it
Why do you think a fish can’t breathe for long out of the water?
Where in the world?
Amazon River basin, South America where the giant arapaima has tiny gills and gulps air with a part that works like a lung. Crocodile icefish in Antarctic waters have blood with no red blood cells at all — 3.119° S, 60.0217° W.
Where these facts come from
Every number on page 43, and who measured it
This page explains how a fish gulps water and pushes it over its gills.
How do fish breathe? — Ask Dr. Universe, a science-answers site from Washington State University
askdruniverse.wsu.edu/2018/01/07/how-do-fish-breathe
We checked it on 31 October 2026
This page tells us about lamellae, the 75% oxygen fact, and why whales have no gills.
The American Museum of Natural History in New York
amnh.org/learn-teach/resources-for-learning/hall-of-ocean-life/water-vertebrates-breathing
We checked it on 31 October 2026
This page tells us that gills are full of blood vessels.
About Bony Fishes (Superclass Osteichthyes): Respiration — Q?rius, the Smithsonian's science learning center
qrius.si.edu/taxonomy/term/12176
We checked it on 31 October 2026
This page tells us that crocodile icefish have no red blood cells.
Antarctic Adaptations — The Antarctic and Southern Ocean Coalition, a group that works to protect Antarctica
asoc.org/learn/antarctic-adaptations
We checked it on 31 October 2026
This page tells us how the arapaima gulps air.
Arapaima — The Smithsonian's National Zoo
nationalzoo.si.edu/animals/arapaima
We checked it on 31 October 2026
Has this changed?
How fish breathe is not going to change.
Scientists keep finding fish that do it in surprising ways, like the icefish with no red blood cells.
Words from this page
- gill
- The feathery part of a fish that takes oxygen out of the water.
- oxygen
- The gas in air and water that animals need to breathe.
- lamellae
- The thousands of tiny folds on a fish’s gills where oxygen gets in.
Try it!
Watch a fish's gill covers open and close.
1. Find a fish in a tank at home, at a pet store or at an aquarium. 2. Watch the spot just behind its head. 3. Count how many times the gill covers open in 15 seconds.
What happens: the gill covers open and close again and again, as water flows out over the gills.
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