Fish live in water through a combination of adaptations: gills take up dissolved oxygen, bodies and fins produce and control movement, and buoyancy systems help many fish stay at a useful depth. Gills and kidneys also help regulate water and salts inside the body. The details vary by species, so no single feature applies to every fish.
How do fish breathe underwater?
Most fish move water into the mouth and across the gills. Oxygen dissolved in the water passes across the gills’ thin exchange surfaces into the blood. Some fish drive this flow by swimming with the mouth open; others pump water over the gills. Gills also help move ions, support acid-base balance and remove nitrogenous waste, so they do more than take up oxygen. NOAA’s overview of finfish anatomy and its technical account of respiration and osmoregulation describe these roles.
Gills are not the only possible route for oxygen uptake. Some bony fish can take in oxygen through the skin or digestive organs, and a few have specialized structures for breathing air. These are variations, not the usual way fish breathe.
How do fins and body shape help fish swim?
Water resists movement, so fish generate thrust by moving the body, tail and fins against it. Body shape and swimming style reflect different needs: speed, maneuverability or sustained travel. The Smithsonian’s Q?rius material on bony-fish locomotion describes several contrasting strategies.
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- Eels send waves along much of the body, helping them maneuver through narrow spaces.
- Trout rely more on the rear body for speed, with less maneuverability as a trade-off.
- Tuna keep the body relatively rigid and use the tail and its connector to power long-distance swimming.
Paired fins, including pectoral fins, can help with movement and control. Their precise contribution differs among fish; fins do not act only as paddles.
How do fish stay at the right depth?
Many bony fish use a gas-filled swim bladder, also called a gas bladder, and adjust its gas content to help manage buoyancy. This makes it easier to remain at a suitable depth without relying on constant swimming. A swim bladder is common among bony fish, but it is not found in all fish.
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Fish without one may use other buoyancy aids. NOAA’s educational background on fish notes oil buoyancy in some fast-swimming fish such as mackerel; NOAA’s anatomy material describes the large livers of sharks as another aid. These are different solutions, not interchangeable features of every species.
How do fish manage water and salt?
Fish must keep the concentration of water and dissolved salts in their bodies within workable limits. The challenge differs between fresh water and seawater, so fish use different combinations of drinking, urine production and ion transport.
| Habitat | Main challenge | Typical response |
|---|---|---|
| Fresh water | Water tends to enter the body, while ions tend to be lost. | Fish replace ions, including through the gills, and produce abundant dilute urine. |
| Seawater | Water tends to leave the body, while salts can accumulate. | Fish drink seawater and remove excess ions, with gills and kidneys contributing to balance. |
These processes are part of osmoregulation. Fish that tolerate changing salinity can adjust their ion-transport responses; a 2022 NOAA repository record describes endocrine signaling and gill transport responses in fish exposed to tidal salinity changes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What makes arapaima different?
Arapaima from the Amazon basin can breathe atmospheric air when pools have low oxygen. The Smithsonian National Zoo explains that the fish has small gills and a modified swim bladder that opens into the mouth and functions like a lung. It is a specialized adaptation to particular conditions, not a typical feature of fish generally. Smithsonian National Zoo: Arapaima
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