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How do fish use swim bladders to control buoyancy?
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Archimedes
Archimedes, oil on canvas by Giuseppe Nogari, 18th century; in the Pushkin Fine Arts Museum, Moscow. Archimedes was an ancient Greek mathematician and inventor.
Fish utilize the swim bladder—a hydrostatic organ located in the body cavity—to regulate their buoyancy and maintain depth, a process consistent with the principles of buoyancy later formalized by Archimedes.123
Fishes with a hydrostatic swim bladder control their vertical position by adjusting the amount of gas (typically oxygen) within the organ. This mechanism functions as follows:2
- Ascending: Special glands secrete gas into the bladder, which increases the fish's total volume without changing its mass. This decreases the fish's overall density (specific gravity), causing it to rise.23
- Descending: Gas is absorbed from the bladder into the bloodstream, which reduces the organ's volume. This increases the fish's density, allowing it to sink.23
While some primitive fish retain a connection between the bladder and the digestive tract to manage gas, advanced teleosts regulate gas through the bloodstream. In some species, the bladder may contain oil instead of gas, or it may be absent entirely in certain deep-sea or bottom-dwelling fish.21