Swim Bladder Helps Maintain Buoyancy

Biological Strategy

Ray-finned fishes

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Modify Buoyancy

Buoyancy is an upward force exerted by air or liquid on a solid object that works against the object’s weight. A hawk glides through air and a duck floats on water due to buoyancy. Some living systems (such as fish eggs) can remain sedentary and therefore maintain the same buoyancy at all times. But most must adjust their buoyancy because they can’t survive long unless they change position. These living systems require strategies to not only be buoyant, but also adjust buoyancy level. They often modify buoyancy by adding or decreasing lift, or by changing their weight. For example, birds like condors that soar for a long time shift the directional orientation of their wingtip feathers to manage buoyancy. Fish alter the amount of air in their swim bladders to increase or decrease their weight, thus altering their buoyancy.

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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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Swim bladders of fish at depth help maintain buoyancy by regulating gas levels.

Image: Alter Welt / CC BY SA - Creative Commons Attribution + ShareAlike

“Many teleost fishes have a large gas-filled bladder (often called a swim bladder) in their body cavity, which eliminates the weight of the fish in water. Neutrally buoyant fishes can hover in the water and swim with much less energy. However, there is only one depth at which a fish has neutral buoyancy, so it is important for the fish to be able to regulate the amount of gas in the swim bladder to maintain neutral buoyancy at different depths.” (Liem 1998:17)

“Gas is efficient in giving lift, since its density is very low, and most teleosts possess gas-filled swimbladders…Fish swimbladders cannot significantly resist changing in volume as the fish swims up and down in the water and the ambient pressure changes; indeed, the swimbladders of almost all teleosts obey Boyle’s law perfectly. So if a fish with a gas-filled swimbladder is to remain neutrally buoyant at different depths, it must secrete or absorb gas to keep the swimbladder at constant volume as the ambient pressure changes. To regulate the mass of gas within the swimbladder in this way requires complex mechanisms of great physiological interest.” (Bone et al. 1995:79-80)

“Gas enters the swimbladder via blood capillaries that run into a modified area of the inner wall, the gas . In actively secreting swimbladders, the surface of the gland is covered with a foamy mucus…” (Bone et al. 1995:90)

“But the swimbladder must be filled from dissolved gases in the blood, and it must not lose gas through redissolution into the blood. So secretion of gas from blood into swimbladder faces a big barrier, and swimbladder gas will all too readily go into solution in the fish’s blood and thence out into the ocean. Two devices stand in the way. First, a layer in the swimbladder wall provides a very effective barrier to the passage of oxygen (Lapennas and Schmidt-Nielsen 1977). Second, blood leaving the so-called gas gland in the wall of the swimbladder passes through an exchanger (fig.5.2) in which blood leaving the swimbladder loses excess dissolved gas specifically to blood moving toward the swimbladder (Scholander 1954).” (Vogel 2003:99-100)

Last Updated August 8, 2017

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

17/06/2013 | Vogel S

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Reference

Book section

Introducing Fishes

Encyclopedia of Fishes. eds. Paxton J. R., Eschmeyer, W. N. pp. 14-19. Academic Press, San Diego, CA. | 01/01/1998 | Liem KF

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Reference

“Gas is efficient in giving lift, since its density is very low, and most teleosts possess gas-filled swimbladders…Fish swimbladders cannot significantly resist changing in volume as the fish swims up and down in the water and the ambient pressure changes; indeed, the swimbladders of almost all teleosts obey Boyle’s law perfectly. So if a fish with a gas-filled swimbladder is to remain neutrally buoyant at different depths, it must secrete or absorb gas to keep the swimbladder at constant volume as the ambient pressure changes. To regulate the mass of gas within the swimbladder in this way requires complex mechanisms of great physiological interest.” (Bone et al. 1995:79-80)

“Gas enters the swimbladder via blood capillaries that run into a modified area of the inner wall, the gas gland. In actively secreting swimbladders, the surface of the gland is covered with a foamy mucus…” (Bone et al. 1995:90)

Book

Biology of Fishes

Chapman & Hall, London, UK. 332 p. | 30/06/1995 | Bone Q; Marshall NB; Blaxter JHS

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