Swim bladder

Swim bladder

The swim bladder, also known as the gas bladder, fish maw, or air bladder, is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming. The swim bladder is evolutionarily homologous to the lungs.

Anatomy and function

The swim bladder is located in the dorsal portion of the fish, slightly below the vertebral column. It is derived from an outpocketing of the digestive tube. It consists of two types of tissues: the pneumatic duct connecting the swim bladder to the esophagus, and the swim bladder proper.

The swim bladder works by varying the volume of gas it contains, in order to adjust the buoyancy of the fish. If the fish needs to go deeper in the water, it can contract the bladder, reducing its volume and thus increasing its density relative to the water, causing it to sink. Conversely, if the fish needs to rise in the water, it can expand the bladder, increasing its volume and thus decreasing its density relative to the water, causing it to float upwards.

Evolution

The swim bladder is believed to have evolved from the lungs of ancestral fish, which used them to breathe air in oxygen-poor water. Over time, the function of the lungs in these fish changed from respiration to buoyancy control. In some species of fish, the swim bladder has again been repurposed to function as a resonating chamber, to produce or receive sound.

See also

• Buoyancy
• Fish anatomy
• Fish physiology

External links

• FishBase glossary entry on swim bladder

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