Swim bladder

A gas-filled organ in many bony fishes that helps regulate buoyancy. It can also contribute to breathing or sound production in some species.

Physostomous fish: A fish whose swim bladder remains connected to the digestive tract by a pneumatic duct. Its open duct allows gas to enter or leave the bladder through the gut.
Buoyancy: The upward force exerted by a fluid on an immersed object. The swim bladder adjusts how much water a fish displaces relative to its mass.
Accessory breathing organ: An anatomical structure that supplements or replaces gill-based oxygen uptake. Some fishes use the swim bladder to take oxygen from air or water.
Liver oil: Oil stored in an animal’s liver, sometimes contributing to buoyancy. Many sharks gain lift from oil-rich livers rather than a gas-filled bladder.
Physoclistous fish: A fish whose swim bladder has no connection to the digestive tract in adulthood. It regulates bladder gas through blood vessels rather than swallowing or releasing air.
Hydrostatic pressure: Pressure exerted by a fluid at rest, increasing with depth. Changing pressure compresses or expands gas in a fish’s bladder.
Weberian apparatus: A chain of small bones linking the swim bladder to the inner ear in otophysan fishes. It carries bladder vibrations to the hearing structures, improving sensitivity to sound.
Dynamic lift: An upward force generated by movement through a fluid. Some fishes rely on swimming-generated lift instead of maintaining neutral buoyancy at rest.
Gas gland: A specialized tissue that secretes gas into the swim bladder from the blood. It adds oxygen to the bladder, helping maintain buoyancy at depth.
Osmoregulation: The control of water and dissolved-ion balance in an organism. Bladder gas regulation relies on blood chemistry and occurs alongside fluid-balance systems.

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