Swim bladder
gas-filled organ that contributes to the ability of a fish to control its buoyancy
The swim bladder, gas bladder, fish maw, air bladder or sound is an internal gas-filled organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift via swimming, which expends more energy. The ventral position of the swim bladder means that the center of mass is above the center of buoyancy, reducing stability but improving maneuverability. Additionally, the swim bladder functions as a resonating chamber to produce or receive sound.
The swim bladder is evolutionarily homologous to the lungs of tetrapods and lungfish, and some ray-finned fish such as bowfins have also evolved similar respiratory functions in their swim bladders. Charles Darwin remarked upon this in On the Origin of Species, and reasoned that the lung in air-breathing vertebrates had derived from a more primitive swim bladder as a specialized form of enteral respiration.
Some species, such as mostly bottom dwellers like the weather fish and redlip blenny, have secondarily lost the swim bladder during the embryonic stage. Other fish, like the opah and the pomfret, use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom-dwelling sea robin can use their pectoral fins to produce lift while swimming like cartilaginous fish do.
The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used by sonar equipment to find fish.
Cartilaginous fish such as sharks and rays do not have swim bladders, as they belong to a completely different evolutionary clade.
The public source identifies “Swim bladder” as gas-filled organ that contributes to the ability of a fish to control its buoyancy. This brief keeps that definition visible, then builds a research path around Swim, bladder and gas-filled.
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This entry incorporates text from “Swim bladder” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.