The Collaborative International Dictionary
Swim \Swim\, n.
The act of swimming; a gliding motion, like that of one swimming.
--B. Jonson.The sound, or air bladder, of a fish.
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A part of a stream much frequented by fish. [Eng.]
Swim bladder, an air bladder of a fish.
To be in the swim, to be in a favored position; to be associated with others in active affairs. [Colloq.]
Wiktionary
alt. (context anatomy usually of fish English) A gas-filled sac within the bodies of most fish that variously acts as a float, lung, sound-producing organ or an aid to hearing. n. (context anatomy usually of fish English) A gas-filled sac within the bodies of most fish that variously acts as a float, lung, sound-producing organ or an aid to hearing.
WordNet
n. an air-filled sac near the spinal column in many fishes that helps maintain buoyancy [syn: air bladder]
Wikipedia
The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth without having to waste energy in swimming. Also, the dorsal position of the swim bladder means the center of mass is below the center of volume, allowing it to act as a stabilizing agent. Additionally, the swim bladder functions as a resonating chamber, to produce or receive sound.
The swim bladder is evolutionarily homologous to the lungs. Charles Darwin remarked upon this in On the Origin of Species.
In the embryonic stages some species, such as redlip blenny, have lost the swim bladder again, mostly bottom dwellers like the weather fish. 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.
The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used in sonar equipment to find fish.
The cartilaginous fish (e.g. sharks and rays) do not have swim bladders. Some of them can control their depth only by swimming (using dynamic lift); others store fats or oils with density less than that of seawater to produce a neutral or near neutral buoyancy, which does not change with depth.
Usage examples of "swim bladder".
Deep inside his swim bladder he could hear the sounds of the other fish in the pool in which he swam.
That was all that came from his mouth, forced out by his painfully squeezed swim bladder.
Instead the sac merely shrank a little, like the punctured swim bladder of a fish.