Swim bladder

Charles Darwin remarked upon this in On the Origin of Species,[3] and reasoned that the lung in air-breathing vertebrates had derived from a more primitive swim bladder as a specialized form of enteral respiration.

In the embryonic stages, some species, such as redlip blenny,[4] 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 gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used by sonar equipment to find fish.

In early life stages, these fish must rise to the surface to fill up their swim bladders; in later stages, the pneumatic duct disappears, and the gas gland has to introduce gas (usually oxygen) to the bladder to increase its volume and thus increase buoyancy.

The resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffuses partly into the swim bladder.

For instance, the eel Synaphobranchus has been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and 0.4% argon in its swim bladder.

In some deep sea fishes like the Antimora, the swim bladder maybe also connected to the macula of saccule in order for the inner ear to receive a sensation from the sound pressure.

According to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary generation from an ancient and unknown prototype, which was furnished with a floating apparatus or swim bladder.

As an adaptation to migrations between the surface and deeper waters, some fish have evolved a swim bladder where the gas is replaced with low-density wax esters as a way to cope with Boyle's law.

[16] Correspondingly, these fish also have both heterocercal and stiff, wing-like pectoral fins which provide the necessary lift needed due to the lack of swim bladders.

This turned out to be due to millions of marine organisms, most particularly small mesopelagic fish, with swimbladders that reflected the sonar.

The swim bladder is inflated when the fish wants to move up, and, given the high pressures in the mesopelagic zone, this requires significant energy.

[25] Indeed, lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms.

The estimated global biomass of lanternfish is 550–660 million tonnes, several times the annual world fisheries catch.

[26] In the East Asian culinary sphere, the swim bladders of certain large fishes are considered a food delicacy.

The vanity price of a vanishing kind of maw is behind the imminent extinction of the vaquita, the world's smallest porpoise species.

Physoclisti can not expel air quickly enough from the gas bladder, the organ most susceptible to sonic damage, thus making it difficult for them to escape major injury.

Physostomes, on the other hand, can release air from their gas bladder expeditiously enough to protect it; nevertheless, they can not relieve pressure in their other vital organs, and are therefore also vulnerable to injury.

[34][35] Siphonophores have a special swim bladder that allows the jellyfish-like colonies to float along the surface of the water while their tentacles trail below.