air bladder

The gas bladder of a Rudd

The gas bladder (also fish maw, less accurately swim bladder 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.^[1]

Species

Gas bladders are only found in ray-finned fish. In the embryonic stages some species have lost the swim bladder again, mostly bottom dwellers like the weather fish. Other fishes 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 cartilaginous fish (e.g. sharks and rays) and lobe-finned fish do not have gas bladders. They can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.

Structure and function

The gas bladder normally consists of two gas-filled sacs located in the dorsal portion of the fish, although in a few primitive species, there is only a single sac. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. By adjusting the gas pressure using the gas gland or oval window the fish can obtain neutral buoyancy and ascend and descend to a large range of depths. Due to the dorsal position it gives the fish lateral stability.

In physostomous gas bladders, a connection is retained between the gas bladder and the gut, the pneumatic duct, allowing the fish to fill up the gas bladder by "gulping" air and filling the gas bladder. In more derived varieties of fish, the physoclisti, the bladder has a gas gland that can introduce gases (usually oxygen) to the bladder to increase its volume and thus increase buoyancy. To reduce buoyancy, gases are released from the bladder into the blood stream and then expelled into the water via the gills.

In order to introduce gas into the bladder, the gas gland excretes lactic acid and produces carbon dioxide the resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffuses partly into the gas bladder. The blood flowing back to the body first enters a rete mirabile where virtually all the carbon dioxide and oxygen produced in the gas gland diffuse back to the arteries supplying the gas gland. Thus a very high gas pressure of oxygen can be obtained, which can even account for the presence of gas in the swim bladders of deep sea fish like the eel, requiring a pressure of hundreds of bar^[2]. Elsewhere, at a similar structure known as the oval window, the bladder is in contact with blood and the oxygen can diffuse back. Together with oxygen other gasses are salted out in the gas bladder which accounts for the high pressures of other gasses as well^[3].

The combination of gases in the bladder varies; in shallow water fish, the ratios closely approximate that of the atmosphere, while deep sea fish tend to have higher percentages of oxygen. 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 gas bladder.

Physoclist gas bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. Physostomes can "burp" out gas, though this complicates the process of re-submergence.

In some fish, mainly freshwater species, the gas bladder is connected to the labyrinth of the inner ear by the Weberian apparatus, which provides a precise sense of water pressure (and thus depth), and improves hearing.^[4]

Evolution

Gas bladders are evolutionarily closely related (i.e. homologous) to lungs. It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g. lungfish, gar, and bichir) and into the gas bladders of the ray-finned fish.^[5] In embryonal development, both lung and gas bladder originate as an outpocketing from the gut; in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in the more "primitive" ray-finned fish, and is lost in some of the more derived teleost orders. There are no animals which have both lungs and a gas bladder.

The cartilaginous fish (e.g. sharks and rays) split from the other fishes about 420 million years ago and lack both lungs and gas bladders, suggesting that these structures evolved after that split.^[5] Correspondingly, these fish also have a heterocercal fin which provides the necessary lift needed due to the lack of swim bladders. On the other hand, teleost fish with swim bladders have neutral buoyancy and have no need for this lift.^[6]

Human uses

Fish maw display in a Melaka shopping mall

In some Asian cultures, fish maw of certain large sea fishes is considered a food delicacy. It is usually served braised or in stews. Fish maws are also used in the food industry as a source of collagen. Fish maw can also be made into a strong, water-resistant glue. Fish swim bladders are also used to make isinglass for the clarification of beer.

Similar structures in other organisms

Siphonophores have a special gas bladder that allows the jellyfish-like colonies to float along the surface of the water while their tentacles trail below. This organ is unrelated to the one in fish.^[7]

References

Bibliography

• Carl E. Bond, Biology of Fishes, 2nd ed., (Saunders, 1996) pp. 283-290.