The swim bladder (also called the gas bladder or air bladder) is a flexible-walled, gas-filled sac located in the dorsal portion of body cavity. This organ controls the fish's buoyancy and in some species is important for hearing. Most of the swim bladder is not permeable to gases, because it is poorly vascularised (has few blood vessels) and is lined with sheets of guanine crystals.
A fish swimming in the water expends less energy if it is neutrally buoyant (that is, it neither sinks nor floats). If this fish starts to descend, the increased pressure from the water surrounding the fish results in a compression of the gas inside the swim bladder. The fish becomes negatively buoyant and will tend to sink. Conversely, if a fish swims into shallower water, there is a decrease in water pressure and so the gas in the swim bladder expands, and the fish tends to float upwards. The swim bladder helps to solve the problems associated with variations of pressure, and thus buoyancy.
If the fish becomes positively buoyant, and starts to float upwards, gas diffuses out of the swim bladder into the blood. This occurs at a site known as the oval. The gas in the blood is then removed from the body into the surrounding water at the gills.
Conversely if the fish becomes negatively buoyant, and starts to sink, air enters the swim bladder at a region called the gas gland. The way the fish does this involves three processes; the acidification of the blood, an increase in the concentration of lactate and hydrogen ions and the movement of blood through a complex structure called the rete mirabile (literally, the wonderful network). These complex processes are not discussed here. Refer to the reference below for more information.
Not all fishes have a swim bladder. Sharks for example do not have a swim bladder, and many species such as the Grey Nurse Shark, use a different strategy which includes having a large oily liver and specialised body shape to maintain buoyancy.
Buoyancy organ possessed by most bony fish. The swim bladder is located in the body cavity and is derived from an outpocketing of the digestive tube. It contains gas (usually oxygen) and functions as a hydrostatic, or ballast, organ, enabling the fish to maintain its depth without floating upward or sinking. It also serves as a resonating chamber to produce or receive sound. In some species the swim bladder contains oil instead of gas. In certain primitive fish it functions as a lung or respiratory aid instead of a hydrostatic organ. The swim bladder is missing in some bottom-dwelling and deep-sea bony fish (teleosts) and in all cartilaginous fish (sharks, skates, and rays).


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.
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 robins can use their pectoral fins to produce lift while swimming. The cartilaginous fish (e.g. sharks and rays) 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 consists of two gas-filled sacs located in the dorsal portion of the fish. 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 the 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 see fish like the eel, requiring a pressure of hundreds of bar. 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 wel.
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.



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.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 more "primitive" teleosts, and is lost in the more derived 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. 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.