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. Without swim bladders to modulate buoyancy, most cartilaginous fish can only control depth by actively swimming, which produces dynamic lift; others store up lipids with specific density less than that of seawater to produce a neutral or near-neutral buoyancy, which cannot be readily changed with depth.
Structure and function
The swim 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 pressurising organ 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 swim bladders, a connection is retained between the swim bladder and the gut, the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air. Excess gas can be removed in a similar manner. In more derived varieties of fish (the physoclisti), the connection to the digestive tract is lost. 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. This process begins with the acidification of the blood in the rete mirabile when the gas gland excretes lactic acid and produces carbon dioxide, the latter of which acidifies the blood via the bicarbonate buffer system. The resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffuses partly into the swim bladder. Before returning to the body, the blood re-enters the rete mirabile, and as a result, virtually all the excess carbon dioxide and oxygen produced in the gas gland diffuses back to the arteries supplying the gas gland via a countercurrent multiplication loop. 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 bars. Elsewhere, at a similar structure known as the 'oval window', the bladder is in contact with blood and the oxygen can diffuse back out again. Together with oxygen, other gases are salted out in the swim bladder which accounts for the high pressures of other gases as well. 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 swim bladder. Physoclist swim 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. The swim bladder in some species, mainly fresh water fishes (common carp, catfish, bowfin) is interconnected with the inner ear of the fish. They are connected by four bones called the Weberian ossicles from the Weberian apparatus. These bones can carry the vibrations to the saccule and the lagena. They are suited for detecting sound and vibrations due to its low density in comparison to the density of the fish's body tissues. This increases the ability of sound detection. The swim bladder can radiate the pressure of sound which help increase its sensitivity and expand its hearing. In some deep sea fishes like the Antimora, the swim bladder may also be connected to the macula of saccule in order for the inner ear to receive a sensation from the sound pressure. In red-bellied piranha, the swim bladder may play an important role in sound production as a resonator. The sounds created by piranhas are generated through rapid contractions of the sonic muscles and is associated with the swim bladder. Teleosts are thought to lack a sense of absolute hydrostatic pressure, which could be used to determine absolute depth. However, it has been suggested that teleosts may be able to determine their depth by sensing the rate of change of swim-bladder volume.
Evolution
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