The swim-bladder, also known as an air bladder, is an air-filled sac situated just under the backbone at the top of the abdominal cavity.
This thin walled sac may function in several ways, for example as a buoyant float, a sound producer and receptor, and in some fish as a respiratory organ. The organ therefore also acts as an aid to "hearing" by amplifying water-borne sounds. It therefore serves as a resonating chamber to produce or receive sound.
The combination of gases in the bladder varies between shallow water and deep water fish. The cyprinids and therefore Koi are physostomes because the pneumatic duct is retained in adult stages and the fish are able to gulp air to fill the gas to fill the gas bladder or they can dispose excess gas to the gut. The pneumatic duct is a connection of the swim bladder to the gut. The pneumatic duct is controlled at its entrance by a sphincter-like arrangement of striated muscle. By inflating / deflating the swim-bladder, the fish can adjust its position in the water and maintain neutral buoyancy.
The addition and removal of gases is a mechanism by which the density of the fish can be made equal to that of the surrounding water at a given depth. Due to the dorsal position it gives the fish lateral stability The amount of gas in the swim bladder is controlled by a network of fine capillary network that secrete gas into the air bladder as well as reabsorbing it. Because of the pneumatic duct, Koi are also able to regulate swim bladder content by ingesting or expelling air. Gases in the swim bladder follow Boyle's law. As pressure increases, the volume of gas decreases. However, in order to maintain neutral buoyancy, fish must maintain the volume of gas in the swim bladder. For small, temporary changes in pressure, the volume of gas in the swim bladder will change according to Boyle's law. However, when a fish experiences more persistent changes in water pressure, the fish will actively add or remove gas from the swim bladder.
The swim bladder can be very important in fish health, as it is susceptible to various kinds of infections and malformations. Swim bladder problems can be very common where fish lose their equilibrium and are unable to maintain their position. This can manifest itself in the fish swimming awkwardly, laying upside-down either on the bottom or top of the water, or unable to maintain a horizontal position in the water. The swim-bladder can be affected by bacterial, fungal or viral diseases. In addition the swim-bladder may malfunction, leading to over or under inflation. Clearly anything which affects the proper functioning of the swim-bladder, like tumours and organ failures, will also affect the fish's equilibrium.
Treatment is very difficult, mainly because it is virtually impossible to diagnose the cause and secondly there are only a few conditions that will respond to treatment. It is however always worth considering a course of antibiotic injections in case a bacterial infection is involved. An attempt should also be made to see whether the fish is defecating, in case the problem is being caused by an intestinal blockage. If an intestinal blockage is suspected, it will be worthwhile to try an Epsom salts bath (70g / litre for 5 minutes) which will purge the fish. If these treatments do not work, there is little else that can be done.
It also interesting to note that pH swings may have an influence on the effectiveness of the air bladder to control the buoyancy of a fish. The gas that fills the bladder is mostly oxygen, carried by hemoglobin in the blood of the fish. In all animals most of the oxygen in blood is bound to the protein hemoglobin, which is inside the red blood cells. A unique property of the hemoglobin in fish with swim bladders is that it enables the fish to fill or empty its swim bladder. When the blood surrounding the bladder becomes slightly acidic the hemoglobin releases oxygen into the bladder. If the blood becomes less acidic the oxygen is reabsorbed by hemoglobin.
By: Chris Neaves