Because a weightless, or buoyant, body requires a minimum of energy to keep it at a given depth, and because a weightless body requires less energy than a weighted body to move at a given speed, many fishes have evolved means of reducing their body weight, or density, relative to the density of water. A fish whose total body density equaled that of water would be effectively weightless, neither rising nor sinking. Because fat is less dense than water, one method of reducing body density would be to increase the proportion of fat within the body. Theoretically, about one-third of a fish's body weight would have to be made up of fat in order to make the fish weightless in seawater. This condition is approached in some species of deep-sea sharks having very large livers that contain a great amount of squalene, a fatty substance that is significantly less dense than seawater.
Another method of reducing density is to include gases within the body. Many fishes have a gas-filled bladder that serves this function. The gases within the bladder are similar to those in air but are present in different and widely varying proportions. The degree of body volume that must be taken up by gas in order to achieve weightlessness depends mainly upon whether the fish is freshwater or marine. Fresh water is less dense than seawater and consequently provides less buoyancy. Freshwater fishes, therefore, require a larger gas bladder than do marine fishes to keep them from sinking. According to calculations, the capacity of a gas bladder should be about 7% of body volume for a freshwater fish and 5% for a marine fish. In actual measurements, freshwater fishes' gas bladders have been found to range from 7 to 11% of body volume, while those of marine fishes have ranged from 4 to 6% of body volume.
If the gas bladder contained an unchanging quantity of gas, the fish possessing it would be weightless at only one depth. The reason for this is that as pressure increases with depth, the gas in the bladder is compressed, decreasing the bladder's volume and increasing the relative density of the fish. The fish would then have to use considerable energy to stop its increasingly denser body from sinking. Conversely, when a fish rises from great depths and pressure is decreased, the volume of the bladder expands and the fish becomes too light to remain at a given depth without considerable effort (in extreme cases, such as with certain deep-water fishes, excessive expansion of gases in the bladder can cause the bladder to burst). The quantity of gas within a fish's bladder must therefore be adjustable. If, as in the carp, the gas bladder is connected by a duct to the gullet, gas may be expelled through the mouth and gill cavities as the fish rises, and, in a similar manner, gas may be added to the bladder by swallowing air at the water surface.
For most fishes, however, coming to the surface to gulp air prior to going deeper is impractical, and in many fishes the gas bladder has no connection to the outside. In these fishes there must be another means of adjusting the quantity of gas within the bladder. This is done by transferring gases from the gas bladder to adjoining blood vessels and back again.
Deflating the gas bladder is a passive operation, working under the higher pressures building up within the gas bladder. This pressure forces the gas into surrounding blood capillaries, which then carry the gas away. These blood vessels may be scattered through the walls of the gas bladder, they may be confined to a compartment at the rear of the bladder, or they may be restricted to a region at the top of the bladder, which is separated from the bladder by a constrictive muscle and is known as the oval organ.
Inflating the gas bladder is an active, or dynamic, operation because it is done against the high pressures within the bladder. The gases may be forced into the bladder by blood vessels covering large areas of the bladder walls, or, more commonly, by a combination of two units known as the gas gland and the rete mirabile. The gas gland is a modification of the inner lining of the bladder; the rete mirabile is a dense bundle of capillaries arranged side by side in countercurrent fashion. Blood leaving the gas bladder would be carrying a gas, such as oxygen, at a high pressure equal to that within the gas bladder. Blood arriving at the gas bladder would be carrying oxygen at quite a low pressure, equal to that within the water passing over the gills. The oxygen therefore diffuses from the outgoing blood into the incoming blood. This process is repeated continuously, with greater and greater concentrations and higher and higher pressures of oxygen collecting at the junction point of the outgoing and incoming capillaries, the gas gland. The gas gland may facilitate this buildup by secreting lactic acid, which acts to increase the pressure of oxygen within the blood. When the pressure of the oxygen in the gas gland exceeds that within the gas bladder, the oxygen diffuses into the bladder.
Distribution - Anatomy - Circulation -
Body Temperature - Water Balance - Swimming
Lateral Line System - Evolution - Reproduction
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