Performance of the gas bladder is an area that seems counter-intuitive for many people. There are two things to remember. First, for a fish to remain neutrally buoyant (the point of the gas bladder), the gas bladder must always remain the same size. It would only change size as the fish grows or changes mass in some other way, like eating a large meal. Second, as the fish swims up and down in the water column, the hydrostatic pressure changes greatly (1 atmosphere (A) for every 10 m of depth, see Ch. IIA). Since the gas bladder is comprised of relatively soft, pliable tissue, the bladder pressure pushing out will always equal the hydrostatic pressure pushing in. Unless the amount of gas in the bladder is increased or decreased, it will change size as the hydrostatic pressure changes. The only way for it to remain the same size as the hydrostatic pressure on the fish increases or decreases is for the fish to increase or decrease the amount of gas in the bladder. The reason that this is counter-intuitive is that most people would guess that a fish would deflate, release gas from the bladder, to go deeper and inflate, add gas to the bladder to come up, but in fact it is the opposite.
When a fish swims down there will be more pressure pushing in on the fish and squeezing the gas bladder smaller. At 10 m, there will always be 2 A pressure in the bladder; the pressure in the elastic bladder pushing out must equal the pressure pushing in. If the fish does not put more gas into the bladder as it swims down, the bladder will shrink, the gas compressing until it reaches 2 A, and the fish will be less buoyant. It will then begin to sink, which will further compress the bladder and accelerate the sinking, until the fish is resting on the bottom more dense than the water. The reverse would occur when the fish swims up. If it did not decrease the amount of gas in the bladder, the decreasing hydrostatic pressure would allow the elastic bladder to balloon making the fish more buoyant and it would pop up to the surface overinflated and unable to swim down. So, fish inflate (put more gas into the bladder) as they swim down and deflate (resorb gas from the bladder) when they swim up.
An actual example of this phenomenon, occurs in deep water fishing. A fish hooked and brought up rapidly is unable to deflate the gas bladder quickly enough and the fish is bloated with a greatly expanded bladder, sometimes protruding out of the mouth. If the fish is released without the angler deflating the gas bladder it will flounder on the surface, unable to swim down. The need to add and subtract gas from the bladder as the fish moves up and down in the water column limits the speed with which a fish can move up and down. There must be enough time for the fish to adjust bladder pressure through the rete and gas gland or resorptive area.
Agnathans and elasmobranchs don't have gas bladders, neither do some bottom dwelling teleosts such as the darters (small percids). Darters live on the bottom, often in swift water. The absence of a gas bladder makes them negatively buoyant (heavier than water) and helps them stay in place. It should be noted, however, that this is the exception, and that in most cases, it is adaptive even for bottom dwelling fish such as catfish (ictalurids and silurids) and suckers (catostomids) to be neurally buoyant. Sharks lack air bladders, but because of a cartilagenous skeleton and a large, fatty liver, are only slightly heavier than sea water. Still, they must swim continually or slowly sink. Angled pectoral fins and a heterocercal tail help push them up as they swim forward.
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