Written by Gwen S. who works in Drs. Foster and Smith’s Technical Support Department.
Most fish have an organ called the swim bladder. Since fish have a density that is heavier than water, they need this swim bladder to be able to stay afloat when they stop swimming. Otherwise, when the fish stops swimming, they would use approximately 20% of its energy to prevent sinking.
The swim bladder, which is a flexible gas filled sac that has very few blood vessels and it is lined with guanine Crystals that make it impermeable to gases, starts as an outgrowth from the oesophagus (gullet) or the digestive tract in the larval stage. There are two types of swim bladders, the Physostomous and Physoclistous.
The Physostomous swim bladder remains connected to the gut of the fish allowing the fish to control the amount of gas in the swim bladder. Fish with this type of swim bladder are usually found in shallow water. They will expel bubbles as they swim closer to the surface of the water and then they will swallow air at the surface before diving back down into the water. This air is passed into the guts and then they will force it into the swim bladder. Sometimes if a deep water specimen is caught and quickly brought to the surface, their swim bladder may expand to fill their throat and it is possible that it may actually expand beyond their mouth.
The Physoclistous swim bladder is where the swim bladder is completely closed up allowing no gas to be moved from the gut to the swim bladder. These fish are able to control gas exchange or the amount of gas in their swim bladder through the capillaries that are found in abundance where the membrane of the swim bladder is very thin. An interesting fact about fish with this type of swim bladder would be that they are actually able to produce sounds from the swim bladder that they will use to defend their territory. The sounds being made will sound different depending on whether the swim bladder is inflated or deflated. It can also reflect sonic energy to the ear bones.
It is very important for most fish to be able to control the amount of pressure within the swim bladder to remain at a certain depth in the water without effort. If a fish is swimming deeper into the water, it will have to change the pressure in the swim bladder. For every 10 meters in depth, the pressure from the water will increase by one atmosphere (14.7 psi). At the surface of the water, a fish will experience one atmosphere and that is doubled at ten meters down.
Some fish that are born without swim bladders; such as sharks. These cartilaginous fish control their depth only by swimming or storing fats/oils in their liver. Then there are others that in the larvae stage have a swim bladder but as they mature, it is lost, such as the Hawkfish. Most Hawkfish or other similar fish remain at or near the bottom and only leave their perch or burrow to get food. However, without the swim bladder these fish are unable to swim long distances.
Read more about Swim Bladder on Wikipedia.com.
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