How Fish Maintain Buoyancy
What is a Swim Bladder and How Do Gas Bladders Work?
Many people find themselves feeling awkward or clumsy underwater, sinking when they want to float or vice versa. SCUBA divers use buoyancy jackets to help them navigate the watery depths but even these don’t give them the agility fish have underwater. Obviously fish are specially adapted to their environment and one body part that particularly helps with buoyancy is the swim bladder, also known as the gas bladder.
What is the Gas Bladder?
The name gas bladder can be misleading as it very unlike a human bladder and has nothing to do with urinary waste products. Rather, a fish uses its swim bladder to exchange gases such as oxygen and carbon dioxide to help it maintain buoyancy. In fact, the gas bladder is considered to have evolved from a lung-like organ according to the Encyclopaedia Brittanica's “Swim bladder”. MSN Encarta describes in its entry “Lungfish” how this fish still uses its swim bladder to breathe.
Who Has a Swim Bladder?
Having a swim bladder can be useful, as a fish with one doesn’t have to expend energy just to stay put. Most bony fish have a swim bladder according to the Encylcopaedia Brittancia but cartilaginous fish such as sharks do not. Jay Calkins writes in the NOAA article “Sharks” that as sharks have no swim bladder they sink when they are not moving – thus most sharks are constantly moving. Stephen Szedlmayer, Fisheries Specialist at Auburn Univeristy” writes in “Flatfish of Alabama” that flatfish which live on the ocean’s bottom do not have swim bladders.
How Does the Swim Bladder Maintain Buoyancy?
As a fish swims deeper into the ocean it experiences greater pressure on its body. At sea level people feel the weight of one atmosphere of air pushing on their bodies; 10 meters below sea level the weight of water exerts another atmosphere’s worth of pressure upon the body. Roughly every 10 meters down equals another atmosphere added. As a fish swims upwards or downwards it reacts to changes in pressure by adding or removing air from its swim bladder. To remain neutrally buoyant – that is to not float or sink – the fish will add air to its bladder when descending and remove air when ascending according to Bernd Pelster’s December 2001 “The Generation of Hyperbaric Oxygen Tensions in Fish” published in News in Physiological Sciences
The Science Behind Gas Bladder Buoyancy
The swim bladder is similar in function to a SCUBA diver’s Buoyancy Control Device (BCD) which is essentially a vest that the diver can release air into or out of. Dennis Graver explains the science behind the BCD in the third edition of his book “Scuba Diving”. Graver notes that increasing an object’s volume without a significant change to its weight will increase buoyancy, this is due to how much water the object displaces relative to its weight. As a fish swims upward, the gases within its body expand because less pressure is being exerted. The gases thus take up more volume – but do not change in weight. This makes the fish more buoyant and without a swim bladder the fish would float straight to the top. However, removing gas from the swim bladder while ascending maintains the swim bladder’s volume and thus the fish’s neutral buoyancy.
Why Fish Don't Sink
A fish maintains its gas bladder volume when descending by adding gases to the bladder. As a fish swims downward, more pressure is exerted on its body and the air within compresses or takes up less volume. The fish maintains its volume by adding gases. Pelster notes that a fish does not have to think about adding or removing air from its swim bladder. Rather, the body senses differences in pressures and the gases move through simple diffusion. Pelster explains the specifics of the biology involved.
Most Fish Have a Swim Bladder
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Sharks Do Not Have Gas Bladders
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Many Bottom Dwelling Fish Have No Swim Bladder
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