What is Swim Bladder?

Have you ever wondered how fish manage to stay afloat all day long yet they still have enough energy left over to go about their every day business?  One particular organ, the swim bladder, is responsible for this unique phenomenon in numerous fish.   Without a swim bladder, the fish, with body tissues denser than water, would sink.

This is a balloon inside it that it can fill with gas to create buoyancy and counteract its own weight. the swim bladder has muscles around it that it can use to contract the size of the swim bladder (and therefore reduce the volume, so the fish displaces less water has less buoyancy  - so now the weight is larger than the buoyancy  so the fish sinks). Think of a bony fish as working just like a submarine!

If they want to go up, they relax the muscles in their swim bladder, which increases in volume, displaces more water so the buoyancy  pushing up is higher than the weight and up it goes.
There are two types of swim bladder in bony fish, some are joined to the stomach so excess gas can vent off and they can take air from the surface, this is good since they can change depth quickly, the other type has a swim bladder with that is totally closed, it fills the swim bladder with gas that diffuses out of the blood, so if it goes too far up too quick it cannot reabsorb the gas again in time and it gets decompression illness (like a diver) e.g. Snappers, if they're dragged up from depth too quickly they can be injured, so for catch and release some fishermen insert a hollow needle carefully to bleed off the gas and release the fish - a high % survive to be caught again.

Sharks don't have a swim bladder their skeletons are made from cartilage rather than bone, so they're more flexible, they also have a HUGE liver full of oil. This means that the oil provides a lot of the shark's buoyancy (oil floats on water remember) about 70% of the sharks buoyancy  comes from this. however this is not all of the buoyancy  needed to keep the shark floating.

The shark's fins provide the rest of the buoyancy just like an airplane's wings, the water flowing over them create lift due to the shape of the fins in cross section (they are an aerofoil - as water flows over the top curve of the fin/wing it accelerates to arrive at the same time as the water flowing the shorter route, as a fluid accelerates it's pressure drops so the wing is "sucked" up by low pressure above it and higher pressure below) - This is called the "Bernoulli" effect after the Italian scientist who discovered it - its the reason airplanes fly!

So the fins make up the rest of the buoyancy, keeping it afloat. Some sharks, notably the sand tiger can take up some of the balance of this buoyancy  by taking air into their stomachs allowing them to hang almost motionless barely swimming!.

The swim bladder’s main function is that of a hydrostatic organ. Neutral buoyancy, the ability of an organism to use little or no energy to stay at particular levels of water, is achieved through the expanding and shrinking of the swim bladder due to varying gas pressures. The fish can enjoy a sense of near weightlessness when its swim bladder gas capacity is around 5-7% of its total body volume.  Freshwater fishes require a larger swim bladder than those in salt water because freshwater is less buoyant than salt. 

There are two other functions of the swim bladder that are of great importance to the fish.  One is its connection with the ear to facilitate hearing abilities in the water and the other is its sound producing capabilities.

The evolution of the swim bladder is one of the main reasons that teleosts have been so successful.  With this highly functional organ, fish can have more precise control over their movement while expending minimal amounts of energy.

Advantages of Swim bladder
One clear advantage of having a swim bladder is that little to no extra energy is necessary in order to remain stationary at a constant level of water.  Only a slight control by use of the pectoral fins is required to balance out the propulsive force of water exiting the gills.  Fish with no swim bladder on the other hand, such as mackerels, sharks, and rays must expend energy by constantly swimming in order to keep from sinking. 

Another advantage of swim bladders is oxygen storage.  Physoclists and physostomes alike may occasionally use the oxygen present within their bladder as an emergency backup in times of urgent need, although, this emergency store can only be of aid for a few minutes.

Finally, swim bladders in some fish are known to increase hearing abilities.  With the presence of inner ear- swim bladder connections, these fish have exhibited greater sensitivity to sound, however it is not yet clear whether there is also an increase in frequency selectivity.

Disadvantages of Swim bladder
One disadvantage of having a swim bladder is that neutral buoyancy can only be achieved at a small range of depths specific to certain fishes.  If a fish swims below its buoyancy range, it will have to expend greater energy in the exercise of swimming in order to keep from sinking. 

On the other hand, if a fish swims above its upper buoyancy level, it becomes overly buoyant.  Its swim bladder would expand to such a great capacity that the fish might be thrown out of control if it does not compensate for this increased buoyancy by vigorously swimming downward.  The fuller the swim bladder gets, the more tipsy the fish is-- much like a large helium balloon. 

Therefore, many fish that do not have a swim bladder, such as the Atlantic mackerel, have greater depth flexibility and speed in moving through columns of water.  A swim bladder would only serve as a constraint for these fish which are continuously active predators. 

One other disadvantage of having a swim bladder is that oftentimes the bladder serves as an acoustical target which sounds can be bounced off of.  This might enable predators to more easily locate the fish.

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