Saturday, 2 January 2010

Gas Bladder

Swimming is an act that requires much energy. A fish swims all the while without rest. How does it manage to do so?
~~G. VENKATARAMAN Thanjavur, Tamil Nadu

True. Swimming is an act that requires a lot of energy and that is why obese people are advised to swim besides regular exercises. But it is not true that a fish swims all the while without rest. In fact, it does not swim vigorously unless alerted. It mostly rests in suspension or moves occasionally but quietly. Such acts do not demand much energy.

Energy is needed in an act, according to laws of physics, only when such an act performs a finite amount of work and work is said to have been accomplished only when a body is displaced against some opposing force. A body under equilibrium or in a state of suspension hardly does any work or any work is done upon.

A normal fish has a unique anatomy that includes a gas bladder or swim bladder which offers an ability to the fish to control its buoyancy. The fish is, thus, able to stay in suspension at any desired depth and ascend or descend in quite pond water without having to spend much energy in such acts. The only energy it needs in such controls is just to inflate or deflate the gas bladder to the desired density (buoyancy).

The wedge-like shape of the fish balances it from gravitational pull while the mild jet power of the gills and the paddling power of the pelvic and pectoral fins help it make the gentle forward movement. The smoothness of the scales gives minimal friction with water matrix. The dorsal, fat and anal fins balance its body from toppling when the caudal fin negotiates steering.

Only when the fish is in fast flowing water and when it does not want to be carried along the flow or when it is hunted by its predator, does it need to do a real swimming (that needs to hold its own body relative to the frictional force of the flow) and to spend some energy. It must have that much energy lest it should be swept away along the currents. For all these acts, the energy needed is not astounding to fish’ capability.

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