How a Swim Bladder Works

The swim bladder is perhaps the major key to the teleosts' success, and it is well worth a digression to explain it. It is an internal bladder filled with gas, which can be sensitively adjusted to keep the fish in hydrostatic equilibrium at any desired depth. If you ever played with a Cartesian Diver as a child you'll recognize the principle, but a teleost fish uses an interesting variant of it. A Cartesian Diver is a little toy whose business part is a tiny upended cup, containing a bubble o...