Swim Bladder

An important organ seen in ray-finned fish (as well as in Sarcopterygii, or lobe-finned fish) but not in cartilaginous fish is the swim bladder. This organ is a sac containing gas; the fish is able to adjust its buoyancy and thus its position in the water by adjusting the amount of gas in the swim bladder. The swim bladder develops as a pouch budding off the embryonic digestive tract, and in chondrosteans (such as sturgeons) and holosteans (such as gars), plus a few teleosts such as eels, it retains an open connection to the esophagus. In most bony fish, the swim bladder is completely closed off, and gas levels in the swim bladder are adjusted by secreting gas into the bladder through a network of capillaries, the rete mirabile (Latin for "marvelous net"). The swim bladder is homologous to the lungs of tetrapods. Both develop in the same way, and some fish with an open swim bladder may use it as a breathing organ.

Although the fishes' swim bladder is primarily an organ for buoyancy control, it also functions in the production and generation of sound. Many fish, particularly those of coastal waters, have muscles that insert into the swim bladder. When the muscles are contracted, the swim bladder vibrates, producing a hollow-like sound. These sounds have been described as grunts, groans, thuds, and barks. Some species, like croakers, make sounds at feeding times, while others make noise during reproductive periods.

The swim bladders of fish can also act as a receptor of sound--one even better than their own ears! Swim bladders can discriminate a wider range of frequencies, can better determine pitch, and are more sensitive to sound than the bony otolith of the ear. Most fish have some kind of connection between the inner ear and the swim bladder.