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. 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.
Go to marine mammal acoustic article.