How does a croaker make the noise for which it is named?
Fishes of the Family Scianidae are commonly known as ?drumfish? or ?croakers? because of the ability of many of them to produce sounds ranging with the species from a grunt to a funny ?booming?. This is accomplished by the highly modified swim bladder, which is capable of being operated as a resonating chamber for the production of sound. A few Scianids, such as the northern kingfish, Menticirrhus saxatilis, completely lack the swim bladder and are therefore incapable of producing a sound
The croaker, Micropogon undulatus, produces its characteristic noise not by the use of vocal cords, but by adaptation of various organs which are modified for sound production. In the croaker the apparatus consists of two long, thin muscles which are located on the sides of the air bladder. When these muscles are caused to vibrate rapidly a sound is produced. This sound is picked up and amplified by the air bladder into an audible croak which may be heard through six feet of water or more. Other sound producing mechanisms are found in many marine animals including the porpoise and toadfish. The porpoise makes a whistling noise by forcing air through its blow hole. In addition to this sound the porpoise produces another sound, similar to that when one rubs a fingernail over a balloon. The toad fish, Ospanus tau, produces a musical singing sound by use of a heart-shaped air bladder. These fish, can be heard singing to one another, possibly a love call - back and forth across an inlet.