From Brain to Boatwhistle in a Toadfish

Because of the distinctive sounds which it it makes, the oyster toadfish, Opsanus tau, been the subject of more than its share of studies of fish communication. The male toadfish produces a sound like a boatwhistle that is associated with mating and nesting. Both males and females make grunting sounds which appear to be aggressive or territorial in nature. The source of both these sounds is the fish's swimbladder, which contains a pair of the fastest contracting muscles in any animal. In a grunt the swimbladder contracts 90 times a second; and in the boatwhistle 200 times a second resulting in a higher pitched, longer sustained sound. Michael L. Fine, who did his doctoral work at the Graduate School of Oceanography, has studied the toadfish's unique vocalizing from three points of view: the structure of the swimbladder itself, geographic variations in toadfish sounds, and the areas of the fish's brain that trigger the swimbladder mechanism.

First, Fine measured the swimbladders of 144 toadfish and demonstrated that after a period of equal growth rate, the swimbladders of males grew faster than those of females. The relationship of weight to width of the bladders was the same for males and females suggesting that these sound-producing organs are structurally similar, and that the difference in sounds made by the male and female may be the result of neural and hormonal differences rather than has variations in structure of the swimbladder.

Fine then continued his work with a study of different geographical areas and at different seasons and by electrically stimulating the toadfish brain to find out how the variation in calls made by fish from the acoustic system is organized.

Using earlier recordings made by Marie P. Fish and William Mowbray as well as his own tapes, Fine studied sounds made by toadfish in Delaware, Virginia and South Carolina as well as Rhode Island. He found that both pitch and duration of the boatwhistle varied in relation to the time of year so that there was no distinctive Virginia or Rhode Island boatwhistle. The frequency of the boatwhistles increases to a peak in early summer and then decreases in the middle of July about the time the mating season ends. The duration of the call decreases at the same time as the frequency. Temperature has an influence on frequency but the duration of the call is independent of temperature. There are also some regional differences. In one type of vocalization elicited by brain stimulation, the Florida toadfish has no warm-up period as does the Rhode Island toadfish and its grunts are faster. Also the farther south the toadfish lives, the higher pitched is its boatwhistle. Duration of the call varies irregularly up and down the coast.

In an attempt to determine the areas of the brain which control the various sounds, Fine stimulated the brains of anesthetized toadfish electrically. Sounds have been elicited by this method in birds, frogs and monkeys, but this toadfish and a related species are the only fishes on which it has been successfully attempted.

Fine was able to elicit sounds in 209 of the 436 electrode tracks that he tried in 29 fish from Virginia, Florida and Rhode Island. The various types of sounds in a range from simple to more complex he called: "one-to-one" responses (undifferentiated pulses at the same repetition rate as the stimulus frequency), buzz grunts (mechanic al-sounding individual pulses but following the stimulus less closely), knock grunts, burst grunts, and boatwhistles. The last three were under greater brain control and sounded like normal toadfish vocalizations. Knock grunts are short, while burst grunts are longer and more musical. The sound highest in the order of complexity is the boatwhistle.

Fine charted the areas of the brain responsible for sound production. He determined that there is not a separate "boatwhistle area" and "grunt area," and the response elicited at a particular site is not constant. Indeed a 2/10 of a millimeter difference in depth can elicit a different response in the same area. He also stimulated a female toadfish to produce a boatwhistle which is only produced by males in nature. Often electrical stimulus evoked no sound. Since the swimbladder is doing the same thing -contracting - in a grunt and a boatwhistle but at different rates of speed, one might expect that a faster paced stimulus could obtain a boatwhistle from the same site in the brain as a grunt. The faster the stimulation the more rapid was the grunt but a still faster pace did not result in a boatwhistle.

The spots where responses were elicited were marked with dye and then the brain was dissected. As a result Fine was able to chart a nerve pathway from the diencephalon forward in the brain, through the midbrain, the medulla and out the spinal cord to the swimbladder which produces the sound.

Says Fine, "The toadfish can provide a model for understanding the functioning of vertebrate brains. The brains of mammals are more complicated and thus more difficult to study and we cannot experiment freely on human beings. "

Return to Maritimes Readings List
Return to Fish Sounds home page