Fish and Sound

Fish also communicate with sound. One species in particular caused a stir in Sausalito, California in the 1980s. A successful antipollution campaign had clarified the waters of the bay, and made the houseboats that line the shore newly fashionable. But suddenly residents began losing sleep because of a loud hum that appeared to come from the water. Some blamed a newly laid power line. Others thought the sewage works had secretly resumed pumping at night. But the cause was, in a sense, the clear water.

Cleaning up the bay had produced a perfect environment for toadfish, a slimy, bizarre-looking relative of sea robins, sculpins and midshipman. Turns out the toadfish hums a love song to attract mates. And males hum in chorus during the breeding season, sometimes continuing for an hour. This chorus of love songs had sparked the concern of houseboat owners.

Toadfish also make two other sounds. Grunts, warnings that apparently tell rival males to back off or potential predators to stay away, last only two tenths of a second. The so-called boat whistle lasts nearly a second, may attract females as well as humming does, and might identify individuals. In the distantly related bicolor damselfish, females can distinguish individual male chirps and males can tell the chirp of their nearest neighbor from the vocalizations of more distant males. Fish sounds function mainly in mate attraction, but are also used in school coordination, and they travel well under water. Scientists have observed fish responding to a sound signal from half a mile away.

Fish produce sounds in a variety of ways, often with organs much less specialized for the task than the vocal apparatus of other vertebrates. Grunts appear to come from grinding teeth, the sound amplified by the air-filled swim bladder. And special muscles in or near the swim bladder itself can cause it to vibrate like a drumhead.

Neurobiologist Andrew H. Bass, of Cornell University, has studied sound production in the midshipman, sometimes called the singing fish. A set of special drumming muscles attach to the walls of the midshipman’s heart-shaped swim bladder. As Bass expected, females—who don’t sing—have smaller swim bladders and smaller drumming muscles. But Bass found a surprise. Some males sport female-sized swim bladders and weak drumming muscles. Furthermore, these males resemble females in body size and shape. These “midshipwimps,” who make up less than 10 percent of Bass’s study population—don’t sing or build nests, and their similarity to females continues all the way to the anatomy and cellular structure of their brains. But don’t let their nonmasculine build fool you. They mate and pass on genes quite successfully.

The small midshipman males employ a strategy seen in many fish and in other animals as well. They are so-called satellite or sneaker males. The majority of males follow the usual route—eat and grow to a large, attractive size, compete for space, build and guard a nest and grunt your heart out. They reap the reward of a nest full of eggs laid by several females. But sneaker males slink around the edges of territories, slipping in to deposit sperm (sneak spawning) while normal males are busy guarding and showing off. Their sperm do not fertilize many eggs in a single nest, but they can sneak spawn in several. In a second sly strategy, some small males release their milt into the water surrounding a nest and fan the sperm-laden water toward the nest with their fins.

Fish hear—or feel—sound in two ways. Some have small bones connecting the inner ear to the swim bladder, creating in effect a single large ear. Fish have no outer ears, and therefore no need for the middle ear bones that connect the eardrum to the inner ear. But they do possess an inner ear similar to those of other vertebrates.

But fish also detect vibrations in the water with a unique lateral line system similar in many ways to our inner ears. Where organ of hearing in our inner ears forms a coil, that of fish lies stretched out along its side. The lateral line tube stretches the length of a fish, and sometimes branches around its head. The tube connects to the water by way of small pores in the skin and scales. Mucus fills the tube, just as in our cochlea. When a pressure wave strikes the fish, it jiggles the mucus and bends small hairs that project into the mucus in bunches. The hairs trigger nerve impulses, which travel to the brain. While fish cannot determine the location of a sound detected through the single swim bladder, they can locate sounds detected by way of the lateral line.