Since ancient times, fishers have known that certain fish are able to produce sounds underwater. But not until 1942, the beginning of World War II, did modern scientists get an earful using underwater sound listening devices placed along U.S. coastlines to protect important harbors from enemy submarines. Hydrophones picked up noises at the entrance to Chesapeake Bay in Virginia that sounded like a chorus of frogs and crickets. The stunned scientists concluded that the sounds originated from marine life activity. Following that incident, marine life noises have been recorded and analyzed on a worldwide basis, resulting in the stratospheric growth of scientific water-sound technology.
When I first started scuba diving, I didn’t notice the natural cacophony over the rush of bubbles emanating from my regulator exhalations. At some point, I began holding my breath so as to listen more intently to the background clatter. When I further solidified my auditory observations by the quieter activity of skin diving, I finally realized that underwater sounds did not require high-frequency equipment. I alone could bear witness to the concerto. During the day, unseen snapping shrimp hide their tiny selves in the reef so, although I do not see them, I do hear their steady crackling. The feisty orange garibaldi damselfish sound is easy to discern. If I get too close to this sonic fish, particularly during its nesting season, it will warn me away with its unmistakable thumping noises. I oblige immediately. On a skin dive one evening, I dove down thinking all would be still, but the underwater world never sleeps. Creaky doors kept opening and closing. Actually, it was the unsettling sound made by spiny lobsters.
How do creatures without vocal cords make such diverse and intriguing sounds? For some members of the arthropod family, the snapping shrimp, one claw is freakishly enlarged and can produce a very loud popping sound. A collapsing bubble outside the claw causes its characteristic clack. It clamps its claw so fast that a water jet gushing from the claw first loses and then gains pressure, causing an air bubble in the jet to swell and collapse with a pronounced “snap!” The imploding bubble generates shock waves that stun nearby prey and ward off other shrimp. Thousands of claws popping simultaneously produces a din reminiscent of a microphone picking up sounds of food being fried in a huge vat of hot oil.
A garibaldi produces a low-frequency thumping by grinding its teeth, which are located in the gill or throat region. The sound is amplified by the swim bladder, an expandable, gas-filled sac that helps fish maintain buoyancy. A clawless spiny lobster produces a rasping sound using its antennae. The sound created is not unlike the way a violin makes sound. To generate sound vibrations from a violin, the bow “sticks and slips” over the strings due to friction. When a spiny lobster moves its antennae a certain way, a piece of soft tissue rubs against a smooth, stiff file near its eye and produces sound.
Making sounds is no random act. There’s a lot of listening happening, too, though you won’t find an identifiable outer ear anywhere on an invertebrate or a fish. Interestingly, fish inner ears are similar to those of other vertebrates, including mammals. Ocean animals use their hearing to detect predators, find prey and communicate to potential mates. One loud, manmade sound in the ocean emanates from seismic air guns, devices used to search for oil deposits.
To examine what damage may be done to fish hearing by human-induced noises, researchers placed rockfish (snapper) in cages at varying distances from such a gun and exposed the fish to various decibels and repetitions. Later dissection showed hair cells in the fishes’ ears were either ripped away or dying. Normally, sensory hair cells repair themselves when damaged, but even months later the fishes’ hearing showed little to no improvement. Of course, those fish were trapped, and wild fish could get away, right?
Further studies showed that some fish exposed to such debilitating sound displayed disoriented swimming behavior, meaning that many may be unable to swim away. Relatively recently, squid and octopi have been recorded to produce a faint popping noise as a defensive mechanism, yet more compelling reasons to break our oil addiction.
Sound is at least as important to marine animals as it is to land animals. Having this sense means being aware of events all around, no matter where attention is focused. Contrary to earlier thinking, the special qualities of the undersea world actually emphasize sound because sound travels much farther underwater than it does in air. And considering that much of the world’s ocean is dark and murky, the value of vision is reduced or even altogether eliminated. Accordingly, sound offers marine critters more information potentially critical to their survival. To hear some of these sounds, visit http://www.dosits.org/gallery/intro.htm, and click on “dusky damselfish” to hear thumping, “snapping shrimp” to hear crackling, “spiny lobster” to hear creaking and “air gun” for manmade noise.
— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to firstname.lastname@example.org.