The Symphony of the Reef

Sound travels farther and faster in water than in air, so countless marine organisms have evolved to produce and detect it. The constant cycle of life and death on coral reefs plays out amid a rich cacophony.

Over the past few millennia, humans have spent countless hours listening to rhythmic and melodic compositions; but there is music in nature, too. The natural din that emanates from a pristine environment can be — like man-made music — discordant, soothing or rousing. Environmental soundscapes may go unnoticed, but they can reveal purpose and meaning if listened to carefully.

The sporadic calling of birds, the snapping of shrimp or the whine of cicadas is not just meaningless background noise. Research conducted in the past 50 years indicates that the racket made by animal populations has evolved to facilitate communication, navigation, reproduction and hunting. Evidence acquired within the past two decades points to sound being essential to the survival of many organisms — particularly those that dwell underwater.

Sound is basically an energetic pressure gradient that sets in motion molecules of the conducting medium. The pressure gradient pushes together surrounding molecules and then allows them to settle back into their original positions as the gradient (noise) moves past. Water is much denser than air, and thus sound generally passes far and fast through it — five times faster than through air.

Angling for an Advantage
Beneath the lapping waves, and typically unnoticed by humans, a variety of marine organisms employ complex organs to both create and distinguish sounds. The perception of noise underwater is not necessarily a simple thing, but since sound travels so efficiently underwater, numerous marine creatures have developed methods of sensing it. In generation after generation, all sorts of marine creatures have gained reproductive advantages from beneficial mutations that promoted the creation of or sensitivity to sound. Invertebrates, fish, reptiles and mammals in marine environments have a variety of methods for sensing vibrating water molecules, and, just as important, they've developed important adaptive reactions.

Some species' sensory organs are similar to those seen in humans. Specialized cells called neuromasts on the lateral lines of fish, for example, have nerve structures much like those found in mammals' cochleas, the auditory portions of the inner ears. Other organisms have evolved distinctive structures; fish use swim bladders to generate noises, and mollusks, echinoderms, crustaceans and cnidarians can detect sounds using balance sensory receptors called statocysts.
A Raucous Soundscape
Any diver who listens attentively knows the ocean is not the hushed and still environment it may appear to be. Waves, thunder claps, howling wind and rain create a clamorous natural backdrop. Tides and currents resonate as they sweep across coral, sand, kelp or rocky bottoms. Rumbling volcanoes and seismic events add to the acoustic milieu. Include the countless sources of biological noise in the sea, from miniscule crustaceans to the world's largest beasts, and the ocean becomes a raucous soundscape of natural music.

On a healthy coral reef, fish of all shapes, sizes and families grunt, grind, sing and scrape to manufacture sounds used to delineate territory, form bonded pairs and hunt. More than 1,000 species of fish make and use sound in one way or another. Crustaceans make noise for defensive and, possibly, courtship purposes, but the unknowns far outweigh the data. A variety of crabs, lobsters, shrimps and other crustaceans have developed noise-making capabilities as diverse as those employed by terrestrial insects.

The crackling made by barnacles as they open and close and move their articulated appendages can be detected for miles. The predominant sound coming from coral reefs is the incredibly loud popping of tiny bubbles (cavitation) generated by hordes of small snapping shrimp for hunting and communication. Mussels can produce sound by stretching and breaking the byssal threads that attach them to the substrate. Urchins have been observed making crackling noises by clicking their sharp spines as they move. This crackling can also be caused by the urchin's test (the shell surrounding its body cavity) rubbing against its Aristotle's lantern (feeding apparatus).
Uses of Sound
Over the past 20 years marine scientists have employed listening technologies originally developed by the military. With these previously classified instruments and methods, researchers have been able to sample biological soundscapes throughout the marine environment; however, the natural compositions have not been easy to decipher. Singing whales, courting fish, unexplained ticking and humming, rumbling, grinding, groans and thuds can all be heard as marine species live, hunt, bond and procreate. But massive gaps persist in our understanding of the meaning of these sounds and their ecological role. The importance of sound to individuals, populations and entire communities of marine life remains largely unknown.

One poorly understood aspect of biological sound in the sea is marine organisms' auditory perception. We don't know the extent of the marine creatures that are adapted to use sound in one way or another. Fish such as groupers may use noise to establish territory for hunting or to attract mates. Schooling fish such as bigeye trevally use sound to synchronize the swimming patterns of the school and, perhaps, for navigation.

It is now known that the larvae of many fish and invertebrates employ sound as a navigational tool for finding the appropriate habitat in which to live out the next phase of their lives. Some organisms that incubate on reefs may imprint on reef noise; in others the attraction may be innate. Anemonefish spend their larval phase in the open sea far from reefs. On the verge of metamorphosing into their brightly colored juvenile forms, drifting anemonefish larvae detect the dissonance of noisy reefs, and they vigorously swim toward the nearest or loudest.

Recent studies have also led scientists to believe that reef fish have the ability to use underwater sounds coming from different habitat types to guide their nocturnal movements. Some nocturnal fish feed in deep, dark waters at night but return to the protective confines of coral reefs as day breaks. The acoustic differences among habitats may cue the fish to return to their preferred microhabitat during twilight hours. New data have also shown that minute coral larvae (planulae) can distinguish noises generated by bustling reefs. The sounds attract the larvae, which swim, using cilia, toward an appropriate settling site on the reef.
Signs of Life
In the past few years marine scientists have discovered that the healthiest reefs, which concentrate the most life in a given area, are also the noisiest. These noisy reefs may act as magnets, attracting more fish and invertebrate larvae than less-diverse reefs nearby. Pelagic species, which spend their adult lives in deep, open water, can detect the noise of boisterous reefs as well, but they actively avoid the reefs, preferring the feeding grounds of the open ocean.

A study performed at three well-managed marine protected areas (MPAs) in the Philippines found that the MPAs are significantly louder than overfished reefs where algae and urchins dominated. It appears that fish and invertebrates are probably able to locate reefs using sound as well as discriminate between the quality of thriving and damaged reefs. As the field of marine-soundscape ecology grows, more surprises emerge.

Arthur Myrberg of the University of Miami noted, "[S]ound production is important in the lives of fishes, and it is possible that we humans may be able to make use of that information as well." Several years ago scientists from the U.S. National Oceanographic and Atmospheric Administration (NOAA) and the University of Hawaii developed the Ecological Acoustic Recorder (EAR), which records the sounds of coral reefs. The hope is that EARs can demonstrate the disparity between healthy and stressed reefs and make for an inexpensive monitoring method. EARs might also contribute to identifying and managing spawning aggregation sites, since many commercially valuable species generate sounds in the course of reproduction. Whether this technology will be an effective, unobtrusive and inexpensive means of keeping tabs on the world's reefs remains to be seen.
Human Noise
It is now known that for at least some and maybe most marine life, sound is essential to livelihood. This fact should be incorporated into the development and management of fisheries and other marine resources. Because such a wide variety of creatures adapt to their surroundings through sound perception, it is likely that anthropogenic noise has a greater impact on the ocean environment than previously supposed.

It is difficult to determine the impact of human sound on marine ecosystems since its immediate effects go unseen. Accounting for soundscape ecology in the design of future technologies could drive the development of acoustic transducers (used for oceanographic, defense, geophysical and marine-life applications) toward more sensitive receivers rather than more powerful transmitters. This same approach could also be applied to seismic exploration, which involves the use of low-frequency sounds to probe the geology of the deep seafloor. Noise generated by the next generation of commercial vessels could be reduced through the use of anti-fouling technologies applied to hulls and low-cavitation or noncavitating vortical drives in place of today's loud, high-cavitation propulsion systems.

Biological sound undoubtedly conveys an extraordinary volume of information in marine ecosystems, and it's only beginning to be understood; the purpose of most of it remains a complete mystery. Phillip Lobel of Boston University is one of the world's experts on fish bioacoustics. His startling prediction is that "future research will find fish matching the complexity of communication we see in birds." Cracking the codes in marine-life clatter may help illuminate the evolution of communication, hearing, mate detection and territory defense, but at present all this remains speculation.

Far from being a place of placid silence, the wet world below the waterline is a magnificent concert hall filled with rich, meaningful and evolving biological music that researchers hope to understand more fully.
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© Alert Diver — Fall 2013