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.
AD
LIFE AQUATIC
40
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fall 2013
