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,
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T E X T A N D P H O T O B y E t h a n D a n i e l s
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.
the Reef
