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SPRING 2014
LIFE AQUATIC
Divers love corals, but how much do we really know
about them?
Coral Competition
Corals are animals known as anthozoans, literally
“flower animals,” but their delicate appearances belie
very aggressive attitudes. Corals are not nice. To survive
they must defeat competitors that try to occupy the
same space. In the absence of zoning laws and property
rights it helps to have chemical and biological weapons.
For example, corals may extend sweeper tentacles
loaded with stinging cells called nematocysts, waving
them in the direction of anything they sense nearby.
Many corals have a sting so potent it can be felt — and
not soon forgotten — by divers, producing painful welts
that may require medical attention.
Corals also produce allelopathic substances, letting
them waft into the water to suppress the health and
happiness of their neighbors. Many soft corals produce
terpenoid compounds that not only protect their
territory but also make them taste bad to predators.
If chemical and biological warfare don’t do the trick,
some stony corals may use an even more practical
option: Eat and digest thy neighbor. Stringlike digestive
filaments cast out from the mouth or directly through
the corals’ sides quickly decompose what they touch.
In the several coral species that live unattached,
an alternative defensive approach is moving to a new
location. Despite having no brain, no eyes and no
legs, they somehow sense when things aren’t right; in
a coordinated way they inflate and deflate the polyp
to literally walk, or they use the currents to move
like a sailboat. Such free-living corals may avoid a
toxic neighbor, flip themselves upright when turned
over, unbury themselves when covered in sediment
or pounce on another coral to smother it. Despite all
these defenses, corals don’t always win their battles.
Some grow faster or are more potent than others, and
some sponges and many types of algae have chemical
and biological arsenals that can kill corals.
Coral Feeding
When we think of corals feeding, the image of a wall
of mouths comes to mind. Coral polyps extend their
tentacles to trap plankton from the moving water, but
there are other food sources and modes of feeding,
too. In places with little water movement corals feed
on suspensions of fine particles and larger organic
aggregates that fall from upper layers of the water
column; this is called marine snow, and corals use
hairlike cilia to move the particles into the polyps’
mouths. Coral mucus is referred to as a molecular
trap because it captures not only particulate but also
dissolved organic material that serves as food for both
the coral and its commensal bacteria. Mucus shed into
the water also traps food and rains back down onto the
corals as nutritious organic particles.
Symbiosis and Feeding
Many corals form a symbiosis with photosynthetic
dinoflagellates commonly known as zooxanthellae.
They produce food for the corals and receive essential
nutrients in return. However, this is more than a
simple two-way partnership — it establishes a whole
community called a coral biome. Though corals are
not nice with some neighbors, it seems they have
the secret lives of
corals
B y J U L I A N S P R U N G
KEN NEDIMYER
STEPHEN FRINK
JOE CAPARATTA
