Biology
 |
| Anatomy
of a coral polyp. |
The
building blocks of coral reefs are the
generations of reef-building , and other
organisms that are composed of calcium
carbonate. For example, as a coral head
grows, it lays down a skeletal structure
encasing each new polyp. Waves, grazing
fish (such as parrotfish), sea urchins,
sponges, and other forces and organisms
break down the coral skeletons into fragments
that settle into spaces in the reef structure.
Many other organisms living in the reef
community contribute their skeletal calcium
carbonate in the same manner. Coralline
algae [i.e zooxanthelate,filamentous algae]
are important contributors to the structure
of the reef in those parts of the reef
subjected to the greatest forces by waves
(such as the reef front facing the open
ocean). These algae contribute to reef-building
by depositing limestone in sheets over
the surface of the reef and thereby contributing
also to the structural integrity of the
reef.
Reef-building or hermatypir
corals are only found in the photic
zone (above 50 m depth), the depth to
which sufficient sunlight penetrates
the water for photosynthesis to occur.
The coral polyps do not photosynthesize,
but have a symbiotic relationship with
single-celled algae called zooxanthellae;
these algal cells within the tissues
of the coral polyps carry out photosynthesis
and produce excess organic nutrients
that are then used by the coral polyps.
Because of this relationship, coral
reefs grow much faster in clear water,
which admits more sunlight. Indeed,
the relationship is responsible for
coral reefs in the sense that without
their symbionts, coral growth would
be too slow for the corals to form impressive
reef structures. Corals can get up to
90% of their nutrients from their zooxanthellae
symbionts.
Although corals are
found growing in most areas of a healthy
coral reef, the elevation of the reef
flat relative to sea level (and considering
tidal range) imposes significant constraints
on coral growth. In general, only a
small number of hardy coral species
can thrive on the reef flat, and these
cannot grow above a certain height because
the polyps can withstand only limited
exposure to the air at low tide. Of
course some reef flats carry a meter
or so of water over the surface, and
then coral growth can be prolific. It
is the upward growth of coralline algae
on the outer part of the reef flat that
ultimately results in an overall rise
in the surface elevation of a reef,
which slopes gently downward in towards
the shore or lagoon and very steeply
downward in the seaward direction. Prolific
growth of these algae is a response
to water motion bringing in inorganic
nutrients and removing waste products.
The damaging effects of exposure at
low tide on the algae is ameliorated
somewhat by constantly breaking waves
on the reef edge. Nonetheless, it is
the case that mature reefs are in equilibrium
with both sea level and wave regime
with respect to their elevation, and
excess production of limestone moves
away from the margin to expand the reef
laterally and fill in low areas.
The more prolific growths
of corals are to be found in water deeper
than where the bottom is exposed at
low tides: on the frontal reef slope
(forereef), in lagoons, and along reef
channels that bisect the flat. Under
conditions of clear, moving seawater,
corals provide the bulk of the skeletal
material comprising the reef and the
structural complexity that results in
a high diversity of reef associated
fishes and invertebrates.
