Page 36 - Coral Reef Teachers Guide
P. 36
Coral Forest Teacher’s G u i d e What and Where are the Coral Reefs?
This means that most coral reefs are less Reef-building or hermatypic corals live only
than 10,000 years old. As communities es- in the photic zone (above 50 meters), the
tablished themselves on the shelves, the depth to which sufficient sunlight pene-
reefs grew upwards, pacing rising sea lev- trates the water, allowing photosynthesis
els. Reefs that rose too slowly could be- to occur. Coral polyps do not photosynthe-
come drowned reefs, covered by so much size, but have a symbiotic relationship with
water that there was insufficient light. zooxanthellae; these organisms live within
Coral reefs can also be found in the deep the tissues of polyps and provide organic
sea away from continental shelves, around nutrients that nourish the polyp. Because
oceanic islands and as atolls. The vast ma- of this relationship, coral reefs grow much
jority of these islands are volcanic in ori- faster in clear water, which admits more
gin. The few exceptions have tectonic ori- sunlight. Without their symbionts, coral
gins where plate movements have lifted the growth would be too slow to form signifi-
deep ocean floor on the surface. cant reef structures. Corals get up to 90%
of their nutrients from their symbionts.
Healthy tropical coral reefs grow horizon-
tally from 1 to 3 centimeters per year, and Reefs grow as polyps and other organ-
grow vertically anywhere from 1 to 25 cen- isms deposit calcium carbonate, the basis
timeters per year; however, they grow only of coral, as a skeletal structure beneath
at depths shallower than 150 meters be- and around themselves, pushing the coral
cause of their need for sunlight, and cannot head’s top upwards and outwards. Waves,
grow above sea level. Live coral are small grazing fish (such as parrotfish), sea ur-
animals embedded in calcium carbonate chins, sponges, and other forces and organ-
shells. It is a mistake to think of coral as isms act as bioeroders, breaking down cor-
plants or rocks. Coral heads consist of ac- al skeletons into fragments that settle into
cumulations of individual animals called spaces in the reef structure or form sandy
polyps, arranged in diverse shapes. Polyps bottoms in associated reef lagoons. Many
are usually tiny, but they can range in size other organisms living in the reef commu-
from a pinhead to 30 centimeters across. nity contribute skeletal calcium carbonate
in the same manner. Coralline algae are
important contributors to reef structure
in those parts of the reef subjected to the
greatest forces by waves (such as the reef
front facing the open ocean). These algae
strengthen the reef structure by depositing
limestone in sheets over the reef surface.
Coral reefs form some of the world’s most
productive ecosystems, providing complex
and varied marine habitats that support
a wide range of other organisms. Fringing
reefs just below low tide level have a mutu-
ally beneficial relationship with mangrove
forests at high tide level and sea grass
meadows in between: the reefs protect the
mangroves and seagrass from strong cur-
rents and waves that would damage them
or erode the sediments in which they are
rooted, while the mangroves and sea grass
protect the coral from large influxes of silt,
fresh water and pollutants.
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