Coral Reefs
Not all corals build coral reefs. Those that do not are called 'soft corals' and often live at great depths
in cold seawater. Stony corals are
reef-building species. Coral reefs are
built as calcium carbonate (CaCO3) exoskeletons of one generation of stony corals are
secreted on the exoskeletons of preceding generations. It requires millions of years for massive
reefs—such as those found in warm, shallow waters of the
Most reef-building activities are the result of stony coral living in a mutualistic relationship with a group of dinoflagellate protists called zooxanthellae (zooxanthellae live in the tissues of various marine invertebrates, including jellies and nudibranchs). Stony
corals depend on photosynthetic activities of zooxanthellae
as a principal source of carbohydrates. Predatory activities are mainly a source of
protein for polyps. Zooxanthellae
also promote exceptionally high rates of calcium deposition. As zooxanthellae
carry on photosynthesis, they remove carbon dioxide (CO2) from the
polyps’ environment, raising the pH. These pH
changes induce the precipitation of dissolved CaCO3 as aragonite
(coral limestone). The 90 m depth limit
for reef building is believed to correspond to the limits to which sufficient
light penetrates to support dinoflagellate
photosynthesis.
Certain algae, called coralline algae, live outside the coral organisms
and create their own calcium carbonate masses.
These algae contribute to the reef by cementing together larger coral
formations.
Reefs can extend hundreds of meters below the ocean’s surface; however,
only the upper and outer layer includes coral animals and algae. Most of the reef consists of exoskeletons of
previous generations of stony corals.
(The depth of the reef mass is evidence of changing oceanic levels
during glacial periods and of the subsidence of the ocean floor.) In addition to the outer layer of
photosynthetic and cnidarian life-forms, the reef
supports a host of other organisms, including fishes, mollusks, arthropods,
echinoderms, soft corals and sponges.
The exceptionally high productivity of reef communities depends on the
ability of reef organisms to recycle nutrients rather than to lose them to the
ocean floor.
There are three types of coral reefs: 1) Fringing reefs are built up
from the sea bottom so close to the shoreline that no navigable channel exists
between the shoreline and the reef. This
reef formation frequently forms a narrow, shallow lagoon between the reef and
the shore. Surging water creates
frequent breaks and irregular channels through these reefs. 2) Barrier reefs are separated from shore by
wide, deep channels. The Great Barrier
Reef of Australia is 1,700 km long with a channel 20 to 50 m deep and up to 48
km wide. (The