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Under Construction
Coral Reefs
Coral reefs can be made up of hundreds of different species of coral. There are two main types: “hard”
coral with an outer skeleton of calcium carbonate (CaCO3) and “soft” corals that embed bits of CaCO3
inside their bodies. Although it comes in many shapes and sizes, all coral is composed of tiny individual
polyps. A polyp is a tiny animal that looks like an upside-down jellyfish.
 In soft corals, each polyp
contains little spikes of CaCO3 that help hold many polyps together in structures that look like fans or
whips. In hard corals, polyps sit inside little
cups which they build out of calcium
carbonate. Many of these cups are cemented
together to make up a coral colony. Reefs
are formed when hundreds of hard coral
colonies grow next to and on top of each
other. Since most species of coral polyps
stay deep within their calcium carbonate cups
during the day, the casual observer may think
of coral as inanimate rocks. At night,
however, the polyps emerge, and wave their
tiny stinging tentacles in the water to catch
microscopic organisms called plankton.
What makes coral polyps so unique is that plankton is only part of their diet. Each polyp harbors within its
body special algae called zooxanthellae. These one-celled plants use sunlight and carbon dioxide to
conduct photosynthesis, a process that produces oxygen ... and other nutrients needed by the polyps. In
return, the algae get protection and a constant supply of carbon dioxide and other raw materials they need
for photosynthesis. Such a mutually beneficial relationship is called symbiosis. Without this special
relationship, it is likely that there would be far fewer animals in clear, tropical waters since they typically
cannot support life. It is important to realise that the fish, crabs, snails, worms and other reef creatures
depend on the health and growth of the coral reef for their existence.
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Fire Coral
Divers beware - it stings |
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Soft Coral
Picture by Neil Sutcliffe |
New Corals
To start a new reef, coral polyps can reproduce sexually, using sperm and eggs. Male polyps send sperm
into the water where it enters female polyps and fertilizes the eggs within.
Fertilization is internal in corals
known as “brooders.” whereas “Broadcasters” shed their eggs and sperm into the water, and fertilization is
external.
Baby corals, called planulae develop and leave the polyps, floating on the current until they
reach a suitable surface where they can attach and begin to grow into new coral polyps. Sexual
reproduction allows corals to distribute themselves. To simply add onto an existing colony, polyps
undergo asexual reproduction in which new polyps “bud” off the parents and form their own CaCO3 cup
right beside the older polyp. Polyps formed this way are exact copies of each other, creating entire
colonies of coral with exactly the same genes.
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Brain Coral
Picture by Neil Suttcliffe |
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Sarcophyton soft coral
Picture by Neil Suttcliffe
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Coral Growth
Some reef corals are capable of growing 15 centimeters (6 inches) in a year. Massive corals such as star
coral and brain coral grow considerably slower, typically only 1/8 inch to 3/4 inch per year. As old corals
die, new ones usually settle and grow over the dead skeletons. Many generations of settlement,
competition, growth and death result in structures like the Great Barrier Reef in Australia, which is
hundreds of feet thick and millions of years old.
True reef-building corals can only grow where the water is clear, warm and shallow. Average water
temperatures typically do not fall below 20 degrees Centigrade (68 degrees Fahrenheit) and it is generally
no deeper than 100 meters (325 feet). Most corals grow in depths of less than 40 meters. These
conditions are met in tropical waters near the equator, on the eastern sides of continents and around
oceanic islands.
When a reef forms close to shore it is a fringing
reef. As the reef matures, the oldest corals near the
shore may die and the reef will become an offshore
barrier reef with a lagoon in between it and the
shore. When corals grow around a volcanic island
an atoll results as the island gradually subsides,
leaving only a ring of coral visible near the sea
surface.
Changes in sea level can also expose pieces of a
fringing or barrier reef, turning them into small coral
islands like the Florida Keys.
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Carnation Coral |
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Red Cauliflower Coral (Dendronephthya) |
Threats
Both hard and soft corals are vulnerable to unusually strong waves (e.g., those formed by a hurricane) as
well as dramatic changes in the temperature and saltiness of the water. Predation by fish, snails, worms,
crabs, shrimp and starfish, and overgrowth by fleshy algae can also kill corals. Parrotfish, for example,
have strong teeth with which they break through the CaCO3 cup to the polyp inside. Corals also compete
against each other for light and space. The faster growing corals usually dominate. However, slow
growers like brain coral are better at surviving physical disturbances such as storms. Over thousands of years, corals have evolved ways to defend against the natural threats they face.
Extending polyps only at night, using toxic chemicals (fire coral is an example) and producing huge
numbers of larvae all help corals survive and prosper. Unfortunately, these adaptations may be of little
use when it comes to threats from humans.
Human activities also threaten corals
in a variety of ways, including pollution, deforestation,
fishing and collecting.
Pollution
There are basically two kinds of pollution that damage a reef.
- Increasing water cloudiness - Zooxanthellae must have absolutely clear water in order to
get enough sunlight and still remain embedded in the tissues of the polyp. Sediments stirred up by
boaters and divers, washed off land by rain or expelled by oil drilling rigs can kill a reef by depriving it
of light. (Polyps can also be smothered by blankets of sediment.)
- Chemical -
Fertiliser runoff and sewage discharge fill the water with nutrients that allow algae to grow faster than the
corals, overgrowing and smothering the polyps. In addition, reef organisms are poisoned by heavy
metals, pesticides and oil. Even low levels of oil can slow down reproduction by making it difficult for
microscopic larvae to swim and settle. Slower reproduction means that reefs cannot repair damage as
quickly as usual.
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Staghorn coral (Acropora sp.) - may grow to be a meter in diameter in just seven years.
This is one of the fastest growing types of coral and is thus often dominant
on newly forming reefs. |
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Sea fan (Gorgonia sp.) - There are hundreds of different species of gorgonian's many of which have a plant
like appearance. Sea fans
usually grow at a right angle to the water current which helps them to catch as much food as possible. Picture by Neil Suttcliffe
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Tubastrea |
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Whip Coral
Picture by Neil Suttcliffe
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Fungia |
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