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Corals in General
Coral Reef Formation
Types of Reefs
Biological Diversity and Economical Value
Corals of Mauritius
Threats and Stresses on Mauritian Corals
Coral reefs are made from masses of lime of carbonate built up from the sea floor by the accumulation over thousands of years of the skeletons of minute animals called polyps. Algae called zooxanthellae live in the tissues of the polyps in a mutually beneficial way (symbiosis). Eventually, the coral reef breaks the surface of the water.
Reef building corals grow best in shallow, sunlit water up to a depth of 12 metres though they can still construct reefs from the sea floor up to 40 metres deep.
Reef building corals prefer sea water of normal salinity (between 30 and 40 parts per thousand), with temperatures ranging from a maximum to 28 degrees in summer to a minimum of 15 degrees in winter.
A second group of corals exist that grows in thickets and develop on banks rather than on reefs. They tend to be found on the outer, deeper and colder parts of the continental shelves and platforms. These organisms can live in minimum winter temperatures ranging from 4o to 15o C and at depths of about 60 to 200 metres. There are no such corals in Mauritius but they can be found along the eastern Atlantic shelf edge from Norway to Cape Verde islands for example.
A third group of corals can be found in even colder seas, where
temperatures range form 2o to 6o C. They consist of small, solitary
corals and are to be found on the abyssal floors of the oceans
and on continental shelves around the Falkland Islands, Patagonia
As coral reefs can only exist in shallow, warm and sunlit waters of tropical or sub tropical regions, their range is restricted to between 30 degrees North to 30 Degrees South. They occupy a mere 0.2% of the oceans' surface.
Coral polyps are minute marine animals that live in colonies each
attached to the other by living tissues. Each coral polyp secretes
around itself a protective limestone cup that is linked to other
cups. Linked together this assembly of cups gives rise to a tremendous
number of different shapes and sizes.
As most corals are immobile they reproduce sexually by releasing
eggs and sperms in the water at given periods of the year. This
is known as spawning. Corals spawn at about the same time during
the night a couple of days after the full moon. In some regions
spawning occurs in October or November and in other regions that
happens in March or April. For most corals, fertilisation is external.
A larva called a planula results from this fertilisation.
The next generation of coral reefs originates from this free swimming larva called a planula. When it settles on the sea bed it reproduces asexually by budding and produces up to four polyps each of which, in turn, reproduces asexually giving rise to still other polyps, each secreting its protective cup of lime connected to the cups of neighbouring polyps. As the colony expands and grows upwards and outwards the original polyps die off to be replaced by new ones which settle on the remains of their ancestors.
Few planula are successful in establishing new colonies as numerous predators like fish and invertebrates take a heavy toll.
Given time a huge structure, the coral reef, is built made up of the lime skeletons of dead polyps on top of which is found a thin layer of live polyps.
Corals come in different shapes, sizes and colours, some are solitary
like the mushroom coral, others colonial like the brain, stag
horn or table corals. They also have varying growth rates, for
example the Acropora sp. has been reported as growing at the rate
of 10 to 20 cm a year, whereas the massive brain coral grows at
a very low rate: half a centimetre to one centimetre a year.
A symbiotic relationship exists between the reef forming corals and zooxanthellae. The microscopic algae lives inside the bodies of the reef forming coral and benefits from constant and optimum conditions in a secure place. The algae makes use of carbon dioxide produced by the polyp during respiration and uses it during photosynthesis to make carbohydrates some of which the polyp will use as a source of energy. Oxygen given off by the algae during photosynthesis as a by product, is used by the polyp during respiration. This close relationship between the polyp and the algae enables them to be more successful than if they were on their own.
The zooxanthellae algae, which can be green, brown or yellow is
responsible for the variety of colours exhibited by corals. The
massive blanching of corals witnessed throughout coral communities
world wide during the late eighties happened when the symbiotic
algae died leaving only the polyp and the carbonate skeleton.
Though the coral polyp obtains most of its food from the algae, it also feeds on drifting plankton by paralysing the planktonic animal with stinging cells found on its tentacles and passing the plankton to its guts through a mouth. This type of feeding occurs mainly at night when the density of plankton is the highest, while during the day the polyp remains retracted inside its skeleton.
As coral reefs are mainly made up of lime ( calcium carbonate
) they play an important role in the global cycle of calcium capturing
half of the calcium flowing into the ocean every year. This calcification
by corals enables scientist to infer considerable information
about past marine environmental conditions like sea level, siltation
There are basically three tyres of coral reef formation.
The fringing reefs
The barrier reefs
The patch reefs
Fringing reefs develop around oceanic islands close to the shore. The reef flat tends to be shallow on the land ward side and gives rise to a reef crest emerging out of the water.
Barrier reefs tend to develop along the edge of the continental shelf. A wide and deep lagoon separates the coast from the reef.
Patch reefs tend to form on the sea bed of the continental shelf
that is raised and close to the surface.
Coral reefs are biologically very diverse ecosystems, as diverse as tropical rain forests. It is believed that although covering less than 0.2% of the surface of the oceans, coral reefs harbour up to a quarter of all marine species, but the true figure could be even higher.
The reef's structure provides shelter to a wide range of organisms
like fish, crabs and invertebrates. Furthermore, wave action and
different organisms, in search of food and shelter, attack the
whole structure. Fragments of different sizes results ranging
from blocks to fine mud which then accumulates within the reef,
shifted to calm areas by waves or currents. This accumulation
of sand and mud provides habitat for sea grasses, blue green algae
and mangroves found close to the shore.
Sea grasses form an important part of the vegetation to be found on coral reefs. They tend to grow in sheltered places on the reef flats or in shallow water in between land and fringing reefs. Sea grasses are not algae but proper plants with roots and tiny flowers that bloom under water. Pollen is carried from the female flowers to the male ones by currents. They provide ideal shelter for a number of marine animals, and as such they are very important, but few animals feed on sea grasses.
Coral reefs are also very productive ecosystems. Up to 25% of
the fish catch from developing countries or 10% of the amount
of the fish caught world wide come from coral reefs. These two
figures are ample evidence of the importance coral reefs have
for humanity as a whole.
Reefs are also of great value to tourism, especially in Mauritius
where this industry depends entirely on clear lagoons and sandy
In spite of its considerable importance, coral reef ecosystems are now degrading throughout the world at a very rapid rate. It appears that 10% of coral reefs world wide are now lost beyond recovery and that a further 30% may collapse within the next 10 to 20 years and up to a staggering 60% may be gone in the coming 40 years.
The main local threats to coral reefs are eutrophication due to sewage pollution, over fishing and sedimentation ( the result of erosion, deforestation and poor land use). Tourism with excessive hotel and beach developments, the dredging of bathing and ski areas and anchor damage by pleasure crafts and fishing vessels are now also becoming major threats to coral reefs.
Coral bleaching occurred massively during the 80's, including in Mauritius, and it caused depressed coral growth, coral mortality and loss of aquatic life. It is thought that a variety of causes were responsible, including siltation and changes in salinity or, most likely, abnormally high sea water temperatures, approximately 30o C.
Further threats can be termed as global. For example global warming
which also may be one of the causes of coral bleaching. Depletion
of the stratospheric ozone layer resulting in an increase in ground
level ultra violet light may have as yet unforeseen consequences
on coral health.
In Mauritius, 150 km of fringing reefs surround the island, except along parts of the southern and western coasts, whereas off Grand Port, in the south east, there exist a short stretch of barrier reefs.
At the last count there were 36 genera of hard corals and 90 species
in the waters of Mauritius.
Corals in Mauritius are under threat from a multitude of factors,
industrial sewage, hotel development,
the operation of boats in the lagoon,
sewage and solid waste.
Please follow the hyper links to the relevant files.
Since 1980, the Mauritius Marine Conservation Society (M.M.C.S) has sunk a total of 10 ships for the purpose of creating artificial reefs around the coast line. It can take a long time, from 10 years to a century, for the sunken ship to be completely covered by corals and algae. But within weeks of the ship being sunk, small juvenile fishes take refuge inside it. Within a period of five years, soft corals and algae develop on the ship, eventually the small fish attract the larger ones and from then on it can be said that an artificial reef has been created.
The ships sunk by the M.M.C.S. since 1980
|Name of Ship||Date Sunk||Location||Depth Sunk|
|Water Lily||1980||Trou Aux Biches||24|
|Emily||1981||Trou Aux Biches||24|
|Tug II||1981||Flic En Flac||20|
|St Gabriel||1987||Flic En Flac||38|
|Kei Sei 113||1987||Flic En Flac||36|
|Stella Maru||1987||Trou Aux Biches||22|
|Silver Star||1991||Grand Baie||39|
Source: Le Week End 8th September 1996
As most ships are sunk in waters less than 40
metres deep, the wrecks are accessible to a wide range of amateur
scuba divers. This is of importance with regards to the increased
popularity this sport is taking among mauritians and tourists.
Furthermore, it has been noted that eventually these artificial
reefs attract commercially valuable fin fishes.
(1) The Illustrated Encyclopaedia Of Wildlife, Volume 61
Last Update: Thursday, February 05, 1998