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| Some
living hard corals can be seen at the coral rubble area. They
are found further out towards the sea where they are seldom
exposed at low tide. |
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'Sclero'
means 'hard'. The hard corals are the largest group
in the Class Anthozoa.
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|
Many of the corals found on Chek Jawa are adapted to grow in murky
waters. Here is one possible reason why there
are fewer living hard corals on Chek Jawa than around the Southern
Islands.
Hard corals belong to the same group as sea anemones. While sea anemones
are large solitary polyps, corals are tiny polyps that are linked
each other in a colony. Each coral polyp produces a tiny external
skeleton made up of calcium carbonate. This skeleton protects them
from danger and provides support. Huge coral reefs are made up of
the joined skeletons of countless tiny polyps, living ones growing
over the skeletons of dead ones. The various shapes and forms of hard
corals arise from the way the polyps join to one another.
Hard food: Hard corals are carnivores.
The coral polyps have tiny tentacles with stinging cells to capture
food. Large polyps might take small fish, while smaller ones feed
on plankton or collect finer particles using mucus films and strands.
Most coral polyps only extend their tentacles to feed at night, and
remain retracted in their skeletons during the day.
All reef-building hard corals also have an additional source of food
as they harbour zooxanthellae (symbiotic single-celled algae) inside
their bodies. The algae can carry out photosynthesis and shares the
food and oxygen produced with the coral polyp. The coral in turn provides
protection and simple minerals. It is believed this additional source
of nutrients from the zooxanthellae help hard corals produce their
hard skeletons and thus expand the size of the colony faster.
Coral Babies: Hard corals generally
reproduce asexually: new polyps bud off and enlarge the colony. A
piece of living hard coral that breaks off may continue to grow into
a separate coral.
However, they also reproduce sexually. A polyp may produce sperm or
eggs, but usually only either one at a time. Eggs and sperm are released
simultaneously for external fertilisation. Mass spawning occurs once,
sometimes twice, a year in reefs. During this time, many species may
spawn at the same time, resulting in an upward rising 'snow' of eggs
and sperm. The eggs develop into free-swimming larvae that drift with
the plankton before settling down to start a new colonies. Here is
a fascinating photo
of a coral larva on Image
Quest 3-D Marine Library
Role in the habitat: Hard corals
are important reef builders. Coral reefs provide homes for small animals
and are a haven and nursery for ocean-going creatures. Reefs also
protect the shoreline from strong waves, storms and erosion.
For more about human uses and status and threats to hard corals, see
cnidarians in general.
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Disk coral
Turbinaria sp.
Disk coral
Turbinaria sp.
Green boulder coral
Turbinaria sp.
Brown boulder coral
Porites sp. |
See
also ...
Cnidarians in general
Links
What
is a coral? on the CRC Reef Research Centre: introduction to hard corals,
types of corals, how they reproduce and grow and links.
About coral
reefs on the Coral Reef
Alliance website: quick facts on all aspects of corals including what
they are, what they eat, how they reproduce and the reefs they form; and
threats to
coral reefs.
An introduction to coral reefs
from the Coral Reef Ecology Home Page by Dr Teresa Turner on the University
of the Virgin Islands website: brief facts on coral anatomy, types of reefs,
zooxanthellae, what they eat, how they reproduce and threats to reefs.
Coral Reef website of the National
Oceanic and Atmospheric Administration of the US Department of Commerce:
factsheet on threats to reefs and lots of links.
Coral
Reefs on the Florida Keys National Marine Sanctuary website: an introduction
to coral reef growth and reproduction, types, zonation, roles in the ecosystem
and economy, threats. Including links to more info.
Other references
- Tan, Leo W. H.
& Ng, Peter K. L., 1988. A Guide to Seashore Life. The Singapore
Science Centre, Singapore. 160 pp. online
version
- Lim, S., P. Ng,
L. Tan, & W. Y. Chin, 1994. Rhythm of the Sea: The Life and Times
of Labrador Beach. Division of Biology, School of Science, Nanyang
Technological University & Department of Zoology, the National University
of Singapore. 160 pp.
- Barnes, Robert
D. & Ruppert, Edward E., 1996. Invertebrate Zoology. Harcourt
College Publishers. 6th Edition. pp. 1056, G-1-16, I-1-30.
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