corals and coral reefs
updated Oct 2016
you learn only 3 things about them ...
Although a hard coral may look like a dead rock, it is
a living animal. Don't step on it.
Hard corals are a colony of many tiny animals. Thus they
grow slowly. Don't break them!
corals are rainforests of the sea. Their presence allows
other marine life to find shelter and food in the area.
Hard corals reefs are commonly seen on many of our Southern shores.
Some are also found on our Northern shores. At low tide, they are
often mistaken for non-living rocks or dead corals. Many of them may
actually be alive! Please don't step on them.
Does Singapore have any reefs left?
Much of our reefs have been affected by land reclamation and coastal
development. These works have not only reduced live coral coverage
by about 65% since the 1980's, but also resulted in murky waters which
reduced sunlight penetration from 10m in the 1960's to about 2m today.
But Singapore's remaining reefs still has about half as many coral
species as the Great Barrier Reef although our reefs are only 0.01%
Most of our amazing reefs are hidden from view in the sediment-laden
waters, due to on-going coastal development. But during low tide,
the water clears up and some of our reefs are revealed. Ordinary people
can view our rich shores without having to swim or dive. Some of Singapore's
best reefs are just half an hour away from the city centre!
What are hard corals? Hard
corals produce a hard skeleton and belong to Order Sclerectinia,
'Sclero' means 'hard'. This
is part of Class Anthozoa that includes soft corals and sea
anemones. All of these belong to Phylum
Cnidaria, which includes jellyfishes
and hydroids. There
are about 3,600 known species of hard corals, making them the largest
group in the Class Anthozoa.
Each hard coral is a colony of tiny animals called polyps. Each polyp
produces a hard skeleton. What you see as a hard coral is the joined
up skeletons of countless tiny polyps.
More about the polyps: Each
polyp is very similar in structure to sea
polyp comprises a tube-like body column. One end of the tube has a
disk with the mouth in the centre (and is thus called the oral disk),
usually ringed with 6 (or multiple of 6) tentacles that are smooth
and unbranched. The other end of the tube ends in a pedal disk that
to the base of its skeleton.
The polyps in a colony are connected to one another by living tissue
that covers the entire surface of the colony. So please don't step
on living corals as you will damage the tissues, even though the polyps
Most hard corals have tiny polyps 1-3mm in diameter. But some hard
corals such as mushroom corals are enormous solitary polyps.
More about the skeleton:Each
tiny coral polyp produces a tiny external skeleton made up of calcium
carbonate. Called a corallite, this skeleton protects them and provides
of the polyp's body column is usually hidden in the corallite. The
polyp can retract into its corallite to hide from predators or to
avoid drying out when the colony is exposed at low tide.
The various shapes and surface patterns of hard corals arise from
the way the corallites are arranged. Here's more about some
common shapes and textures of hard corals.
hard coral polyps are tiny, they may produce a stony structure that
is several metres in diameter, weigh tons and be made up of hundreds
of thousands of polyps. Huge coral reefs are made up of the skeletons
of these tiny polyps, living ones growing over the skeletons of dead
ones. It is estimated that one square metre of living hard corals
produce 10kg of new calcium carbonate a year!
Most hard corals grow attached to a hard surface. But mushroom
corals lie unattached as adults.
How does the colony grow bigger? New calcium carbonate is constantly secreted by living tissues. But
each polyp generally has a fixed adult size. Thus with time, the corallite
becomes deeper. Periodically, the polyp lifts its base and builds
a new floor, sealing off a little space below. As the colony grows,
there develops a 'condominium' of abandoned floors, with the living
polyps only on the top floor!
The colony also grows as new polyps may bud off from existing polyps.
The buds may arise from the oral disk, column or base of the 'parent'
polyp. Or new polyps may arise from the common tissue in between existing
What do they eat? All hard corals
are carnivores. Those with small polyps feed on plankton or collect
finer particles using mucus films and strands. Some hard coral polyps
lack tentacles (e.g., members of the Family
Agaridae) and rely entirely on mucus to gather suspended food
particles from the water. Hard corals can produce a large quantity
of mucus. Larger polyps may capture small fish. Some coral polyps
only extend their tentacles to feed at night, and remain retracted
in their skeletons during the day. Yet others feed both day and night.
The polyps of all reef-building hard corals harbour microscopic, single-celled
algae (called zooxanthellae). The
polyp provides the zooxanthellae
with shelter and minerals. The zooxanthellae
carry out photosynthesis inside the polyp and share the food produced
with the polyp. The white colour of the skeleton is believed to assist
in photosynthesis by reflecting light onto the zooxanthellae.
It is believed
the additional nutrients provided by the zooxanthellae are vital to
hard coral health and growth. Thus
clear waters that let sunlight through for photosynthesis is important
for healthy reef growth. Many of the hard corals on our shores, however,
are adapted to murky waters.
A study suggests some
corals glow in the dark to help or to protect the zooxanthellae.
Called flourescence, this happens when pigments in the coral polyp
transform solar radiation into less damaging wavelengths. In this
way, polyp pigments act as a sunscreen to prevent damage to the zooxanthellae
within the coral polyp. Corals may fluoresce even during the day,
but the sunlight is so strong that you can't see it.
Fiercely Territorial: There is
competition among hard corals for the best locations for gathering
food or getting sunlight for their zooxanthellae.
Although they can't move about, hard corals can defend their territories
against other encrusting animals that might try to settle near them
or grow over them. Some can produce filaments or special long tentacles
(called sweeper tentacles) to clear the surrounding area of pesky
Coral Babies: Hard corals can
reproduce asexually by budding. 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. Some corals release their
eggs and sperm all the same time. Called broadcast spawners, these
mass spawning events usually occur once a year, a few nights after
full moon. During
this time, many species may spawn at the same time, resulting in an
upward rising 'snow' as eggs and sperm drift
to the water surface where fertilization occurs. After
a few days, the embryos will have developed into free-swimming larvae that eventually settle down on a suitable surface.
A stylised depiction of the reproductive cycle of Acropora. From "Effects of sediments on the reproductive cycle of corals R. Jonesa, G.F. Ricardoa, A.P. Negria, 2015 on ScienceDirect"
Does mass spawning
happen in Singapore? Yes it does! More on the wild shores of singapore blog.
While bazillions of eggs and sperms are released during a mass spawn,
most don't make it. Hordes of marine creatures gorge on the spawn,
from fishes and crabs to jellyfishes. As the tiny coral larvae develop,
they have to survive the countless predators that constantly sieve
the water for plankton and edible bits. As well as many other challenges
that we are still learning about. Excessive sedimentation, for example,
can interfere with fertilisation and other aspects of coral larvae
survival and successful settlement.
Role in the habitat: The partnership
between hard corals and zooxanthellae
allows hard corals to thrive in clear nutrient-poor tropical waters.
Like trees in the rainforest, hard corals provide the basis of life
on the reef. Their hard structures provide shelter for small animals,
a nursery for ocean-going creatures and protect the shoreline from
strong waves, storms and erosion.
Human uses: Living coral reefs
are worth far more to humans when they left alone. Reefs bring in
tourists which generate business beyond the shore (e.g., hotels, restaurants
and travel-related industries). Reefs are also homes to a bewildering
variety of creatures, some of which protect themselves with toxins
or other chemicals that may have pharmaceutical applications. A few
hard coral species are being used in surgery because their internal
structure is similar to human bone. Used in bone grafts, especially
in facial reconstruction, small coral implants quickly become infiltrated
by blood vessels.
Humans often take for granted the usefulness of coral reefs as nurseries
for important seafood and in protecting shorelines. We do not realise
these services that they provide, until we have lost the reefs.
Status and threats: Many of our hard corals are listed among the threatened animals
of Singapore (see cnidarians in general for a list of these threatened corals).
Human destroy reefs in many ways.
Destructive methods are used to capture live reef animals for the
aquarium trade or 'live' fish trade for restaurants. Cyanide is sometimes
used which kills corals. Or bomb fishing techniques that destroy reefs.
corals are mined for building materials or harvested for the
aquarium trade. Like other creatures harvested for the live aquarium
trade, most die before they can reach the retailers. Without professional
care, most die soon after they are sold. Those that do survive are
unlikely to breed successfully.
Coral reefs are
also affected by boaters who throw their anchors carelessly, and thoughtless
divers and shore visitors who damage fragile features.
Land reclamation buries reefs. While coastal development and dredging
raises sediment levels in the water which block out sunlight needed
by the zooxanthellae partners of hard corals and affects hard coral
Global warming may raise the temperature and acidity of the ocean
and result in more extreme weather conditions. These will seriously
affect the health of coral reefs.
Like other creatures of our shores, trampling by careless visitors
and over-collection by hobbyists also have an impact on local populations.
is coral bleaching? Zooxanthellae
contribute to the colour of coral polyps. When
coral are stressed, there may be mass loss of
zooxanthellae in a hard coral colony. As a result, the underlying
white skeleton shows through the now transparent polyps. The coral
colony thus appears white or 'bleached'.
Without the food provided by the
lost zooxanthellae, the polyps will be
stressed and prone to diseases. Skeleton production and reproduction
are also affected. Once the cause of bleaching is removed, however,
polyps may eventually regain zooxanthellae
(which live freely in the water) and thus recover their colour and
health. But prolonged bleaching can seriously damage large sections
of a reef. Bleaching doesn't only happen to hard corals, but may also
affect other animals that have a similar relationship with zooxanthellae,
such as sea anemones, soft corals and giant clams.
believed to cause bleaching include: temperature fluctuations (too
high or too low), excessive exposure to ultraviolet light, excessive
sedimentation in the water, changes in salinity (such as due to flooding),
pollution, oil spills and disease. It is generally believed that bleaching
is related to unusual prolonged temperature increases in the seawater.
Hard corals harbouring zooxanthellae live close to the upper limit
of temperature tolerance. Thus a temperature increase of even 1-2
degrees centigrade can result in bleaching. It is believed that global
warming will lead to more frequent occurrences of mass bleaching.
Where can we explore coral reefs in Singapore? Labrador has the last large mainland reef. There are also reefs at Sentosa, St. John's Island, Kusu Island and Sisters
Islands and Pulau
Kusu Island has living reefs
just minutes from the city centre
Kusu Island, Jul 04
Each polyp creates a tiny skeleton
Raffles Lighthouse, Jul 05
Pore corals have tiny corallites.
Sisters Island, Jan 06
disk corals have large polyps
with short body columns.
Sentosa, Jun 07
With long polyps that hide the hard skeleton,
the Anemone coral is indeed often
for a sea anemone.
Sisters Island, Dec 05
is a giant solitary polyp!
Sisters Island, Feb 07
Some corals flouresce
Kusu Island, Jun 04
Sisters Island, Nov 05
coral after bleaching.
Sisters Island, Ju 10
of fishes on a hard coral.
Sisters Island, Nov 05
produce mucus to protect themselves.
Pulau Semakau, Mar 05
produce slime to clean themselves.
Sisters Island, Dec 10
mucus produced during coral bleaching.
Pulau Biola, May 10
of a living reef at Sentosa
for the Integrated Resort.
Sentosa, Jul 07
Reefs of Singapore by Loh Tse-Lynn of the Marine Biology Lab,
NUS. A comprehensive site with lots of photos and maps, tons of
info on all our southern islands, and all about reef conservation
efforts in Singapore.
- Bleach Watch Singapore blog and facebook page.
corals (Order Madreporia) Tan, Leo W. H. & Ng, Peter K. L.,
Guide to Seashore Life. The Singapore Science Centre,
Singapore. 160 pp.
Reef Alliance: 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.
Reef website of the National Oceanic and Atmospheric Administration
of the US Department of Commerce: factsheet on threats to reefs
and lots of links.
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.
media reports on coral reefs on the wildsingapore news blog.
- From the
wild shores of singapore blog
- Danwei Huang,
Karenne P. P. Tun, L. M Chou and Peter A. Todd. 30 Dec 2009. An
inventory of zooxanthellate sclerectinian corals in Singapore
including 33 new records (pdf). Raffles Bulletin of Zoology
Supplement No. 22: 69-80.
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
- Veron, Jen.
of the World
Australian Institute of Marine Science, Australia. 3 volumes.
- Chou, L.
M., 1998. A
Guide to the Coral Reef Life of Singapore. Singapore Science
Centre. 128 pages.
Harry and Daniel Knop. 2005. Corals:
Indo-Pacific Field Guide
IKAN-Unterwasserachiv, Frankfurt. 305 pp.
Eric H. 2001. Aquarium
Corals: Selection, Husbandry and Natural History
T.F. H Publications. 464 pp
- Wee Y.C.
and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
National Council on the Environment. 163pp.
- Ng, P. K.
L. & Y. C. Wee, 1994. The
Singapore Red Data Book: Threatened Plants and Animals of Singapore.
The Nature Society (Singapore), Singapore. 343 pp.