Hard corals: animal, vegetable or mineral?

Hard corals are commonly seen on many of our shores. These odd stone-like organisms hardly look like animals. Could they be plants? Or simply colourful rocks? What ARE hard corals?

Are they animals?

A hard coral is made up of many small animals called polyps. The polyps usually look like tiny sea anemones, with a tube-like body column topped with tentacles.
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 and rely entirely on mucus to gather suspended food particles from the water. Hard corals can produce a large quantity of mucus.

Or vegetable?

The polyps of many hard corals harbour microscopic, single-celled algae (called zooxanthallae) inside their bodies. The algae undergo photosynthesis to produce food from sunlight. The food produced is shared with the polyp, which in return provides the algae with shelter and 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.

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.

Coral bleaching results when the there is mass loss of symbiotic algae (zooxanthallae) from the coral polyps.

The algae give colour to the polyps. Without their algae, the polyps are colourless and the underlying white skeleton shows through. Thus large patches of the colony appear colourless or white.Factors 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 and disease. It is generally believed that bleaching is related to unusual prolonged temperature increases in the seawater.

Or mineral?

Each polyp produces a hard skeleton. Called a corallite, this skeleton protects them from danger and provides support. A large hard stone-like coral that you see is a colony of countless tiny polyps and their tiny skeletons!
Out of water, the polyps are often retracted, leaving only their hard skeletons. Thus, they are often mistaken for non-living rocks or dead corals. But they are alive!

The various shapes and surface patterns of hard corals arise from the way the polyps join to one another.

Do we have living corals on our shores?

Much of our reefs have been affected by land reclamation and coastal development. These not only reduced live coral coverage, but also resulted in murky waters which reduced sunlight penetration.

Most of our amazing reefs are hidden from view in the sediment-laden waters due to on-going coastal development.During low tide, however, the water clears up and some of our reefs are revealed. Ordinary people can then 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! More about our city reefs on wildsingapore.