Scleractinia

Order Scleractinia (commonly known as Hard Corals)

Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Anthozoa
Subclass:	Hexacorallia
Order:	Scleractinia Bourne, 1900 [1]

Where seen?
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.

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 skeleton:
Each tiny coral polyp produces a tiny external skeleton made up of calcium carbonate. Called a corallite, this skeleton protects them and provides support. Most 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.

Although 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.

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 polyps.

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 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 thezooxanthellae. 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.

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 intruders.

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 coral larvae that drift about and eventually settle down to form new coral colonies.

Role in the habitat:
The partnership between hard corals andzooxanthellae 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.

Credit: http://www.wildsingapore.com/wildfacts/cnidaria/coralhard/coralhard.htm