Hard Corals
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It has some 359 individual species of hard corals from more than 70 genera. This compares with a global maximum of about 450 species in Indonesian and Philippine waters. While most of the coral species on the Great Barrier Reef are found in other parts of the world, ten species are considered endemic.
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Hard coral - Acropa sp | Brain coral | Hard Coral - Acroporidae sp |
About coral
Corals are closely related to jellyfish and are very simple animals. However, the majority of corals are colonial with many single polyps making up one colony. A coral colony can be thought of as being similar to an apartment building made up of several units, with each tennant holding their head out the window to catch passing food. Each polyp has its own mouth, stomach and tentacles and reproductive organs. All feed and breed individually.
Some corals are composed of only one polyp and are called solitary corals. These include the mushroom corals (Family: Fungiidae).
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Hard coral - Avia lizardensis | Hard coral - Coelentrate sp | Mushroom coral |
Growth
The framework of a coral reef is composed of the calcareous skeletons of many corals. As a coral grows new polyps replace old polyps which become the calcareous skeleton.
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Tabletop coral | Hard coral - Porites sp | Staghorn coral |
Corals need:
Humans need food, water and shelter to survive. These things are also important to corals but they have other specific needs:
- Preferably clear water, although some corals are able to survive in in-shore muddy waters (For example Goniopora sp., Galaxea sp.).
- Shallow water (approx 0-30m) where they receive adequate sunlight.
- Low nutrient levels.
- Water temperature range 16-35 degrees Celcius.
Feeding
Corals employ a variety of methods to obtain their food.
Corals use their tentacles to filter and capture plankton and small fish from the water column. Nematocycts lining the tentacle skin help to paralyse the food so that the tentacles can deliver it the the mouth.
Hard corals, however, gain most of their energy from the tiny algae called zooxanthellae (pronounced zoo-zan-thel-lee) which live inside their skin. The zooxanthellae use the coral as a safe place to live, and in return, can provide the coral with up to 80 % of its energy. This is called a symbiotic relationship.
This is why it is important for corals to live in shallow waters where they can get lots of sunlight.
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This picture taken through a microscope shows a soft coral polyp (Lobophytum compactum) Green shows the polyp tissue, while the red represents the zooxanthellae. |
Different corals
Generally there are two main types of corals - hard and soft. Hard or stony corals have six (or multiples of six) smooth tentacles, while soft corals have eight feathery tentacles around their mouth. Hard corals have a hard skeleton, which is the part you see when a coral dies and is the part that is artificially coloured and sold in shops. Soft corals do not.
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Turret coral | Hard coral - Turbinaria sp. | A colourful reef scene |
Corals and colour
Some corals have pigments in their tissues that give them their orange, yellow, green, blue, red and purple colours. Others get their golden-brown colour from the algae (zooxanthallae) that live within their tissues.
As light passes through water it is refracted, or bent. Some colours, such as reds and yellows are lost, ie, you can no longer see them.The colours left are the blues and greens; this is why the ocean appears a blue colour. The corals you see in many pictures only appear so colourful because the photographer has used artificial lights to capture all the colours of the spectrum.
Coral health
When corals are not 'well' they get stressed and loose their zooxanthellae and end up a white colour. This is known as coral bleaching. The coral actually looks dead but often it is not. High or low temperatures and lack of sunlight can cause a coral to bleach but they usually regain the algae and their colours in a few months.
Reproduction
Corals can reproduce in two ways: by asexual reproduction, known as budding, and sexual reproduction which can be broadcast spawning (most common), or brooding.
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This picture shows the process of settlement from the pelagic larval stage to the benthic juvenile stage |
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Coral spawning |
In broadcast spawning, corals expel eggs and sperm into the water during one or two nights of the year. When an egg is fertilised by the sperm it develops into a planula, which floats around in the water for a while before settling on the ocean floor. After it has settled, it starts to bud and the coral colony develops.
Coral predators
Parrotfish, butterfly fish, angelfish, sea slugs, snails, worms and the crown-of-thorns starfish all eat corals.
Coral relationships
Crabs, shrimps, fish, eels and worms all live under or within a coral colony for protection.
Recognising coral
The easiest way to identify hard corals is by their appearance i.e., boulder, branching, plate, table, vase, bushy, solitary. They can also be described as rock like, heavy and solid feeling. Hard corals can also be shape and dangerous.
Protecting coral
Humans can damage corals by hitting them with anchors, walking on them and polluting the water they live in. Storms can injure or even kill many corals. Diseases can also kill them.
It is important to treat corals well so they can be around for a very long time. Never remove or break off a piece of coral. When reef walking, never walk where you are not supposed to and stick to the sandy areas. When you are in a boat never anchor near a coral reef because the anchor could get caught in the coral and destroy it.
