What is coral?
Each piece of reef is made up of many coral polyps, which are actually animals, and the rock (calcium carbonate or limestone) that they attach themselves to. Coral reefs are made up of billions and billions of coral polyps. These tiny coral polyp animals, not much bigger than the head of a pin or match, are able to build the largest living growing structures on Earth, namely “The Great Barrier Reef” and our own “Ningaloo Reef”.
Polyps are considered very basic, primitive animals. They have a mouth surrounded by tentacles, a body cavity or gut, they extract food, and with it calcium, from the water. (To picture this, imagine a jellyfish in an egg cup backwards). The calcium undergoes a chemical reaction within the polyp gut to become calcium carbonate, which is continually secreted by the polyp in solution form, producing the rock formations that make the reef and house the polyps themselves. It is then hardened by the water and becomes solidified calcium or limestone.
So coral polyps not only make themselves a house, but a place to live, hide, sleep and feed for every other coral reef inhabitant. Does that sound basic and primitive to you?
Having Friends Around For Dinner - A Study In Sybiosis
Corals, like any living thing, need energy to survive, which they get by eating whatever they can catch when they extend their tentacles armed with little harpoons called stinging cells containing toxins which they fire at the rising Zooplankton (animal plankton) in the early evening to capture, then they eat it. They take it into their gut and digest it with enzymes. They expire carbon dioxide (CO2), as do all animals.
Much more important to the polyp, and therefore to the reef’s development, is that living inside the body tissue of a coral polyp, sometimes even from the larva stage, is a minute single cell algae (plant) known as Zooxanthallae (pronounced zoo-zanth-a-lay).
These minute algae produce up to 98% of the energy compounds the polyps need in return for a safe place to live. This is why coral is found in warm clear and sunlit waters so its faithful friend the algae can continue to provide the coral food through photosynthesis because without the algae’s food source it couldn’t survive. This give-and-take relationship, where 2 different species live together for a mutual benefit, between the animal (polyp) and plant (zooxanthallae), is called symbiosis.
Not only does the zooxanthallae algae provide most of the polyp’s food, it also provides the coral with its colour.
Coral’s natural colour is white, for the same reason our bones are – it is all made from calcium carbonate. Any colour coral has comes from the particular type of algae living inside its stomach - different algae provide different colourings.
Location, Location, Location
There are 2 major coral types, soft and hard coral. The reef you mostly see is hard coral.
What you see when you snorkel around these amazingly colourful corals, with all their wonderful and weird shapes and sizes is not as it appears, soft and leafy but in fact, very hard, sharp and brittle solid rock. For this reason we ask you not to touch, stand on or lean your weight on the coral while you are swimming. Some of the formations have taken hundreds of years to grow. If you need another reason, you will get cuts and scratches which are very hard to heal.
Hard corals are the reef forming corals, soft corals don’t contribute to the reef forming process much at all. However, soft corals have the ability to kill the hard corals. They produce chemicals called Turpines which are lethal to hard corals. The soft corals manoeuvre around - very, very slowly, but they do have the ability to manoeuvre. They select a likely place to stay, release their chemical, kill an area and attach themselves to it.
But why doesn’t the soft coral take over all the hard coral?
The hard corals are much longer living and the soft corals are far more easily storm and wave damaged, knocking them from their perch, and tumbling them around until they simply disintegrate. So soft corals mostly live at depths hard coral doesn’t like where they are protected from wave action.
For hard corals to grow successfully, they need certain geographical features to be present. Firstly they need a hard base to anchor onto. They glue themselves down for life (that is except for mushroom / fungi species).
They need relatively shallow, warm water 18°C and above and most importantly, plenty of sunshine. Most coral reefs develop between 30° north and south of the equator. Coral Bay is about 23° south, at about the same level as Rockhampton in Queensland and Alice Springs.
The Ningaloo Reef owes its existence to a warm water current called the Leeuwin Current. This current originates in the Pacific Ocean, flows over the north of Australia, through the Arafura Sea, down the eastern edge of the Timor Sea, down this west coast of Australia through the Indian Ocean, and right around to the southern coastline where it fades out into the Southern Ocean. The fact that this current flows down the western coast of this country makes it unique on the globe. There is no other large land mass on earth that has a warm water west coast current, and consequently there is no other significant coral reef situated on the western edge of any large land mass on earth.
And so close to the shore!