Australia | Coral Reef

As we move further up the west coast of Australia, and approach the Tropic of Capricorn, corals are more in evidence, and start to form reefs. The coral reef is a very different ecosystem from that of the cold waters of the south. 

The coral reef is like a gigantic self-contained organism, in which energy passes from one layer to the next, with just two essential ingredients - the sun and the sea. The corals are specialists in poor waters like these, provided they are clear. 

If we dive down just a few metres, to where there is less sunlight, we can see that the variety of species quickly diminishes. 

Nearer the surface, the coral world flourishes in all its splendour. Over two thousand types of animals live here, with strange relationships among the different species, all of them adaptations to ensure survival in such a competitive society. 

This is a pair of clown fish, who choose to live where others die. They confidently swim between the stinging tentacles of this anemone, knowing no harm will come to them. The tentacles are armed with poisonous cells, which would mean certain death for any other fish. The clownfish are immune because the anemone does not recognise them as foreign bodies, they are like part of the family. 

The entire reef is full of specialists ready to eat anything edible.
 
The trigger fish like this one can, with their tough mouths, attack any reinforced structure, no matter how strong it is, and the parrot fish can even crush the corals. 

If you’re among polyps, the best thing is to look like them, and we have already met the master of imitation. The cuttlefish also lives here, just one more piece in the intricate puzzle of  the reef. And, as always, they have to hide from the local marauding hungry mouths – in this case the grouper. There are members of the serranid  family in virtually all the waters of the world. Their rear fins are a sure sign they can accelerate very rapidly if necessary. 

One of the most dangerous killers is not, however, particularly large. Nor does it hide, because it wants everyone to know who he is, so they will leave him in peace. 

It is a sea snake of the laticauda genus, a reptile which evolution has returned to the sea, adapting its anatomy to the new medium. One and a half metres in length, it hunts fish by surprising them in their lairs, and injecting its deadly venom into them. Its flattened tail serves as a fin, making it easier to move around this strange, unpredictable landscape full of holes and cavities. Though it lives in the water, this species must periodically return to land, to mate and lay its eggs. Like all snakes, it is dependent on the temperature of the environment it lives in, and so can only survive in warm waters like these. 

The laticauda knows that all the creatures of the reef live very close to, or hidden inside the structures that form this bizarre environment. 

In their constant wanderings around the coral reef, it is common for two of these snakes to meet. When that happens, it is important their natural aggression should be inhibited, until they recognise each other. And so they embrace, and remain still, until they have made sure. Once they have checked each other out, they separate and each one goes on his way. 

But the corals are animals, and would not be able to take advantage of the energy from the sun, were it not for the fact that inside them live tiny algae called zooxanthellae. These produce oxygen and carbon, and generate the limestone that forms the hard skeleton of the corals. The more sunlight, the more they grow. 

Oblivious to life of the coral reef, the largest fish in the world returns south in search of the plankton it feeds on. It is the whale shark, a colossus 18 metres in length, peaceful, inoffensive and curious. 

It slowly swims around the sea with its mouth open, filtering tiny crustaceans and fish. Its life is a mystery – in fact, it was only discovered by science relatively recently. With him, we are returning to the cold waters where we began our story.