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Off the east coast of Australia, up to 200 kilometres offshore,
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a line of surf rises out of the open ocean.
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Beneath the surface, creating these breaking waves,
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is the most magical marine environment on Earth.
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It's one of seven recognized wanders of the natural world.
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Great Barrier Reef
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Stretching for over 2,000 kilometres up the tropical coast of Australia,
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it is quite simply the largest living structure on the planet.
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It's made up of almost 3,000 different reefs,
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each one with a different personality.
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And it's so much more than just coral.
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But the Great Barrier Reef is most famous for its underwater world.
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Here, there is more life than almost anywhere else on Earth.
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Many of the creatures are exquisitely beautiful,
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but some seem straight out of science fiction.
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While others can kill in an instant.
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And just when you think you know the reef, it changes.
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Every second, every hour, every day and every year.
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It's a world of continual surprises.
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This is the amazing story of an ever-changing natural miracle.
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The Great Barrier Reef.
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Coral reefs only live in clear sunny waters of tropics.
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they cover less than one percent of the world's oceans.
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They are such special places, they contain a quarter of all known marine life,
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and the Great Barrier Reef is by far the biggest of them all.
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The sheer number of creatures that live here makes the reef as hectic
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as any human city, with a rhythm of almost perpetual rush hours.
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Imagine all this activity, this continuing ebb and flow of life,
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continuing for over 2,000 kilometres.
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It's hard to believe that such a huge solid structure
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hasn't always been here,
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but compared with the rest of Australia this reef is very young.
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If we can go back, just 20,000 years in a instant,
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we see it was a very different place.
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It wasn't even underwater.
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it was covered with forests of eucalyptus and paperbark,
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and it was a land abounding with the animals of the Australian plains.
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Aboriginal people would've lived here too,
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hunting the wildlife in areas that are now deep under water.
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Then at the end of the last ice age, ice at the poles melted
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and sea levels rose all around the world,
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flooding this low-lying coast.
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This happened only 10,000 years ago.
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In fact, stories of The Great Flood
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are still passed down in aboriginal culture today.
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As the sea rose,
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corals began to grow on the rocky fringes of the continental shelf,
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creating the Great Barrier Reef we see today.
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These shallow tropical waters are clear and warm...
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perfect conditions for corals to thrive.
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Sheltered behind this long strip of reef, a lagoon was born.
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An area of protected water larger in size
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than the whole of Great Britain.
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And a new coastline too, with shallow sandy waters.
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The rising sea also cut off areas of high ground,
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creating the 600 islands that dot the lagoon.
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Some are little more than rocks.
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Others substantial mountains covered in woodland.
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The Great Barrier Reef is so large that it can be seen from space,
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quite an achievement, considering the size of the creatures that built it.
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This vast structure is created by tiny animals called polyps.
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Each polyp is like a tiny upsidedown jellyfish
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sitting in a stony cup.
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They live together in colonies, like underwater tower blocks.
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They respond to touch, temperature, currents
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and the cycles of the sun and moon,
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and their tiny movements combine to give each colony,
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a rhythm of its own.
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The polyps can't build the reef alone.
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For this they need partners that are even smaller.
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Within each polyp's tentacles are millions of tiny brown dots.
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Each is a microscopic plant
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which transforms sunlight into food and energy for the corals.
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It's an almost miraculous partnership that allows the corals
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to turn minerals in the water into limestone,
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building their stony skeletons.
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In this way, each individual colony grows.
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If we were able to watch a reef over several years,
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we'd see a continually growing marine metropolis.
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Its intricate architecture provides homes for thousands of creatures.
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And the most obvious animals here are fish.
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There are coral eaters, plant eaters, plankton eaters,
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the hunters and the hunted.
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Bluefin trevally
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powerful predators that hunt in packs.
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One minute apparently minding their own business,
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the next charging their prey with a sudden rush.
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Working together, they create confusion.
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The trevally depart as quickly as they arrived,
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and the colourful reef fish regroup again to feed.
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Beyond them, silver baitfish never let down their guard.
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They shimmer like a thousand tiny mirrors,
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swirling as one amorphous mass to confuse any attackers.
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It is their only defence against this
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a shark mackerel.
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These are the greyhounds of the ocean.
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They need all their speed and agility against this ghostly school
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that is here one moment and gone the next.
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Despite the dangers,
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small fish have to risk leaving the protection of the reef to feed.
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And perhaps none is bolder than this little wrasse.
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This gigantic grouper is many thousands of times her size
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and an aggressive territorial predator
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with one of the largest mouths on the reef.
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But she's not deterred from approaching.
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What she's about to do seems almost suicidal.
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She's feeding on tiny bloodsucking parasites,
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which is why the grouper allows this.
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Nevertheless, the wrasse has to regularly vibrate her fins
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against the inside of his mouth
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just to remind him not to swallow.
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A single wrasse can eat an incredible 1,200 parasites a day.
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And she's not fussy where she finds them.
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Without this decontamination
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the grouper would quickly become infested.
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This service is so valuable that the grouper is a regular client,
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visiting several times a day.
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Even he has his limits.
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The many fish that live here, create a constantly changing visual spectacle.
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but there is one fish,
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that physically alters the reef's structure itself.
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Bumpheaded parrotfish, each the size of a small sheep.
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They have a tough beak
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and gigantic jaw muscles that work like bolt cutters.
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It might seem like a lot of hardware for a diet of soft algae
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and coral polyps, but the only way to get at them
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is by sheering off chunks of the coral rock.
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A second set of internal teeth
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then grinds the coral into a fine paste.
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A single bumphead can chew up five tonnes of coral every year,
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which they excrete back onto the reef as sand.
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As fast as the reef grows, parrotfish break it down again,
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creating a continually changing environment.
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Constant remodelling of the reef by ravenous parrotfish
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is nothing compared to the effect of the ocean.
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Large waves regularly whip in from the Pacific.
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The constant action of water hitting the reef crest,
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breaks up the coral into rubble.
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Waves and currents move the rubble and sand around
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and shape it into beautiful small islands
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known as coral cays.
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Some come and go in a matter of days.
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Others build and grow and last for years,
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as plants take root and protect them from the elements.
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Today it's rare to find a cay that is free from humans
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and mainland predators, but this is one of the best.
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Raine Island.
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It's home to thousands of birds
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and in the centre of the island they jostle for the best available space.
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Nowhere else on the reef are they found in such numbers and variety.
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Frigate Birds, redfooted boobies and Caspian terns.
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In fact, 84 different species are found here.
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It's also the largest green turtle breeding ground in the world.
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Having mated offshore,
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thousands of females haul themselves
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up the beach to lay their eggs.
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The record is 26,000 turtles in a single night.
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They turn a remote desert island
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into a crowded frenzy of nocturnal activity.
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Wind and waves have created an island
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that for the moment at least,
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has just the right conditions to support this incredible spectacle.
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But the island still changes, every year, every season.
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And the reef itself never stays the same for long.
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Twice a day, life along the length of the Great Barrier Reef
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has to cope with complete upheaval,
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as the rhythm of the moon causes tides to flood and drain the reef.
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During the lowest tides, the water runs off,
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and whole sections of living coral
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that would normally be underwater, are exposed.
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It's almost as though some mighty force has lifted the reef
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right out of the water.
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These are extreme conditions.
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As the exposed corals start to heat up,
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the tiny polyps retract inside their stony skeletons for protection.
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Then they secrete mucus. It acts as a sunscreen,
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and there's no skimping on quantity.
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An area no larger than the size of a coffee table can produce
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five litres of the stuff.
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As the tide retreats to the edge of the reef,
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it cuts off pools of water.
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The most animals, like these sea cucumbers and starfish,
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the best way to survive is by taking refuge in these rock pools.
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With the tropical sun beating down,
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the exposed flat is one of the most hostile environments on the reef.
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For a fish caught out here it should mean certain death.
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But not for this one.
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It's an epaulette shark, it can't breathe out of water.
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But it survives here by shutting down some parts of its brain
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and increasing the blood supply to others.
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Not only that, it can walk on land.
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These abilities make it a specialised reeftop hunter.
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Manoeuvring around the confined space of a rock pool,
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it is master of all it surveys.
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Nothing is safe. Not even animals hiding under the sand.
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It scans for smells,
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and for tiny electric signals
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given off by the bodies of hidden creatures.
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It's detected a crab.
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It doesn't need sharp teeth because it simply sucks up its prey.
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The epaulette isn't the only shark able to
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adapt to a change of the tides.
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When the sea returns, it brings cool oxygenated water back to the reef.
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The relief, though, is shortlived.
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Larger sharks gather at the reef's edge,
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waiting to get at the freshly accessible hunting grounds.
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As the water rises,
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more and more creatures take the opportunity to feed in a new area.
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For the sharks, fish that can normally outrun them
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are temporarily caught in the shallows,
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trapped between sand and surface, with few places to hide.
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Lemon sharks hug the shore, surrounded by schooling bait fish,
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for the moment,
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they seem to have little interest in all this food around them.
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They're simply waiting for help.
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Young trevally.
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They dart for the fish, causing them to panic and break ranks.
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By putting themselves in the midst of the shoal,
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the sharks are in a perfect position to exploit the chaos.
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For a hungry lemon shark, no water seems too shallow.
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The sharks are able to use these special tactics to feed inshore
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because of the rhythm of the tides,
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which change conditions every few hours.
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The cycle of day and night also has a dramatic effect
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on everything that lives on the reef.
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There are many enduring mysteries about the Great Barrier Reef,
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particularly at night,
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this is one of the most enchanting
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Under ultraviolet light, the corals fluoresce.
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It's this amazing lightshow.
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One theory is that these magical colours
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are in some way caused by the coral's natural sunscreen,
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but no one really knows.
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And the same goes for much of the life on the reef.
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Just as in human cities,
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there's a new cast of characters after dark.
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Octopus emerge from their holes to stalk their prey.
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A tiny sole improves his chances of survival
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by mimicking a toxic flatworm.
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Some of the strangest nocturnal creatures are relatives of the starfish,
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like this feather mouthed sea cucumber,
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sifting food from the sand.
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Another is the basket star.
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It comes alive at night, throwing out thousands of arms
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to filter food from the plankton rich current.
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The day shift is still here, of course, just hidden.
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At night many of these fish sleep,
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resting in safe nooks and crannies on the reef.
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Without eyelids, they enter a trancelike state,
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barely moving other than to keep water flowing through their gills
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But they do give off a scent, and that means they can be found.
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This cone snail catches its prey
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in one of the most surprising ways imaginable,
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and specialises in killing fish.
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It sniffs them out whilst they are sleeping.
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The goatfish may be asleep but it's aware of its surroundings.
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First, the snail must sneak close enough
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to bring its secret weapons into play.
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The snail appears to release chemicals that paralyse its victim.
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The goatfish seems powerless to escape, as it's swallowed alive.
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Moments later,
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a venomous barb inside the snail puts the fish out of its misery.
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In a place where even seashells are deadly,
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remaining undetected is essential
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if you want to survive the night.
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And simply hiding is not enough,
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just a hint of your scent,
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wafting into the current, can attract trouble.
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One fish has come up with a very elegant solution to this problem.
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This parrotfish disguise their scent using a sleeping bag made out of mucus membrane.
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No one has ever filmed a parrotfish making its cocoon before.
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It starts by exuding a layer of mucus around its body.
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It then draws water into its mouth
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and pushes it out of its gills,
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blowing up the mucus bubble around it.
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Within half an hour, the fish is completely enveloped
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in a slimy cocoon, which traps its smell.
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But not all reef hunters are fooled.
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White tipped reef sharks do use smell to find their prey,
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if that doesn't work they have another sense.
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They can detect an animal's bioelectrical energy,
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the tiny electrical signals
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given off by even the smallest of its twitching muscles.
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This resting parrotfish risks discovery every time it moves a fin,
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or wafts its gills.
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As the sharks pass, it holds its breath.
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But sharks don't give up that easily.
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White tips are the perfect size and shape
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for hunting in these cramped spaces.
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Their snakelike bodies can even wriggle into crevices,
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and they can fold their dorsal fin flat
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to squeeze through small holes.
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If discovered, a sleeping parrotfish wouldn't stand a chance.
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Once the first shark has caught its prey,
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the smell and commotion attracts others.
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Because of their sophisticated senses
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and remarkable physique, white tips are great reef hunters.
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At night they turn it into an art form.
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Better than all other sharks.
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We expect sharks to be hunters,
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but after dark, the reef itself becomes one giant predator.
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By day corals may be passive and plantlike,
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but by night they become active hunters.
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They snatch tiny animals from the current,
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immobilising them with stinging tentacles.
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Just like their relatives the jellyfish.
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And they're highly competitive.
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Here, two individual polyps are fighting for possession
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of a single unfortunate creature.
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At night the corals don't just hunt.
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When territory is at stake,
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whole colonies go to war with each other.
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Once a coral senses another is too close,
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it launches a barrage of stinging cells,
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and the enemy returns fire.
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It's a war of numbers
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the coral with the greatest firepower will win.
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Eventually, the coral on
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the right forces its opponent to retreat.
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Yet another small part of the reef has been changed.
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Every night this goes on,
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from one end of the Great Barrier Reef to the other.
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Day and night, the reef is always changing,
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but that is nothing compared to the impact of the seasons.
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In summer, warmer water means more food,
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making it the best time to bring new life into the world.
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A pair of noddy terns fly close together
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over an island on the reef.
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These are two mates reaffirming their bond.
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Noddies signal to each continually during courtship,
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both in the air and after landing.
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Once in the trees, the performance changes.
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They bow their heads repeatedly.
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It's pretty clear how they got their name.
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Once they've started courting they don't like any interruption.
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To reinforce their relationship,
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the male has to collect nesting materials.
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And the females are famously fussy,
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so he chooses his present carefully.
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A bent twig, it seems, is just the thing.
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A few more of them and they have a nest, if you can call it that,
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ready for their single chick.
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Noddies are devoted partners,
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and often remain together season after season.
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Beneath the waves, the summer warmth triggers fish to court,
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In bicolour parrotfish, the larger male
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leads a tender underwater ballet to woo a partner.
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The need to breed spreads across the reef.
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These are surgeon fish.
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Single females dart to the surface,
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pursued by groups of competing males.
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As each female releases her eggs,
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the males race to fertilise them, with a puff of white sperm.
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The rise in water temperature also sets the conditions
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for the most remarkable event on the reef.
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On only a few summer nights each year,
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and triggered by a particular phase of the moon, the corals spawn.
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This is the world's largest synchronised breeding event.
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On any one of these nights,
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trillions of eggs and sperm are released,
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flooding the entire 2,000 kilometre length of the reef.
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By spawning at this precise moment
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the corals have caught a lull in the tide,
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allowing sperm and eggs
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of the same species to meet and fertilise.
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They form baby corals, as the tidal currents pick up,
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they're spread far and wide.
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The young corals join billions of other baby creatures
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floating in the plankton.
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A baby grouper, only a centimetre long,
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that will one day grow into a giant, over two metres long.
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Most reef animals have tiny babies like these,
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carried at the mercy of the currents.
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Many have some ability to direct themselves,
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and even the young corals can swim to a degree,
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propelling themselves with microscopic hairs.
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Each one of these could grow into a whole colony
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and start a new coral reef.
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It is remarkable that such miniature, fragile forms of these
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create the lagest living thing on the earth.
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The reef is a dynamic place, in constant flux,
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influenced by the forces of geology,
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weather, wind and waves, the rhythms of the sun and moon.
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Yet its future depends on fragile young life forms
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that must survive in the face of these powerful natural forces,
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and find somewhere safe to settle and grow.
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In the natural world, surely there are few tales that are quite so remarkable.
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And this is just the start of what makes the Barrier Reef so great.
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Only 7% of the ecosystem is coral.
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The rest is covered with rainforest and rivers,
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lagoon, swamp, and shore.
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These are home to some of the world's strangest creatures.
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In the next programme we explore the fascinating worlds
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and wildlife beyond the reef,
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and discover their connections to this most magical place.33406
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