Original subtitles

Off the east coast of Australia, up to 200 kilometres offshore,

a line of surf rises out of the open ocean.

Beneath the surface, creating these breaking waves,

is the most magical marine environment on Earth.

It's one of seven recognized wanders of the natural world.

Great Barrier Reef

Stretching for over 2,000 kilometres up the tropical coast of Australia,

it is quite simply the largest living structure on the planet.

It's made up of almost 3,000 different reefs,

each one with a different personality.

And it's so much more than just coral.

But the Great Barrier Reef is most famous for its underwater world.

Here, there is more life than almost anywhere else on Earth.

Many of the creatures are exquisitely beautiful,

but some seem straight out of science fiction.

While others can kill in an instant.

And just when you think you know the reef, it changes.

Every second, every hour, every day and every year.

It's a world of continual surprises.

This is the amazing story of an ever-changing natural miracle.

The Great Barrier Reef.

Coral reefs only live in clear sunny waters of tropics.

they cover less than one percent of the world's oceans.

They are such special places, they contain a quarter of all known marine life,

and the Great Barrier Reef is by far the biggest of them all.

The sheer number of creatures that live here makes the reef as hectic

as any human city, with a rhythm of almost perpetual rush hours.

Imagine all this activity, this continuing ebb and flow of life,

continuing for over 2,000 kilometres.

It's hard to believe that such a huge solid structure

hasn't always been here,

but compared with the rest of Australia this reef is very young.

If we can go back, just 20,000 years in a instant,

we see it was a very different place.

It wasn't even underwater.

it was covered with forests of eucalyptus and paperbark,

and it was a land abounding with the animals of the Australian plains.

Aboriginal people would've lived here too,

hunting the wildlife in areas that are now deep under water.

Then at the end of the last ice age, ice at the poles melted

and sea levels rose all around the world,

flooding this low-lying coast.

This happened only 10,000 years ago.

In fact, stories of The Great Flood

are still passed down in aboriginal culture today.

As the sea rose,

corals began to grow on the rocky fringes of the continental shelf,

creating the Great Barrier Reef we see today.

These shallow tropical waters are clear and warm...

perfect conditions for corals to thrive.

Sheltered behind this long strip of reef, a lagoon was born.

An area of protected water larger in size

than the whole of Great Britain.

And a new coastline too, with shallow sandy waters.

The rising sea also cut off areas of high ground,

creating the 600 islands that dot the lagoon.

Some are little more than rocks.

Others substantial mountains covered in woodland.

The Great Barrier Reef is so large that it can be seen from space,

quite an achievement, considering the size of the creatures that built it.

This vast structure is created by tiny animals called polyps.

Each polyp is like a tiny upsidedown jellyfish

sitting in a stony cup.

They live together in colonies, like underwater tower blocks.

They respond to touch, temperature, currents

and the cycles of the sun and moon,

and their tiny movements combine to give each colony,

a rhythm of its own.

The polyps can't build the reef alone.

For this they need partners that are even smaller.

Within each polyp's tentacles are millions of tiny brown dots.

Each is a microscopic plant

which transforms sunlight into food and energy for the corals.

It's an almost miraculous partnership that allows the corals

to turn minerals in the water into limestone,

building their stony skeletons.

In this way, each individual colony grows.

If we were able to watch a reef over several years,

we'd see a continually growing marine metropolis.

Its intricate architecture provides homes for thousands of creatures.

And the most obvious animals here are fish.

There are coral eaters, plant eaters, plankton eaters,

the hunters and the hunted.

Bluefin trevally

powerful predators that hunt in packs.

One minute apparently minding their own business,

the next charging their prey with a sudden rush.

Working together, they create confusion.

The trevally depart as quickly as they arrived,

and the colourful reef fish regroup again to feed.

Beyond them, silver baitfish never let down their guard.

They shimmer like a thousand tiny mirrors,

swirling as one amorphous mass to confuse any attackers.

It is their only defence against this

a shark mackerel.

These are the greyhounds of the ocean.

They need all their speed and agility against this ghostly school

that is here one moment and gone the next.

Despite the dangers,

small fish have to risk leaving the protection of the reef to feed.

And perhaps none is bolder than this little wrasse.

This gigantic grouper is many thousands of times her size

and an aggressive territorial predator

with one of the largest mouths on the reef.

But she's not deterred from approaching.

What she's about to do seems almost suicidal.

She's feeding on tiny bloodsucking parasites,

which is why the grouper allows this.

Nevertheless, the wrasse has to regularly vibrate her fins

against the inside of his mouth

just to remind him not to swallow.

A single wrasse can eat an incredible 1,200 parasites a day.

And she's not fussy where she finds them.

Without this decontamination

the grouper would quickly become infested.

This service is so valuable that the grouper is a regular client,

visiting several times a day.

Even he has his limits.

The many fish that live here, create a constantly changing visual spectacle.

but there is one fish,

that physically alters the reef's structure itself.

Bumpheaded parrotfish, each the size of a small sheep.

They have a tough beak

and gigantic jaw muscles that work like bolt cutters.

It might seem like a lot of hardware for a diet of soft algae

and coral polyps, but the only way to get at them

is by sheering off chunks of the coral rock.

A second set of internal teeth

then grinds the coral into a fine paste.

A single bumphead can chew up five tonnes of coral every year,

which they excrete back onto the reef as sand.

As fast as the reef grows, parrotfish break it down again,

creating a continually changing environment.

Constant remodelling of the reef by ravenous parrotfish

is nothing compared to the effect of the ocean.

Large waves regularly whip in from the Pacific.

The constant action of water hitting the reef crest,

breaks up the coral into rubble.

Waves and currents move the rubble and sand around

and shape it into beautiful small islands

known as coral cays.

Some come and go in a matter of days.

Others build and grow and last for years,

as plants take root and protect them from the elements.

Today it's rare to find a cay that is free from humans

and mainland predators, but this is one of the best.

Raine Island.

It's home to thousands of birds

and in the centre of the island they jostle for the best available space.

Nowhere else on the reef are they found in such numbers and variety.

Frigate Birds, redfooted boobies and Caspian terns.

In fact, 84 different species are found here.

It's also the largest green turtle breeding ground in the world.

Having mated offshore,

thousands of females haul themselves

up the beach to lay their eggs.

The record is 26,000 turtles in a single night.

They turn a remote desert island

into a crowded frenzy of nocturnal activity.

Wind and waves have created an island

that for the moment at least,

has just the right conditions to support this incredible spectacle.

But the island still changes, every year, every season.

And the reef itself never stays the same for long.

Twice a day, life along the length of the Great Barrier Reef

has to cope with complete upheaval,

as the rhythm of the moon causes tides to flood and drain the reef.

During the lowest tides, the water runs off,

and whole sections of living coral

that would normally be underwater, are exposed.

It's almost as though some mighty force has lifted the reef

right out of the water.

These are extreme conditions.

As the exposed corals start to heat up,

the tiny polyps retract inside their stony skeletons for protection.

Then they secrete mucus. It acts as a sunscreen,

and there's no skimping on quantity.

An area no larger than the size of a coffee table can produce

five litres of the stuff.

As the tide retreats to the edge of the reef,

it cuts off pools of water.

The most animals, like these sea cucumbers and starfish,

the best way to survive is by taking refuge in these rock pools.

With the tropical sun beating down,

the exposed flat is one of the most hostile environments on the reef.

For a fish caught out here it should mean certain death.

But not for this one.

It's an epaulette shark, it can't breathe out of water.

But it survives here by shutting down some parts of its brain

and increasing the blood supply to others.

Not only that, it can walk on land.

These abilities make it a specialised reeftop hunter.

Manoeuvring around the confined space of a rock pool,

it is master of all it surveys.

Nothing is safe. Not even animals hiding under the sand.

It scans for smells,

and for tiny electric signals

given off by the bodies of hidden creatures.

It's detected a crab.

It doesn't need sharp teeth because it simply sucks up its prey.

The epaulette isn't the only shark able to

adapt to a change of the tides.

When the sea returns, it brings cool oxygenated water back to the reef.

The relief, though, is shortlived.

Larger sharks gather at the reef's edge,

waiting to get at the freshly accessible hunting grounds.

As the water rises,

more and more creatures take the opportunity to feed in a new area.

For the sharks, fish that can normally outrun them

are temporarily caught in the shallows,

trapped between sand and surface, with few places to hide.

Lemon sharks hug the shore, surrounded by schooling bait fish,

for the moment,

they seem to have little interest in all this food around them.

They're simply waiting for help.

Young trevally.

They dart for the fish, causing them to panic and break ranks.

By putting themselves in the midst of the shoal,

the sharks are in a perfect position to exploit the chaos.

For a hungry lemon shark, no water seems too shallow.

The sharks are able to use these special tactics to feed inshore

because of the rhythm of the tides,

which change conditions every few hours.

The cycle of day and night also has a dramatic effect

on everything that lives on the reef.

There are many enduring mysteries about the Great Barrier Reef,

particularly at night,

this is one of the most enchanting

Under ultraviolet light, the corals fluoresce.

It's this amazing lightshow.

One theory is that these magical colours

are in some way caused by the coral's natural sunscreen,

but no one really knows.

And the same goes for much of the life on the reef.

Just as in human cities,

there's a new cast of characters after dark.

Octopus emerge from their holes to stalk their prey.

A tiny sole improves his chances of survival

by mimicking a toxic flatworm.

Some of the strangest nocturnal creatures are relatives of the starfish,

like this feather mouthed sea cucumber,

sifting food from the sand.

Another is the basket star.

It comes alive at night, throwing out thousands of arms

to filter food from the plankton rich current.

The day shift is still here, of course, just hidden.

At night many of these fish sleep,

resting in safe nooks and crannies on the reef.

Without eyelids, they enter a trancelike state,

barely moving other than to keep water flowing through their gills

But they do give off a scent, and that means they can be found.

This cone snail catches its prey

in one of the most surprising ways imaginable,

and specialises in killing fish.

It sniffs them out whilst they are sleeping.

The goatfish may be asleep but it's aware of its surroundings.

First, the snail must sneak close enough

to bring its secret weapons into play.

The snail appears to release chemicals that paralyse its victim.

The goatfish seems powerless to escape, as it's swallowed alive.

Moments later,

a venomous barb inside the snail puts the fish out of its misery.

In a place where even seashells are deadly,

remaining undetected is essential

if you want to survive the night.

And simply hiding is not enough,

just a hint of your scent,

wafting into the current, can attract trouble.

One fish has come up with a very elegant solution to this problem.

This parrotfish disguise their scent using a sleeping bag made out of mucus membrane.

No one has ever filmed a parrotfish making its cocoon before.

It starts by exuding a layer of mucus around its body.

It then draws water into its mouth

and pushes it out of its gills,

blowing up the mucus bubble around it.

Within half an hour, the fish is completely enveloped

in a slimy cocoon, which traps its smell.

But not all reef hunters are fooled.

White tipped reef sharks do use smell to find their prey,

if that doesn't work they have another sense.

They can detect an animal's bioelectrical energy,

the tiny electrical signals

given off by even the smallest of its twitching muscles.

This resting parrotfish risks discovery every time it moves a fin,

or wafts its gills.

As the sharks pass, it holds its breath.

But sharks don't give up that easily.

White tips are the perfect size and shape

for hunting in these cramped spaces.

Their snakelike bodies can even wriggle into crevices,

and they can fold their dorsal fin flat

to squeeze through small holes.

If discovered, a sleeping parrotfish wouldn't stand a chance.

Once the first shark has caught its prey,

the smell and commotion attracts others.

Because of their sophisticated senses

and remarkable physique, white tips are great reef hunters.

At night they turn it into an art form.

Better than all other sharks.

We expect sharks to be hunters,

but after dark, the reef itself becomes one giant predator.

By day corals may be passive and plantlike,

but by night they become active hunters.

They snatch tiny animals from the current,

immobilising them with stinging tentacles.

Just like their relatives the jellyfish.

And they're highly competitive.

Here, two individual polyps are fighting for possession

of a single unfortunate creature.

At night the corals don't just hunt.

When territory is at stake,

whole colonies go to war with each other.

Once a coral senses another is too close,

it launches a barrage of stinging cells,

and the enemy returns fire.

It's a war of numbers

the coral with the greatest firepower will win.

Eventually, the coral on

the right forces its opponent to retreat.

Yet another small part of the reef has been changed.

Every night this goes on,

from one end of the Great Barrier Reef to the other.

Day and night, the reef is always changing,

but that is nothing compared to the impact of the seasons.

In summer, warmer water means more food,

making it the best time to bring new life into the world.

A pair of noddy terns fly close together

over an island on the reef.

These are two mates reaffirming their bond.

Noddies signal to each continually during courtship,

both in the air and after landing.

Once in the trees, the performance changes.

They bow their heads repeatedly.

It's pretty clear how they got their name.

Once they've started courting they don't like any interruption.

To reinforce their relationship,

the male has to collect nesting materials.

And the females are famously fussy,

so he chooses his present carefully.

A bent twig, it seems, is just the thing.

A few more of them and they have a nest, if you can call it that,

ready for their single chick.

Noddies are devoted partners,

and often remain together season after season.

Beneath the waves, the summer warmth triggers fish to court,

In bicolour parrotfish, the larger male

leads a tender underwater ballet to woo a partner.

The need to breed spreads across the reef.

These are surgeon fish.

Single females dart to the surface,

pursued by groups of competing males.

As each female releases her eggs,

the males race to fertilise them, with a puff of white sperm.

The rise in water temperature also sets the conditions

for the most remarkable event on the reef.

On only a few summer nights each year,

and triggered by a particular phase of the moon, the corals spawn.

This is the world's largest synchronised breeding event.

On any one of these nights,

trillions of eggs and sperm are released,

flooding the entire 2,000 kilometre length of the reef.

By spawning at this precise moment

the corals have caught a lull in the tide,

allowing sperm and eggs

of the same species to meet and fertilise.

They form baby corals, as the tidal currents pick up,

they're spread far and wide.

The young corals join billions of other baby creatures

floating in the plankton.

A baby grouper, only a centimetre long,

that will one day grow into a giant, over two metres long.

Most reef animals have tiny babies like these,

carried at the mercy of the currents.

Many have some ability to direct themselves,

and even the young corals can swim to a degree,

propelling themselves with microscopic hairs.

Each one of these could grow into a whole colony

and start a new coral reef.

It is remarkable that such miniature, fragile forms of these

create the lagest living thing on the earth.

The reef is a dynamic place, in constant flux,

influenced by the forces of geology,

weather, wind and waves, the rhythms of the sun and moon.

Yet its future depends on fragile young life forms

that must survive in the face of these powerful natural forces,

and find somewhere safe to settle and grow.

In the natural world, surely there are few tales that are quite so remarkable.

And this is just the start of what makes the Barrier Reef so great.

Only 7% of the ecosystem is coral.

The rest is covered with rainforest and rivers,

lagoon, swamp, and shore.

These are home to some of the world's strangest creatures.

In the next programme we explore the fascinating worlds

and wildlife beyond the reef,

and discover their connections to this most magical place.