Coral Seas The Blue Planet

In all the seas of the world,

the warm waters of the Tropics contain the richest and most colourful communities.

Coral reefs. They may seem like underwater paradise,

but they are perpetual battlegrounds for space.

Even the corals have to fight for it.

In this crowded, frenetic community, every individual has to find its own place, its own way of surviving.

But none of these creatures would be here, if it were not for the coral.

A coral larva drifts in the open sea, floating in a soup of young reef animals.

If just one of these coral larvae settles in a suitable spot and survives,

a new reef will be founded.

In just a few days, the larva changes form and becomes a polyp, similar to a sea anemone.

Identical copies bud off, and gradually a colony develops.

Each polyp surrounds itself with a hard skeleton and from this solid base begins to grow.

It increases in length by 15 centimetres a year.

This branching coral is only two years old.

A mature reef can be thousands.

Corals provide the foundations on which the entire reef community relies.

Some organisms, like the Christmas tree worms, live within the coral.

Others climb out, away from the reef, to filter their food from the water.

As the community grows, intimate relationships are formed

and different creatures become dependent on one another.

Even animals that spend much of their time travelling in the open ocean

return to the reef for a clean.

Coral reefs can be home to astounding numbers of fish.

Here swim the smallest and the largest fish in the sea.

Whale sharks are only visitors.

When currents bring nutrient-rich water up from the deep, they come here to feed.

All these animals are here because of the coral.

This extraordinary complex maze is built layer upon layer

by millions and millions of individual animals...

..polyps.

Each polyp's flesh is supported by a limestone skeleton.

Below the gut...

..is the place where most of the growth occurs.

Here the living tissue deposits an intricate lattice of limestone.

Beneath that, the limestone skeleton is bare, having been vacated by the living coral tissues.

This is the hard structure that forms the foundation of the reef.

And a single reef can extend for many miles.

Coral reefs are only found in the clear, warm, shallow waters of the tropics.

Sunlight is vital to them,

even though they're animals,

because inside their flesh live millions of tiny single-celled algae...

plants.

And all plants need sunlight to photosynthesise sugars.

98% of the food that corals consume is produced by the algae.

Without them, the reef would not exist.

Like any other plant, algae need just the right amount of light,

not too much, not too little.

The corals regulate that

with pigments that we can only see when they are illuminated by ultraviolet light.

Most corals, for protection, spend the day withdrawn into their stoney fortresses.

Even then, they're not safe from the jaws of these butterfly fish.

At night, the corals take in water, expand their tentacles, and emerge to feed.

They collect plankton.

Each tentacle has batteries of stinging cells which fire on contact.

Once the prey is caught, it's passed down to the polyp's mouth.

It's at night when the polyps are extended that they add to the limestone foundations beneath them.

Inevitably, the corals begin to overgrow each other and that means trouble.

When neighbours get too close, they detect one another's presence chemically.

The aggressor, on the right, prepares for battle.

The polyps extrude their guts...

and simply digest their rivals alive.

A no-man's land, a band of white skeleton, is the only evidence of the night's border dispute.

Some corals are targeted by yet more deadly predators.

Predators that can crawl in search of their victims.

Crown of thorns starfish - poisonous, invincible eating machines.

They also extrude their gut and digest coral wholesale.

But some corals have help.

Small crabs living within their branches resist these onslaughts and defend their home.

They launch an attach on the vulnerable underside of the starfish.

Even the crown of thorns will retreat from such a determined attack and this coral is left unharmed.

Hump-head parrot-fish, nearly a metre and a half in length.

Their jaws are so powerful they can bite through rock.

When they descend to feed, the reef itself is under threat.

They are indiscriminate feeders, taking both rock and coral alike in their quest for algae.

These fish play a large part in the erosion of the reef.

The rock and coral they swallow emerges later as a fine sand.

On a single reef, they can produce tonnes of it every year.

This soft sand forms the tropical beaches that we find so alluring.

Over time, the sand builds up to form an island, which is then colonised by animals and plants.

Trees take root.

Birds arrive.

The guano from thousands of terns which have chosen to nest here

enriches the sandy soil, which then can support more plants.

But these terns, like other sea birds, depend on the ocean for their food.

Below water on the reef, there is not only competition for living space,

but a continual contest between predators and prey.

It's the arms race between them that, over millions of years,

has produced today's extraordinary diversity of form.

Jacks are one of the key predators on the reef.

Their weapon is speed.

They seek silversides and THEIR defence is to congregate in confusing shoals of shimmering silver.

The jacks deal with that by herding the silversides onto the reef.

Here the jacks have a better chance of separating individual fish from the shoal.

The jacks can now catch the isolated individuals with lightning attacks.

It's far safer to be hidden on the reef itself, within the tunnels of a sponge, for example.

These tiny shrimp are no bigger than grains of rice.

These shrimps are unique.

It's recently been discovered that they have a sophisticated social system, similar to that of bees.

All members of the colony are the offspring of one female - the queen, who is the only one to produce eggs.

As in a colony of bees, different individuals are specialised for particular tasks.

Some are guards and are armed with especially large and powerful claws.

They are on watch at all times, ready to tackle intruders.

A polychaete worm. For it, a sponge is an excellent hunting ground.

In such a maze of tunnels, attack can come at any time from any quarter.

Once the guards are alerted, the worm loses its advantage.

Better to retreat intact than risk serious injury.

The sponge makes a safe home for the shrimps and supplies them with food,

so that they never need venture outside.

An establishment that provides for all their needs is clearly well worth defending.

Just as shrimps guard their home, other animals defend their hunting grounds.

Glass fish make tempting prey for the red-mouthed grouper.

Its strategy is to swim slowly amongst them until they no longer see it as a threat.

There are other fish here too.

Lion fish are ambush predators, taking their time and watching for the right moment.

But there isn't room here for two predators.

The grouper, braving the lion fish's poisonous spines, tries to evict its rival.

But lion fish are persistent.

This grouper spent many hours simply defending his hunting patch.

Some animals prefer to avoid conflict whenever possible.

These harlequin shrimp, having captured the starfish,

are taking it back to a safe house beyond the reach of competitors and danger.

The problem with starfish is that they have minds of their own and five large, sticky arms.

By the time the shrimps have prised off one arm, another has reattached itself.

Only by manoeuvring the starfish on to its back, can they have any hope of gaining the advantage.

Even so, getting it back home

is a major undertaking.

The starfish is now a living larder.

If the shrimps can hang on to it, it will feed them for days to come.

The top of the reef is usually covered by a thin layer of green algae,

another living larder, and many fish depend on it.

Powder blue tangs defend their right to graze on a particular patch.

But for a larder as well-stocked as this, there is always competition.

When a shoal of convict tangs decide to graze, little can stop them.

Powder blue tangs try to keep them off.

But they are overwhelmed by sheer numbers.

The territory is stripped of algae in minutes.

The blue tangs appear to be fighting a losing battle.

But, eventually, they begin to get the upper hand.

They persist with their attacks until the marauders are well on their way.

When night falls, some very strange creatures creep out of crevices

and crawl over the reef.

This moving bush is an animal,

a basket star, which spreads out its arms to catch the night's plankton.

The reef becomes uncannily tranquil.

The fish retire, hiding themselves where they can.

The marbled rays come out to hunt for prey buried in the sand...

..using electroreceptors to scan the sea bed.

Their activity attracts sharks.

White tips.

At night, when vision is of little use, sharks have a real advantage.

They can still use both smell and electroreception to track fish.

These sharks are also hunting for fish concealed within the reef.

Their slender shape enables them to squeeze through surprisingly narrow gaps.

There is nowhere to hide.

Few animals are safe during these feeding frenzies.

Night after night, the reef animals are subjected to these raids.

But life on the reef is not just about food.

It's also about sex.

There are many different breeding strategies,

but each is aimed at maximising the number of young that will survive.

Every afternoon for two months, brown surgeonfish can be seen streaming across reefs

in the Red Sea.

They all head for the same place, usually some prominent feature.

Here they wait for the light to fade.

Suddenly females within the group make a dash away from the reef to release their eggs.

They're immediately followed by the quickest and closest of the males,

all of whom are striving to fertilise the eggs.

Inevitably others come here to feast on such easy food.

As the surgeonfish spawn, fusiliers move in above to eat the nutritious eggs.

These are just the first of many predators which will feed on the eggs and developing larvae

as they drift in the ocean during the next few weeks.

Other fish are less casual about their eggs.

Banded pipe fish stay close to a small chosen area on the reef.

Every morning at sunrise, the female leaves her sleeping site and swims to find her partner.

For ten minutes or so, they remain together,

reaffirming the bond that is essential to their partnership.

They swim together around his territory in a simple greeting dance.

Throughout the summer, when the female's eggs are ripe,

courtship begins in earnest in the early morning.

It takes time, and after about two hours, they rise together off the sea bed, entwining their bodies.

The male rubs himself against the female

stimulating her to release her eggs.

And now, swiftly, the male takes them.

The eggs, now stuck to his belly, are patted down to ensure that they stay there.

The female then leaves him, but every morning she will return for a session of synchronised swimming,

and so ensure that their bond is maintained.

Ten days later,

under cover of darkness, the male shakes his body,

and the young pipe fish are born.

Only now are they independent of their parents.

Since the male takes charge of the eggs when they're laid,

the female can start immediately producing the next batch.

Without his help, they'd only breed every 20 days, rather than every 10.

So, by sharing the work, they're doubling the number of young they can produce in any one year.

The flamboyant cuttlefish.

Unlike most cuttlefish, this one spends much of its time walking, not jetting, across the sea bed.

This is a male.

He is using his colourful display to try and seduce the larger female,

who seems unimpressed.

Eventually, she concedes.

The final event, the transfer of sperm, is very quick.

TRUMPETING

A singing, male, humpback whale.

Humpbacks are only visitors to the reef.

After a pregnancy that lasted a year, the females come here to give birth and suckle their newly-born young.

Their investment in their single offspring is considerable,

for each female will continue to nurse it for a further 6-12 months.

The males are here to mate.

The lone males sing to establish their relative seniority.

The louder and longer the song, the bigger and stronger the singer.

The better the song, the larger the male, the more mating opportunities he will get.

All these different mating strategies have the same aim -

to ensure that the greatest number of offspring will live long enough to breed themselves.

Corals also reproduce sexually,

but being fixed to the sea bed, they can't move to find a mate.

Somehow they must synchronise their sexual activity

and they do so using the rise in water temperatures of spring and the phases of the moon.

A few days after the full moon, in late spring, when tidal currents are at their weakest,

the corals of the Great Barrier Reef are ready to spawn.

Some corals are male and release clouds of sperm.

Nearby, a female will be releasing eggs.

Other species of coral are both male and female.

These release packages of eggs

already prewrapped in sperm.

Bundles of eggs and sperm float to the surface to mix with others from further along the reef.

Each kind of coral times its release to a certain hour on a certain night.

That maximises the chances of cross-fertilisation.

The fertilised eggs drift away from the reef.

The stormy season brings real danger to the animals of the reef.

Lobsters in the Caribbean sense a change in the water.

The temperature drops and powerful ocean swells disturb the sand.

Under the cover of darkness,

they emerge to run before the storm and risk crossing the exposed sand flats

to seek shelter in deeper water.

Every year, they make this journey.

From all over the reef, lobsters come to join the march.

They conserve energy by travelling in one another's slipstream.

And there is the added benefit of safety in numbers.

By daybreak, they've reached the edge of the deep reef and down they go.

For the rest of the stormy season, they will remain in the shelter of deep water, out of harm's way.

Sometimes during the stormy season, a hurricane builds.

And then the very structure of the reef itself is under threat.

An entire reef can be destroyed by just one big storm.

Hundreds of years of growth gone in a few hours.

Out in the ocean, new life continues to develop.

In time, coral larvae will return to colonise the rubble

and a new reef will grow on the wasteland.

To get pictures of sharks requires some care and a good understanding of shark behaviour.

The Blue Planet team has encountered many of the world's 400 kinds of shark

and by treating them with caution and respect no-one was attacked.

One of the richest areas for sharks is in the Pacific waters of the Central Americas -

the "Island Of The Sharks".

Our teams filmed both day and night.

Cameramen Bob Cranston and Peter Scoones have had a lot of experience with sharks,

and Bob has worked in these waters before.

They hope to film the feeding behaviour of white tip sharks -

a medium-sized species which hunts in packs.

For filming at night, especially sharks in a big area, we needed quite a bit of light.

Little battery lights wouldn't do.

We brought in a generator

and cables coming down to 650 Watt lights.

I don't think the sharks were disturbed at all by the light.

These sharks are clued in to little electronic impulses from each other

and also from the prey.

To reach this remote location our dive boat had sailed 300 miles off the west coat of Costa Rica

to the tiny isolated island of Cocos.

Although it's only six miles long,

it attracts an enormous concentration of marine life.

This is a big event.

This is a place in this vast ocean.

And the deep water currents come in and hit this island,

sea mount, whatever you want to call it.

It upwells,

brings up nutrients for little fish.

The bigger fish feed on the little fish and the sharks feed on them.

The whole food chain is right here.

We're here to film it.

With so many different subjects to choose from

the team made up to four dives a day

and some of the encounters were quite unexpected.

Some sharks keep swimming to breathe

but the white tip reef shark can sit on the bottom

and pump water across its gills by moving its mouth up and down.

One morning we woke up and there was this big ship,

a helicopter on its back deck.

We negotiated to get some flight time

and shoot some aerials of Cocos.

Wonderful, because few have seen Cocos from the air.

In Cocos's micro-climate it rains almost every day.

The water has been churning up the sediment near the shore,

making water visibility unacceptable.

It was time to move on.

This is our last evening at Cocos.

We've had two sunny days out of 18.

Tomorrow we're gonna take a day off and cruise to Malpelo

which is a little spot out in this huge ocean.

It'll be an easy day.

I need one. It's been 20 days of diving every day.

I'm worn out.

Malpelo is a barren volcanic rock 300 miles south of Cocos.

Bob spent the 36-hour journey doing essential maintenance to his diving equipment.

The tiny island of Malpelo is inhabited - and not just by seabirds.

To extend their fishing rights,

Columbia has placed a permanent garrison on the island.

The welcome distraction of visitors must have been a relief.

This precarious rope ladder is the only way onto the island.

Volcanic cliffs rise straight out of the sea,

making the shores treacherous.

Once ashore it's an arduous climb in the extreme heat.

Those guys who climbed Mount Everest,

they had sherpas to carry their gear up the mountain.

It was nice and cool.

The producer picked the hottest day of the year to climb this rock.

The volanic rocks of Malpelo soar almost vertically out of the sea bed.

Deep ocean currents run up against the island

and rise to the surface,

carrying a soup of nutrients.

The result is an oasis of life in a desert of open ocean.

The fertility of Malpelo attracts an enormous number of predators.

One of the most impressive spectacles is called a bait-ball.

A shoal of small fist are corralled into a ball near the surface

and then attacked by predators such as tuna.

But bait-balls are unpredictable, and may only last

a few minutes. Our team were extremely lucky to film this one.

That wasn't a very big bait-ball.

Probably only 400-500 fish, about this big around.

As soon as I jumped in the water, I could hear a roar.

This sound of scales flying... Bap, bap, bap, bap, bap!

Yellowfin tuna - wham! Wham! Wham! Silky sharks - smash! Smash!

And then, in about three minutes,

it's all over. They ate them all.

Malpelo was still to hold one more surprise for our crew,

only discovered when they dived on the other side of the island.

They'd seen the oceanic silky sharks before, but not in such numbers.

The silky sharks are a beautiful shark.

What makes them so fascinating is that there's thousands of them.

I don't think you have anything to be worried about or frightened of

if there's no bait in the water.

If you were to introduce something for them to feed on,

then they start competing with one another,

and then you might smell like a fish, and you could get bit.

But under normal conditions they're a very curious shark.

They come up to you, which makes it good for photography.

They bump and push off your camera.

I think they want to find out what you are and what you're doing here.

It's a very unique place.

You're diving on the edge of reality.

Next week, Blue Planet enters a world governed by the moon -

a world dominated by tides,

that spell life and death in the ocean.