Original subtitles

In the far north,

after three dark months of winter...

a world is waiting...

for a trigger.

The sunshine of spring.

Starfish are the first to respond.

They race for the highest point...

and sensing changes in the water,

with the tips of their tube feet...

they spawn.

Sea cucumbers,

with only their mouths exposed,

now emerge.

They collect as many starfish eggs as they can.

Which is quite a lot, when you've got ten arms.

Now sea pens rise up to claim a share.

The creatures here must grab what they can of this annual banquet.

For the light has also set in motion the greatest transformation of all.

Fronds of kelp, a marine alga,

rise towards the surface,

lifted by their gas-filled bladders.

Soon, a marine forest has materialised...

teeming with life of all kinds.

These green seas are some of the most productive,

but fiercely competitive, waters

in all the oceans.

The southern tip of Africa.

Here, two great oceans collide.

In the shallows, fed by rich currents,

are bountiful forests of kelp.

Barely visible except for the pulsating siphon

through which it breathes, a common octopus,

waiting for prey to pass by.

A crab will do.

The octopus sets off in pursuit.

And then lurks...

with the patience of an ambush hunter.

But the octopus shares the Cape waters

with a great concentration of other predators.

Fur seals...

and sharks.

They all eat octopus...

if they can find one.

And pyjama sharks are experts...

at hunting in the undergrowth.

Time to disappear.

But these tough-skinned little sharks

are small enough to reach deep into crevices.

But the octopus is far from finished.

She slips her tentacles into the shark's gills.

That prevents the shark from breathing.

So, the shark has to let go.

When caught out in the open, and vulnerable,

this octopus does something truly extraordinary...

and never recorded before.

She disguises herself...

with a protective armour of shells.

She's hiding... in plain sight.

The shark can sense its prey...

but the shells confuse it.

In a forest full of hungry mouths,

superior wits allow this octopus to stay alive.

Forests of kelp flourish in seasonal waters around the globe...

particularly along the Pacific Coast of North America.

Here, the biggest kelps of all

grow in vast forests,

stretching for hundreds of miles.

In some places, the giant fronds

rise up to 60 metres tall.

The thickets they create are crowded with life.

Competition here, for space and food, is intense.

A challenging problem for the Garibaldi fish.

He tends to his patch of seaweed,

filled with tiny creatures that he eats.

As with most farmers, his work never ends.

He removes snails and any other grazers

that come to eat his algae...

no matter how big they are.

He has to deal with pests of all kinds.

This can be the worst of them -

sea urchins that can scrape off every vestige of algae from a rock.

Its spines are needle-sharp.

Somehow, the Garibaldi must remove it.

But the problem with sea urchins...

is that they just keep coming.

When evening arrives and the light fades, he has to stop.

He not only needs a rest, he needs to hide.

Because at night, predators prowl through these forests.

A torpedo ray,

capable of stunning its victim with 45 volts of electricity.

While the Garibaldi hides...

the urchins can feed without interruption.

The light returns...

and he finds his farm is once again under attack.

Urchins, like locusts,

have the ability to swarm.

And this can be disastrous, not just for the Garibaldi,

but for the kelp forest itself.

All the vegetation is now under attack.

The urchins move through the forest,

cutting through the kelp fronds

and leaving behind vast barrens.

These urchin armies have felled many kelp forests

along the Pacific Coast of North America.

But help is at hand.

Sea otters.

All other kinds of otters spend much of their lives on land,

but sea otters rarely leave the water.

At first, a newly-born pup is not a very good swimmer,

so its mother spends hours grooming its fur to make it buoyant.

But to provide her youngster with milk and keep herself warm,

she must eat up to 30% of her body weight every day.

She does that by eating shellfish.

And urchins are among a sea otter's favourite delicacies.

In the past, sea otters were hunted so intensively for their fur

that they came close to extinction.

With them gone, many kelp forests were replaced with urchin barrens.

Today, sea otters are protected...

and as their numbers slowly return,

many of the kelp forests are recovering, too.

Now, in some remote places,

sea otters are so numerous

they assemble in huge rafts...

something that hasn't been seen for over a century.

In the sun-drenched shallows off Australia...

kelp is replaced by the sea's only flowering plants,

sea grass.

The most extensive of these marine grasslands

can stretch for over 3,000 square miles.

All across the tropics, they're patrolled by tiger sharks.

They can grow up to five metres in length...

and have powerful crushing jaws.

Green turtles are their prey.

The turtles feed almost entirely on sea grass.

A single one can consume up to two kilos of it in a day.

But they can never rest easy.

Healthy turtles will keep well away from an approaching shark.

And just by keeping the turtles on the move,

the sharks prevent any one patch of sea grass from being overgrazed.

And that has benefits for us all.

A patch of sea grass can absorb and store

35 times as much carbon dioxide

as the same area of a rainforest.

So, the prairies and their sharks are surprising allies

in the fight against a warming climate.

The struggle to survive in our green seas

can have far-reaching consequences.

Once a year, one particular meadow in Australia is transformed.

Around the first full moon of winter,

an army materialises.

Spider crabs.

For the past year, they've been feeding in deeper waters.

Now they march across the sea-grass plains.

Hundreds of thousands of them.

They clamber over one another...

creating great mounds...

nearly 100 metres long...

They're not seeking mates, neither are they laying eggs.

They have come here in order...

to grow.

Like all crabs,

their bodies are enclosed

in a hard, un-expandable shell.

So, to grow, they have to break out of it...

and that allows the soft one that has developed beneath to expand.

It will take days for the new shell to harden.

Its legs are so limp that they won't work properly.

The crab is unprotected and in great danger.

A smooth stingray.

It's huge - about four metres long.

It wants a soft, freshly-moulted crab

that will be easier to eat.

The crabs try to stick together.

But now, disturbed by the ray,

they're scattering.

A newly-moulted crab

is too weak to keep up with the crowd.

The safest place is right in the middle of the pile.

That is why they have all assembled here.

There is safety in numbers.

The vast majority of the crabs escape...

and within the next few days,

they will be ready to return to the depths

and resume their lonely wanderings in search of food.

This is no graveyard...

but the triumph of a 100,000 crabs successfully moulted.

The green seas of Southern Australia

are particularly rich in such spectacular assemblies...

but most of the creatures come together

for a very different reason.

To breed.

The giant cuttlefish,

the largest of all cuttlefish.

They live for just one or two years.

Now, as the Australian summer draws to an end,

they have one last act to complete.

To find a mate.

But there are over 100,000 males,

competing for the arriving females in this one bay.

Among them a giant - a true Goliath.

He probably weighs about ten kilos.

Bands of colour sweep across his skin.

That's how cuttlefish communicate.

This smaller male couldn't possibly take him on.

Beside Goliath, and under his protection,

a female who has just mated with him.

But other rivals are still interested.

It seems a small male wouldn't stand a chance.

The female is now displaying a white stripe

along her side nearest Goliath.

It's a clear signal that she no longer wants to mate with him.

It's all the encouragement that the little male needs.

He's going to have to use trickery.

He tones down his colours

and tucks in his arms.

He's just the right size to mimic a female.

Goliath is deceived.

The small male now displays a white stripe,

just like the real female, to deter his advances.

He slips beside her...

and they mate.

By mating with multiple partners...

the female ensures the greatest genetic diversity for her young.

The sneaky male leaves,

his final act complete.

So, even among giant cuttlefish, it seems...

it's not all about size.

Other males in these Australian green pastures,

take greater responsibility for their young.

A weedy sea dragon.

This is a male and he's carrying a precious cargo.

While mating with the female,

he collected the eggs and attached them to his underside.

Now, he's leaving these seaweed thickets...

and travelling into more open waters

where elaborate camouflage is less effective.

And there are many predators out here.

And this is what the fathers risk their lives for.

Dense clouds of minute shrimp.

Mysids, one of a sea dragon's favourite foods.

They're drawing other sea dragons out here, too.

Finally, it's time for the young to break free.

But algae has grown over these developing eggs...

and it risks smothering them.

Nonetheless, the babies are emerging.

They've hatched successfully.

The fathers return to the tangle of kelp,

where they're virtually invisible...

while the young remain out here.

But they will grow quickly,

surrounded as they are by their ideal food.

Vast numbers of the oceans' young fish

start their lives in the green seas.

One of the richest nurseries of all are the mangrove forests.

Fringing the coastline of the tropics,

they form a natural protective barrier between land and sea...

and are some of the world's most productive forests.

Below the water, their arching aerial roots

give them a firm footing.

Here, there's abundant food for baby fish.

While the tangled roots protect them from bigger fish

and other predators that haunt the channels.

But in Northern Australia,

with the receding tide...

the little fish are forced to leave their shelter.

And now, they are vulnerable.

It's the most deadly assassin in the green seas.

The zebra mantis shrimp -

a male, almost 40 centimetres long.

But he's not hunting just for himself.

He's collecting food for his mate.

She may have been his partner for 20 years.

She relies on him to bring her food...

and puts her energy into her eggs instead.

In a world so full of food,

this would seem a sensible strategy.

But it's also a risky one.

Were her male to disappear,

she could starve.

Something has caught this male's attention.

Perhaps an irresistible odour...

or a distant call.

Whatever the reason,

a male will leave his burrow

and his lifelong mate.

An even larger hole.

Females who have lost their mates

appear to send out distress signals to call in a new male.

A larger female will produce more eggs.

So by mating with her, he will father more offspring.

But infidelity comes at a price.

A larger partner demands more food.

The richer a sea, the greater the competition..

And there is one green sea

that supports more life than all the rest combined.

Unlike the mangrove forests and the sea-grass prairies,

its location is in the open seas

and only temporary and unpredictable.

This greenness comes not from rooted plants...

but from clouds of floating ones.

Billions of microscopic phytoplankton are proliferating.

And in such numbers that they fuel one of the greatest feasts of all.

Off America's Pacific Coast,

hundreds of common dolphins are rushing to a banquet.

They're not the only ones homing in.

So are sea lions.

They're heading for Monterey Bay, California,

where algal blooms have caused an explosion

in plankton feeders.

Anchovies - millions of them.

The dolphins herd the anchovies towards the surface.

Sea birds and sea lions take advantage of the shoal's appearance.

It's a race to grab a share before others arrive.

Humpback whales,

hundreds of them.

With every upward lunge,

they sieve out up to 100 kilos of fish.

They're claiming the biggest share

of one of the biggest feasts on Earth.

So crucial are these tiny plankton,

that almost all marine life ultimately depends upon them.

It's the green seas, not the blue...

that are the basis of almost all life in the world's oceans.

To capture the magnificence of the undersea forests

and the surprising creatures living here, the Blue Planet II team

have developed a series of specialist camera rigs.

From the mega-dome recording half-in-half-out shots

at the top of the towering canopy...

to state-of-the-art time-lapse equipment

that speeds up time to reveal the secret life of the forest floor.

But, of all the creatures living in these forests,

filming one in the waters off the Cape of South Africa

was to prove the greatest challenge of all.

Naturalist Craig Foster has developed a fascination

for its most secretive resident, a common octopus.

There are almost 100 species of shark in these waters,

but that doesn't deter Craig,

who's swum here every morning for the past six years.

When you find that really small, tiny, little oval hole,

then you know it's been killed by an octopus.

And then if you look very, very carefully,

they're often in the vicinity of that.

Craig has witnessed the antics of these octopuses

and wants to share his remarkable discoveries

with Blue Planet cameraman Roger Horrocks.

Roger is immediately charmed by the strength of their personalities.

It's really been astounding to see how individual these characters are.

The common octopus all display different behaviours.

Some were extremely timid,

some were very, very bold,

they have variety and it's really endeared me to them.

After weeks of filming different individuals,

they finally meet one octopus

that's not only seemingly unafraid of the camera,

but willing to perform for it.

She just came right up - whoop!

And then came right through underneath the tripod

in between my legs.

Yeah, that's cool, amazing.

She very quickly just completely accepted both of us.

So, we've actually been getting these really intimate behaviours.

It's amazing.

I've watched this octopus for many months,

it's just trusting us in the water,

carrying on with normal activity which is just so fantastic.

Months later, the team finally film their star octopus on the hunt.

But then discover the grave consequences of sharing the forest

with so many other predators -

pyjama sharks.

Pound for pound, these sharks are far more brazen

and aggressive than a Great White.

They're like guided missiles.

The slightest smell that octopus gives off, that shark will find it.

The pyjama shark actually got hold of the octopus

and, I mean, I just assumed that was it.

After spending so much time with this animal,

it was just so difficult watching her get attacked.

Then, to their amazement, she fights back!

She put her tentacles down,

you can see them coming through the gill, basically,

closed the mouth. The shark couldn't breathe,

and that's what enabled her to escape.

And just witnessing that whole thing was,

you know, it was an incredibly intense moment.

The team are relieved by her escape.

But when caught out in the open,

her next trick astonishes them even more.

It lifted its arms and legs over its head,

and, at the same time, pulled the shell material with it

and created this extraordinary armoury.

Octopus had armoured up and then, when that guy came

through from the back, he could kind of smell something.

He's not seeing an octopus shape,

he's seeing that strange armoury.

Then he was bumping the octopus,

and it was just incredible to see how that octopus

outwitted that shark using the armour,

using all his, his knowledge,

it's just... absolutely phenomenal.

She's a rock star, man. A proper little rock star.

Thanks to Craig's and Roger's dedication,

the octopuses' astonishing behaviours are now known to science.

What else might we find as we continue to explore

these fascinating undersea forests?

Next time on Blue Planet II...

we meet the creatures that live where two worlds collide...

and discover how they cope with the demands

of the ever-changing coasts.