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 siphons

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 bodyweight 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 wafts.

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 a hundred 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

unexpandable 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.

But 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 hundred thousand 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 one hundred thousand 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 the 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 ocean's 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're 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 shoals appearance.

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

Hump Back Whales,

hundreds of them.

With every upward lunge, they sieve out up to a 100 kilos offish.

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.

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.