Original subtitles

Much of our planet

still remains unexplored...

for most of it is covered by water.

Every journey below the surface

can reveal something extraordinary.

More than 1,000 new species are discovered here every year.

The ocean is home to 80% of all animal life

on Planet Earth.

But today,

thriving in this varied world

sometimes demands the most surprising behaviour.

The shallow seas of the tropics.

This may look like a paradise...

but beneath the surface,

it is an arena

for constant life-and-death struggles,

where not everything is quite as it seems.

A predatory lionfish...

on the hunt for shrimp and small fish.

With so many places for its prey to hide,

the lionfish has to be patient.

This shrimp appears tempting.

As the lionfish closes in,

it fans out its fins

to disorientate its prey.

The hunter becomes the hunted...

tricked by one of the shallow sea's

most extraordinary predators...

a clown frogfish.

On its head, it carries a fishing rod,

a modified dorsal fin

baited with a lure, easily mistaken for a shrimp.

But when the fish stop biting,

it's time to pack the rod away

and move on.

And for a frogfish, that takes time.

Rather than swim,

frogfish walk.

Its fins have, in effect, become feet.

It may not look pretty, but what appears to be coral

living on its skin is actually camouflage.

As every good angler knows,

picking the right spot is important.

Jigging the rod sends out pressure waves

to attract the attention of possible prey.

These dragonets are certainly interested.

Almost there.

Hook, line and sinker.

Like all shallow seas,

the cold waters off the coast of North America

benefit from the intensity of the sunshine.

Here, that fuels one of the fastest-growing organisms

on the planet...

giant kelp.

Towering forests can eventually reach

the height of a 12-storey building.

These forests are nurseries for young horn sharks.

As the sun begins to set,

they leave their daytime shelter

and set off into the heart of the kelp forest

to find food.

Barely half a metre long,

this little shark is extremely vulnerable.

There are monsters here.

A giant sea bass weighing over 200 kilograms...

and wolf eels with big teeth

that can crush little red sea urchins.

But the biggest danger comes from other sharks...

houndsharks...

and huge broadnose sevengills.

In this forest,

it pays to keep a low profile.

An angel shark...

one of the most deadly sharks

in the kelp forest,

and certainly the best camouflaged.

An angel shark will lie in wait for days...

but this one has already attracted

too much attention.

It's not welcome here.

Now the coast is clear.

But angel sharks hunt around the clock.

Openings for breathing called spiracles

blow out sand that helps cover its body.

The trap is set.

Staying close to the seabed

is a good way to avoid most predators.

Faster than the blink of an eye.

But it's not over yet.

Horn sharks have a secret weapon.

Sharp spikes keep them

very much off the menu for most.

So this young horn shark...

can hunt in safety...

for now.

Violent winter storms batter these kelp forests.

The plants can be ripped from the seafloor...

and carried away far out to sea.

The open ocean is a vast, featureless desert.

Even a small piece of drifting kelp

can become valuable.

It's just what these fish have been searching for.

Here, shelter, where eggs can be laid unobtrusively,

is rare...

so they make the most of it.

Each female produces up to 1,000 eggs,

and the males rush in to fertilise them.

All this activity attracts the attention of blue sharks.

They can travel thousands of miles through the ocean

in search of food.

And they may not have eaten for weeks.

It seems that they're in luck.

But these fish are no easy meal.

They are flying fish.

The blue shark, however,

are not here for the fish.

They have come for the caviar.

But flying fish lay so many eggs

that the sharks' feasts hardly make an impact.

And in a few months' time,

a new generation will take to the skies.

Floating islands of weed

are always attractive to creatures

looking for shelter out in the big blue.

But in today's ocean

they're being replaced by something else.

Plastic.

12 million tonnes of it end up in the ocean every year.

The deadliest kind

are discarded fishing lines and nets.

Each year, it's likely over half a million animals

are ensnared and killed by these ghost nets.

But some animals

are learning how to take advantage

of the plastic rafts...

and use them for shelter or even as a home.

This male Columbus crab,

barely bigger than your thumbnail,

is a castaway.

Clinging to his net,

he may have been drifting on ocean currents for months.

Now fully grown,

he needs to find a partner.

But where?

Certainly not here,

alone on the net.

Columbus crabs are not good swimmers,

so this male is marooned.

Perhaps a passing loggerhead turtle

could be the chance to hitch a lift.

Made it.

But there is more than one passenger here.

A female Columbus crab.

They waste no time in getting acquainted...

and start to mate.

This pair may well stay together

for the rest of their lives.

The turtle, after all, makes an excellent home.

In return, the crabs provide an onboard grooming service.

Finding a partner in the vast ocean is not easy.

Dawn in the Sea of Cortez.

Mobula rays.

They're gathering here, most likely, to breed.

No-one is sure why they leap,

but the loud splashes

certainly seem to attract more rays.

Other creatures, however,

are also listening.

Orca have heard the splashes.

This female specialises in hunting rays.

But today she is teaching two youngsters

the finer points of mobula ray hunting.

The rays are fast and can outmanoeuvre an orca.

She knows she needs to herd the prey

into a tight group.

But she can't do that on her own.

It'll take teamwork.

And that is today's lesson.

The young orca are quick to catch on.

The rays are surrounded.

They dive...

only to be forced back up again

by the experienced female.

And now,

with the exhausted rays trapped against the surface...

she attacks.

And the young ones follow her lead.

This family are the only orca known to hunt rays like this.

And they are lethally efficient.

But it appears that school is over for today.

For a few battle-scarred survivors,

it's a lucky reprieve.

With the orca gone, the rays can resume their courtship.

And they do so in astounding numbers.

Tens of thousands of rays in a single shoal.

Vast areas of our ocean are yet to be explored.

The least known parts of it are its great depths.

Only highly specialised vessels can take you there.

As you descend, the sunlight rapidly fades.

200 metres down, you enter the twilight zone.

An alien world

inhabited by creatures beyond imagination.

The siphonophore,

with a cloak of lethal stinging tentacles.

Siphonophores repeatedly clone themselves,

and so grow to immense lengths.

This one measures 45 metres

and is the longest animal ever recorded.

With such deadly predators around,

creatures here go to great lengths to hide.

A glass squid.

Completely transparent,

except for its eyes...

and stomach.

So it is extremely difficult to find in the gloom.

Below 1,000 metres,

all traces of sunlight have disappeared.

This is the midnight zone.

Catching food in the perpetual darkness

is not easy.

The gulper eel.

Its giant, extendable mouth

can engulf prey larger than its own body.

Two miles down,

you finally reach the ocean floor.

We know more about the surface of Mars.

The pressure of the water

is 300 times greater than at the surface,

and it's freezing cold.

Yet, amazingly, there are animals living here.

Muusoctopus robustus,

nicknamed the pearl octopus.

Breeding in these extreme conditions

poses a particular problem for these animals.

At these low temperatures,

this female's eggs

could take over ten years to develop.

So she travels to a very special place.

This is the largest known gathering

of octopus in the world.

She joins 20,000 other females...

all here to lay their eggs.

Water, heated deep in the Earth's crust,

rises through cracks in the seabed...

raising the temperature to as much as ten degrees Celsius.

A deep-sea thermal spa.

This warmth significantly speeds up

the development of their eggs.

Even so, they will still take nearly two years to hatch.

Once a female is settled,

she won't move from her spot...

not even to feed.

She will constantly tend to her brood,

keeping them clean

and oxygenating them with jets of water.

Pearl octopus mothers

must surely be amongst the most devoted.

After almost two years,

the ordeal takes its toll.

But the vigil is almost over.

The last of the brood are gently encouraged to leave.

But this will be a final act.

Like all the mothers here,

this octopus is starved and exhausted.

Her life will soon be over.

But this devotion means that her offspring

are among the largest and most developed

of any octopus,

and have the best chance of survival

in the demanding world of the deep.

We are finding more of these strange hot springs

every year.

In some places, they can be astonishingly violent.

Superheated water,

hot enough to melt lead, spouts from the sea floor.

Dissolved minerals condense around them

to form towering chimneys known as hydrothermal vents.

They are home

to unique communities of animals

that all rely on the nutrients in the scalding water.

Some suggest that vents like these

could be the places where life on Earth first began

4 billion years ago.

And there is now evidence of hydrothermal vents

on the moons of Jupiter and Saturn.

Could they, too, be supporting life?

Here on Earth, the influence of these vents

extends far beyond their chimneys.

The essential nutrients carried in these plumes

can rise all the way to the surface,

two miles above.

Here, they stimulate the rapid growth

of tiny marine plants on an astronomic scale.

This phytoplankton is so abundant

that it produces half of all the oxygen on Earth.

It's also food for a host of minute animals -

zooplankton.

Together, they are the basis of the food chains

that sustain nearly all life in the ocean.

In the Pacific waters off South America,

they feed anchovy that gather here in billions.

This is one of the world's most productive ecosystems.

These South American sea lions are on the hunt.

But the anchovy shoal has already been located...

by the planet's most efficient hunters.

A net over half a mile long encircles the shoal.

It can trap over a million fish.

But the sea lions aren't about to go hungry.

They have learnt to look for holes in the net.

Why waste energy

when food comes to you as easily as this?

As more and more sea lions arrive,

some change their tactics.

They line up on both sides of the net...

and wait.

As the net tightens,

the anchovy panic and rush for the surface.

This is the moment

the sea lions have been waiting for.

It's such an effective way of hunting

that the sea lions here

are changing their natural behaviour...

and are raiding the fishing nets

in their thousands.

With the net almost drawn in,

the fishermen beat out a warning signal.

Time is up.

In the confusion, not everyone manages to escape.

The trapped sea lions are in danger of being drowned.

Pups were separated from their mothers.

Then, at the last minute,

the fishermen briefly drop their net...

and the trapped sea lions can escape.

But many are not so lucky.

Around the world,

millions of animals are accidentally caught and die

in fishing nets every day.

And this by-catch, as it's known,

is pushing many species toward extinction.

Animals have evolved in remarkable ways

to the demands of life in the ocean.

But can they now adapt to the new challenge -

living alongside us?

The open ocean is home

to some of the largest

and most extraordinary animals on Earth.

But here, off the Canary Islands,

the Planet Earth team are searching

for one of its smaller and less familiar residents.

We're looking for a tiny creature

called a Columbus crab,

which is only about the size of your thumbnail,

which live on floating objects in the open ocean.

Columbus crabs were once only found on natural debris

like driftwood or seaweed,

but the oceans are changing,

and they've been forced to adapt.

One of the most common items they find

is discarded fishing gear.

Over a million tonnes

is left adrift in the ocean each year,

and Columbus crabs are now making their homes

on these unnatural materials.

Surprisingly, Columbus crabs

are thriving on this new habitat.

But for larger ocean creatures, these nets can be lethal.

Loggerhead sea turtles are especially vulnerable.

Many times they are very injured

and many times they don't even survive.

It's a sad situation.

As a rule, wildlife film-makers observe natural behaviour

and do not interfere.

But this is far from natural.

The team feel they must take action.

Sh, sh, sh!

It seems that the turtle is quite OK.

We'll just finish to cut the plastic

and then we release it.

It's a bittersweet moment,

as this is one of eight turtles they rescued

whilst filming here.

This situation is not natural.

It's something that is being caused by human beings.

Sadly, the crew now face dilemmas like this

on many of their filming expeditions...

including when they travelled

to the Pacific coast of Chile

to show how sea lions are competing for food

with one of the biggest fisheries on Earth.

Expedition leader Fernando Olivares

takes the team to the fleet.

Diving near fishing nets is extremely dangerous,

so the crew must ensure they have the full cooperation

of the fishermen.

Pretty hazardous place to dive.

It's just a case of trying to keep an eye on the divers,

but the sea lions also like to go down and blow bubbles.

So you'll think you're following the divers,

and then a sea lion will pop up.

Taking the utmost care,

the divers get a close-up view

of the sheer scale of the operation.

This one there must have about 15 tonnes of fish in it,

and there's eight or nine boats in this one little bay.

And these are just the small boats.

Out there, there are huge factory ships

that are taking even bigger catches.

How much fish a year from just here?

From a million tonne a year, more or less.

The catches are so vast,

the sea lions have changed their natural behaviour

to collect anchovies directly from the nets.

But it's a risky strategy.

The crew witnessed sea lions caught in nets multiple times.

Whoa, it's intense, mate. There's so many in there.

There's a pup stuck in the net calling to its mother,

and its mother is just on the outside of the net,

calling back.

That is really hard to watch.

No, the pup's really panicking now.

Sometimes fishermen are able to release trapped sea lions...

but not this time.

The pup is in serious danger.

Once again, the crew decide to step in.

Wow.

Nice. Really nice.

It's a relief this time...

but it highlights the new realities

of filming wildlife in today's changing ocean.