Travel to the Depths of Our Mysterious Oceans | 4K UHD | Blue Planet II | BBC Earth

Over half of all animals in the open ocean

drift in currents.

Jellyfish cross entire oceans,

feeding on whatever happens to tangle

with their tentacles.

Some can grow to a metre, even two metres, across.

And when, by lucky chance, they encounter a patch of sea rich in plankton,

their numbers explode.

It's such a successful strategy

that jellies are one of the most common life forms on the planet.

But among the jellies, and looking somewhat like them,

is a rather more complex and sinister creature.

The Portuguese man o' war.

It floats with the help of a gas-filled bladder,

topped by a vertical membrane.

With that serving as a sail,

it maintains a steady course through the waves.

Long threads trail behind it,

some as much as 30 metres long.

Each is armed with many thousands of stinging cells.

A single tentacle could kill a fish

or, in rare cases, a human.

But among its lethal tentacles

lurks a man o' war fish

that feeds by nibbling them.

Whilst this fish has some resistance to the stings,

it must still be extremely careful.

Most other fish are not so lucky.

A tentacle has caught this one and reels it in.

It's already paralysed.

Specialised muscular tentacles transfer the victim to others that digest the catch,

liquefying it with powerful chemicals.

Eventually, all that is left

is a scaly husk.

This voracious man o' war may collect over 100 small fish in a day.

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 seagrass 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, 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 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 100,000 crabs

successfully moulted.

A Sally Lightfoot crab,

one of thousands of shore crabs

just waiting for their moment.

Every day, they gather on the tropical shores of Brazil,

waiting for the tide to go out,

which exposes their feeding grounds.

Seaweed-covered rocks, 100 metres from the shore.

Getting there is a race against the tide.

They leap from rock to rock.

These crabs seem to be afraid of the water.

And for good reason.

A moray eel.

The chain moray is a specialist crab hunter.

Its blunt teeth can easily grip

and crush a crab shell.

It's the crabs' deadliest enemy.

But the crabs' feeding grounds are still a long way off.

They must press on.

Halfway.

But their enemy has other ideas.

Crossing the land

to reset the ambush.

To feed, the crabs must keep going.

But nowhere is safe.

An octopus.

Also a crab killer.

The crabs make a dash for it.

Made it!

Risking life and limb

to graze on these seaweed pastures.

But in two hours' time,

when the tide starts to turn,

they will have to run the gauntlet all over again.

In a far corner of south-east Asia

lies the Coral Triangle,

a cluster of the richest coral reefs in the world.

Undersea cities crammed full of life.

As in any crowded metropolis,

there is fierce rivalry for space,

for food

and for a partner.

But the reef is also a place full of opportunity.

A cuttlefish.

It specialises in hunting crabs.

But a large crab is a dangerous quarry.

It has powerful claws.

The cuttlefish, however, has a remarkable talent.

Its skin contains millions of pigment cells

with which it can create ever-changing colours and patterns.

And that, apparently, hypnotises the crab.

A cuttlefish may be clever, but a shark is bigger.

And it eats cuttlefish.

Time to disappear.

Back to the hunt.

A new target,

but the same mesmerising technique.

For those that manage to establish themselves in these bustling undersea cities,

there can be great rewards.

During the dry season, over half a million terns

crowd on to this remote atoll in the Indian Ocean.

Their chicks are still in their dark, juvenile plumage.

They vary in age.

Whilst the more advanced chicks take to the air,

others aren't quite ready yet.

Those just starting to learn to fly

use the shallow lagoon that occupies the centre of the atoll

as their training ground.

It's difficult for some of them to stay aloft for long.

Giant trevallies.

Usually, they are solitary hunters.

But about 50 of them have come here from neighbouring reefs,

attracted by this abundance of potential prey.

The fledglings stay out of the water if they can.

They even drink on the wing.

If the trevally are to catch one now,

they have to up their game.

So there is a fish here that, amazingly, has a brain

capable of calculating the airspeed, altitude

and trajectory of a bird.

The time comes

when every fledgling has to take to the air

and collect food for itself.

Their parents lead them to the training grounds.

If they are to survive,

they must learn quickly.

After a month of practising over the lagoon,

the youngsters start to leave and take their chances out over the open sea.

Out here on the sand flats,

there is safe accommodation for some.

A family of saddleback clownfish

have found an excellent home.

The tentacles of this carpet anemone can kill,

but the clownfish are immune to the poison,

so they can shelter from danger.

In return, the fish keep the anemone clean of debris.

As with all clownfish,

the family is ruled by a big female.

Her white face marks her out as the boss.

The diminutive male has to prove his worth,

so he works tirelessly,

removing debris and generally keeping on top of the housework.

His greatest challenge is to find a safe place

where the boss can lay her eggs.

But there's nothing solid here for the female to lay them on.

A nearby shell could be the solution.

If only he can move it

to the protection of the anemone.

Too heavy.

Besides, it has a mind of its own.

A hermit crab.

But out here, twice a day,

the anemone is swept by tidal currents,

and they bring in new opportunities.

An old plastic bottle.

Perhaps this will do.

Not heavy enough.

A coconut shell.

It looks just right.

But it's a long way from home.

And he can't move it by himself.

So the pair now work together.

A little adjustment to the anemone's tentacles,

in order to clear a space for it,

and the shell is tucked in.

The female lays.

A safe nursery at last.

He has proved himself worthy to father her young,

and he fertilises them.

Now, he will meticulously tend the eggs,

keeping them clean and healthy until they hatch

in 10 days' time.

The blue shark.

It travels over 8,000 kilometres a year,

riding on the currents, supported by its broad, wing-shaped fins.

This one may not have eaten for two months.

But the currents can carry promising traces of fatty oils from many kilometres away

and will lead it to its next meal.

After days of travel, the smell of food gets stronger.

A dead whale, recently struck by a ship.

This could be a real feast.

But the blue shark must be cautious.

Great white sharks,

10 times heavier than a blue,

are highly possessive around a whale carcass.

Great whites are eager to feed on energy-rich whale blubber,

which we now know forms a major part of their diet.

Once the great white has had its fill,

smaller sharks, like the blue shark, tackle what's left of a carcass.

As the oils from this dead whale spread more widely,

more and more blue sharks appear.

Within days, the carcass will be stripped of its blubber.

Then, no longer kept buoyant by its oil,

it will sink into the depths below.

The blue, with its reserves of fat replenished,

can now survive for another two months without eating.

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 10 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 to complete.

So even among giant cuttlefish it seems,

it's not all about size.

In the underwater forests off northern Japan,

the residents of this sunken wreck are waiting for the summer temperatures

to reach 16 degrees Celsius.

That, for some, is the time for mating.

A kind of giant wrasse called a kobudai.

This is a male.

And in female terms,

he is particularly handsome.

He's a metre long and weighs 15 kilos,

much larger than the diminutive female.

And he is ready to breed.

He attempts to mate with her –

and with any of the other dozen or so females that live in his territory –

whenever he gets the chance.

But females from around 10 years old

take little notice of his advances.

This is because when any large female reaches a critical body size,

she can begin a dramatic transformation.

Over just a few months,

particular enzymes inside her body cease to work

and male hormones start to circulate.

As time passes,

her head expands

and her chin gets longer.

A she has changed into a he.

And with this comes a change in temperament.

The old male who ruled all the females here

is challenged to a face-off.

The more bulbous the head,

the more it intimidates an opponent.

The territory has a new ruler.

Only the largest females transform themselves in this way.

But the change enables them to have more mates,

so they will have many more offspring carrying their genes.

But a new male can't afford to be complacent.

Inside the body of every kobudai female,

there is a new male-in-waiting.

And wave power creates towering fortresses,

like these cliffs in the Arctic,

home to tens of thousands of breeding seabirds.

The faces of the cliffs are accessible only from the air

and have plenty of nooks and crannies for those that can get there.

But to feed, seabirds must still master the ocean world beyond.

The puffin.

He's a fisherman

and a father.

He has a mate for life.

Both share the burden of raising their week-old chick,

their puffling, who needs five square meals a day.

The parents alternate fishing trips.

It's Dad's turn.

When fish stocks are low,

puffins must fly as much as 50 kilometres out to sea

to reach the good fishing grounds.

Once there, they plunge into another world.

Good fishing spots are hard to come by

and they have company –

guillemots.

Like the puffin, their wings are short and good for diving.

Puffins can hold their breath for over a minute

and dive as deep as 40 metres.

A catch!

But it's a long way home.

After an exhausting round trip of almost 100 kilometres,

this puffin's nearly made it.

But there are pirates on this coast.

Arctic skuas.

All around, returning parents are being robbed.

The skuas' long, raked-back wings

make them faster and more manoeuvrable.

Puffins must choose their moment wisely.

A near miss.

A last, desperate burst of speed

and it's made it.

Safely home after a three-hour round trip,

where his patient partner is waiting.

Today, their puffling will eat.

But where fish numbers are in decline,

many puffins now find it hard to get enough food for their chicks.

In the changing seas of today,

it can be even harder to be a successful puffin parent.

A kilometre from the surface,

beyond the reach of the sun.

A giant, black void,

larger than all the rest of the world's habitats combined.

There's life here,

but not as we know it.

Alien-like creatures produce dazzling displays of light.

Nearly all animals need to attract mates and repel predators.

This language of light is so widespread here

that these signals are probably the commonest form of communication

on the entire planet.

And yet we still know little about them.

Hunters illuminate themselves and, by doing so, attract inquisitive prey.

This is a fangtooth.

It has the largest teeth for its size of any fish.

There are pressure sensors all over its head and body,

which can detect anything moving in the surrounding water.

It's the midnight zone's most voracious fish.

But prey use light as a distraction.

A decoy of luminous ink.

Down here, in this blackness,

creatures live beyond the normal rules of time.

Siphonophores are virtually eternal.

They repeatedly clone themselves,

some eventually growing longer than a blue whale.

Down here, it snows.

Continuous clouds of organic debris

drift slowly down from above.

This is food

and a whole variety of filter feeders depend on it.

Jellyfish

and delicate sea cucumbers.

The 1% of marine snow they miss

eventually settles on the seafloor.

Over millions of years,

it forms a layer of mud

up to a kilometre thick.

It's an empty plain

that covers half the surface of our planet.

The deep seabed may at first appear lifeless,

but it's home to a unique cast of mud dwellers.

The sea toad.

It is an ambush predator,

with an enormous mouth

and infinite patience.

This fish has been living for so long here

that its fins have changed into something more useful.

Feet.

They help it shuffle about on the seafloor.

The flapjack octopus.

It hovers just above the surface of the mud

as it delicately sifts through it, searching for worms.

But it can jet away at the first sign of danger.

In the Gulf of Mexico,

these eruptions also release a super-salty liquid –

brine.

It's heavier than seawater

and it accumulates in great pools on the seafloor.

It's difficult to make sense of the sight.

A lake of concentrated saltwater 15 metres deep

at the bottom of the sea.

Around its margin, perhaps even more strangely,

there is a profusion of life.

Giant mussels that can live and grow for a century or more

pack tightly together, dwarfing the shrimps and squat lobsters

that feed around them.

Cutthroat eels, scavengers,

come to the shores of the brine lake

in search of something edible.

Some even venture into the brine.

Spending too long in it can send an eel into toxic shock.

Its only hope is to rise above it.

It manages to escape.

Others are not so lucky.

The brine embalms their bodies,

and the casualties of decades

accumulate around the margins.