Original subtitles

The ocean, the largest habitat on Earth and an

inhospitable place for those of us who live on land.

Yet for a billion years, this was the only place on the planet where life existed.

Today, descendants of those early life forms continue to thrive.

They share the sea with fish, but outnumber them by ten to one.

They have no backbones and have evolved into countless different forms.

Some are huge, large-brained and intelligent.

Others are minuscule, yet build the largest natural structures on the planet.

They are marine invertebrates, the creatures of the deep.

They have colonized every corner of the ocean and have a mind-boggling range of

solutions for the problem of staying alive.

The ocean is by no means uniform.

Differences in depth, temperature, sunlight and currents pose particular challenges.

One and a half miles down, these hydrothermal vents spew out superheated

water at 450 degrees centigrade from cracks in the Earth's crust.

Despite the enormous pressure, the total darkness and the scaldingly high

temperatures, the ancestors of all life may have evolved in a place just like this.

Pompeii worms, so named for their ability to survive volcanic heat.

They share the vents with crabs and two-meter-long tube worms.

They can only survive here because they're able

to feed on bacteria that thrive around the vents.

These colonies are extremely rare.

Not surprisingly, most life thrives nearer the

surface, where feeding is considerably easier.

These are krill, tiny shrimp-like crustaceans.

Swarms can reach astounding numbers, 60,000 per cubic meter.

During the night, they rise towards the surface to feed on plankton.

Here, in the Sea of Cortez off Mexico, the swarms attract hunters of all kinds,

from humpback whales to shoals of predatory fish.

Yet another hunter arrives.

It's one from the deep.

A Humboldt squid.

Two meters long, they have a local reputation as man-eaters.

Alone, they're formidable enough.

But this is a pack of hundreds.

They're highly intelligent hunters.

Their eyesight is exceptional.

They have powerful tentacles, suckers ringed with 70,000 hooks, and a

razor-sharp beak for tearing through flesh.

Now the fish find that they are under attack, and so group together for safety.

But the Humboldt squid work as a team, herding the fish against the rocks.

It's thought that the squid flash red and white not only to confuse their prey,

but also to signal to each other when they are about to attack.

Humboldt squid numbers are growing rapidly.

But they remain mysterious.

As fast as they arrived, they disappear back into the deep.

800 miles from the South Pole, in the shadow of a smoldering Mount Erebus.

Winter temperatures are a punishing minus 40.

This, perhaps, is the last place you might expect to find marine life.

Now it's the beginning of the polar spring.

And for the first time in months, light reaches the sea beneath the ice.

It's extremely cold and completely dark for much of the year.

So conditions are not unlike those of the deep ocean.

Yet, in McMurdo Sound, life flourishes.

The creatures here grow extremely slowly.

But that does mean they can reach a great age and great size.

And they occur in surprisingly large numbers.

Three meter long carnivorous nematine worms,

red sea stars and urchins carpet the sea floor.

This monster worm will eat almost anything and

is constantly scanning the sea floor for food.

Animals are swarming here in such numbers because of this.

A dead seal pup.

Such a great quantity of food may only arrive once in ten years.

But a seal's body won't be easy to eat.

Nematines have a snout like a harpoon that

enables them to puncture the skin of the corpse.

It's harder work for the sea stars.

They feed by pushing out their stomachs through their mouths.

As this sea star presses its stomach against the seal's skin, it secretes

digestive juices that dissolve the seal's tissue.

But that takes time.

These scavengers will feed here throughout the summer until

all that remains of the seal will be a skeleton stripped bare.

The shallows are only a tiny part of the marine world.

90% of it is open water.

Its currents carry life for thousands of miles.

The masters of this nomadic existence are jellyfish.

A life spent drifting in the empty ocean could be a lonely one.

But not for this jellyfish.

Aurelia.

Swarms like this are not accidental.

These individuals all hatched together when

the temperature and currents were just right.

Their timing has ensured that they can make the

most of feeding on a late summer plankton bloom.

Jellyfish have no brain and no blood.

But they do have eye spots that enable them

to tell the difference between light and dark.

And they can move independently of the current by a simple form of jet propulsion.

All jellyfish have stinging tentacles with which to catch their food.

As they pulse their bodies, the tentacles trap plankton,

which is then passed towards their mouth in the center.

A swarm of 100,000 stinging jellyfish might seem a daunting prospect for a predator.

But not for this one.

A huge fried egg jellyfish.

It is a killer.

Its weapons are harpoon-like cells that cover its tentacles.

When they come into contact with a victim, they spear it.

The fried egg then hauls in its prey.

A few lucky Aurelia do manage to pull themselves free.

But for the majority, there's only death.

Jellyfish, like a great many marine

invertebrates, have soft, vulnerable bodies.

They are protected by stinging cells.

But there are other kinds of defense.

Armor plating.

These are spider crabs.

They spend most of their lives in deep water.

But once a year, off the coast of southern Australia, a quarter

of a million crabs set off on a long journey to the shallows.

They're here because they all share a problem.

Each crab has been wearing the same suit of armor for a year now.

And it's getting uncomfortably tight.

So each crab eventually has to shed its shell and produce a bigger one.

Replacing an old shell is understandably a tricky process.

First, the crab grows an entirely new skin within the old shell.

It then flexes its body to force its shell to split along the back.

Before gingerly backing itself out.

The spider crabs are not only here to molt, but to mate.

And they grab the opportunity with considerable enthusiasm.

Coming together in such numbers does, however, have a drawback.

It attracts predators.

A sting ray.

A sting ray.

A sting ray.

Despite being so many potential prey, the sting ray seems to ignore them.

Perhaps there's safety in numbers.

Although not all the crabs are prepared to rely on it.

Alarm spreads amongst the crabs.

But in fact, most of them are safe.

This ray is being very choosy.

It's only interested in the softest shell, the most recently molted crabs.

Once a target is singled out, there is no escape.

When their new armour has hardened, the crabs

return to the comparative safety of the deep.

Leaving behind only their old, empty shells.

Like spider crabs, the behaviour of most marine

creatures is controlled by a very simple nervous system.

But there are exceptions.

This cuttlefish is one of the cleverest animals in the ocean.

She has a very large brain.

In fact, it's larger for her size than that of most fish or reptiles.

Her life is both complex and full of intrigue.

Giant Australian cuttlefish usually live alone.

But once in their short lives, they must come together to mate.

As she approaches the traditional mating grounds, one

of the largest males starts to show interest in her.

She moves him away from his rivals to a quieter spot.

A place where she will be able to lay her eggs in safety.

The male takes the female in his arms and turns her to face him.

Before using one of his arms to pass sacks of sperm to an opening near her mouth.

Once mated, he hovers over her, standing guard until she's laid her eggs.

But he's got a problem.

Males outnumber females four to one.

So keeping her to himself is a constant battle.

This larger rival is more difficult to intimidate.

Cuttlefish can make very dramatic changes to

their skin pattern in order to signal their moods.

Flushes of bright color and stripes that pulse

along his side tell the rival to keep off.

Most rivals back down at this stage.

But not this one.

Although the male's flashing signals get more and more

emphatic, in the end he has no choice but to fight.

Victory.

And the male can return to guard his female.

Cuttlefish are great communicators, but there is a flip side.

They can also be masters of deception.

This male is too small to fight for a mate.

But he has another plan, and it's sneaky.

He approaches the couple cautiously, holding his tentacles tucked up at the

front, mimicking a female that wants to mate.

To complete his disguise, he changes color to appear even more like a female.

The guarding male seems convinced.

Maybe he thinks his luck is in.

Another female to add to his conquests.

The sly, cross-dressing male edges closer

and closer to the female, holding his nerve.

As long as he avoids being grabbed in a mating embrace, the sneak is safe.

At what point the female guesses his true identity is unclear.

But she isn't choosy, and surreptitiously mates

with him right under the larger male's tentacles.

It's time for the female to lay her eggs.

Using the sperm from both males, she fertilizes her eggs

one by one and glues them to a rock in a hidden crevice.

With luck, she will now have a mix of offspring.

Some may become masterful males, and others little sneaks.

She'll have all the bases covered.

The coastal waters of British Columbia.

Home to this four-meter-long Pacific giant octopus.

She is a formidable predator, but at the moment, hunting isn't on her mind.

She has just mated for the first time.

And now she's searching for a safe refuge.

She makes her choice carefully.

This is going to be her home for many months to come.

It's her nursery den.

A hundred thousand eggs hang from its ceiling,

and she's guarding them with her life.

Without her to protect them, they would be eaten by predators or become diseased.

She caresses them with her tentacles, ensuring that

algae don't grow on them and that fish don't eat them.

She constantly keeps the water moving around

them, so they're well supplied with oxygen.

She cares for them for six months.

And during all this time, she doesn't eat a thing.

And now, as they're hatching, she is dying.

One night, as the baby octopus emerge, she jets water

over them for the last time, helping them on their way.

This will be her final act.

This is the only time she will reproduce.

And to give her young their best chance, she sacrifices her life.

Out of the depths comes one of the largest and most aggressive starfish.

In the ocean.

Pycnopodia, a giant sun star the size of a dustbin lid.

It's a hunter.

Each arm is covered by supersensitive tube

feet that can detect prey by touch and smell.

But the sun star is also partial to carrion, and

it detects the carcass of the giant octopus mother.

The miniature suckers on its feet clamp

onto the corpse and drag it out of the cave.

Other scavengers rush to join the feast.

Although it's a fearsome predator, Pycnopodia doesn't have it all its own way.

These sea urchins aren't speedy enough to

escape, but they do have a formidable defense.

The sharp spines are hard to get past.

And what's more, the urchins can move each spine

independently, pinching the starfish's probing arms.

Trapped by an army of urchins, Pycnopodia is spotted by an enormous king crab.

Pycnopodia has more than met its match, and

within seconds the crab rips off one of its arms.

But that is just a temporary inconvenience.

Starfish are able to quickly regrow a lost limb.

The most impressive invertebrates may seem to be the giants, but in fact it's some of

the smallest that can make the biggest impact.

Every square inch of this island has been created by

an ever-growing living superstructure, a coral reef.

It's taken thousands of years to reach this size, and

it all began with creatures smaller than a pinhead.

A reef can't be built just anywhere.

It needs something to give it a firm footing.

A wreck like this provides an excellent foundation.

As soon as it settles on the seabed, the wreck comes under attack from

invaders, plankton, carried here by ocean currents.

These are the microscopic larvae of barnacles, sponges, and most importantly,

corals.

The larvae must attach themselves to the wreck.

Once there, they can develop into young corals called polyps.

But the polyps are very slow growing, and there

is lots of competition from other invaders.

Algae quickly cover the wreck, and that's a problem for the young coral.

Algae attract grazers.

The polyps are in danger of being eaten before they've even got a proper foothold.

If conditions are right, the survivors can go on to build a reef.

Position is critical.

Too deep, and not enough light will reach the corals for them to grow.

Too shallow, and they risk being exposed to the air.

The reef is a great place for them to live.

For the reef to really flourish, it also needs

to be in the path of currents carrying food.

Fast forward half a dozen years or so, and the wreck will begin

to show the first signs of corals visible to the naked eye.

A decade later, and the wreck will be transformed.

Thousands of polyps will form coral heads.

And the reef will be a great place for them to live.

This ship was sunk during the Second World War.

And there has been enough time for a substantial reef to develop.

After decades of growth, different species of

corals dominate particular areas of the wreck.

The fastest growing types grow best on the edges and

overhangs, reaching far out into the water and up to the light.

They need only a small area to establish themselves, yet they can rapidly grow

dozens of plates or branches crammed with polyps to gather as much light as possible.

Slower growing, much more robust corals, like these brain

corals, are better suited to the heart of the developing reef.

It's these that give the reef their structure and permanence.

There could be nearly 500 different species here,

each striving to win a foothold on the rusting hulk.

The pace of life for corals may seem to be so slow that

it's hard to imagine that there is any conflict here.

But as night falls, the mood on the reef changes.

Corals are, in fact, extremely aggressive, and

will fight to the death to expand their territory.

There can be no honourable retreat.

A winner will literally eat its enemy alive.

Along the battlefront, the polyps of both combatants extrude their guts,

long thread-like filaments, over their opponents.

At the fringe, all that remains of the destroyed polyps are their skeletons.