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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 colonised every corner of the ocean,
and have a mind-boggling range of solutions
to 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 super-heated 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.
So named for their ability to survive volcanic heat.
They share the vents with crabs and two-metre-long tube worms.
They can only survive here because they are 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 metre.
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 metres long, they have a local reputation as man-eaters.
Alone, they are formidable enough...
..but this is a pack...
They are 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 smouldering 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-metre-long, carnivorous nemertean 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.
Nemerteans 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.
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 eyespots 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 centre.
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 is only one outcome...
Jellyfish, like a great many marine invertebrates,
have soft, vulnerable bodies.
They are protected by stinging cells.
But there are other kinds of defence.
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 are here because they all share a problem.
Each crab has been wearing the same suit of armour 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 moult, but to mate.
And they grab the opportunity with considerable enthusiasm.
Coming together in such numbers does however have a drawback.
It attracts predators.
Despite there being so many potential prey,
the stingray 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 shelled,
most recently moulted 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 colour 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 colour 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 fertilises 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-metre-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.
100,000 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 are 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.
a giant sunstar the size of a dustbin lid.
It's a hunter. Each arm is covered by super-sensitive tube feet,
that can detect prey by touch and smell.
But the sunstar is also partial to carrion,
and it detects the carcass of the giant octopus mother.
The miniature suckers on its feet clamp on to 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 defence.
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 sea bed,
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 at low tide.
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 that encrust its surface.
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 battle front, 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.
The coral that can digest fastest, wins.
Corals constantly grow over the skeletons of their dead comrades,
building a bigger and bigger reef.
Then, just once a year,
a few days after the November full moon, the corals take part
in a mass spawning event.
Millions of eggs and sperm are released into the water,
and join, to develop into larvae that drift in search of a place to settle.
Eventually, every inch of the wreck's surface will be colonised.
The steel will rust away, and the reef will be on its own.
Most reefs grow without the help of a wreck to start them off.
But, given time, they can create something as huge as this,
the Great Barrier Reef, the largest living structure on Earth.
A coral reef rivals even a rainforest for its diversity of life.
Yet corals like this are found in waters where food is very scarce.
All the creatures here have had to adopt a different
and highly specialised way to gather every nourishing scrap.
Christmas tree worms bore into the coral's skeleton for protection,
swirling out and grabbing food particles with their feathery gills.
Coral barnacles are, in fact, related to lobsters.
They lie on their backs,
waving their feet to gather any food floating past.
Crabs have evolved many different ways of gathering food.
This porcelain crab has a fan of filaments on his front legs.
A boxer crab attaches a tiny sea anemone to each fist.
As well as for defence, he uses their sticky tentacles
to gather passing plankton.
This orangutan crab's whole body is coated with sticky hairs,
in this case, perhaps just a bit too sticky!
This strange creature is a sea cucumber.
It uses its tentacles to grab food from the sediment.
There are hunters here too,
like these nudibranchs, or sea slugs.
Their vibrant colours are a warning that they're toxic.
There are over 3,000 species, many hunt just one specific prey.
Some hunt each other.
This emperor shrimp makes the most of the poisonous nature of its host.
But, it's a rather one-sided affair.
As it feeds, the shrimp gets protection and a free ride.
Other shrimps have developed a more balanced relationship.
Some even solicit for partners.
These dance for their dinner.
And these advertise to passers-by
that they are open for business.
They are a parasite removal team,
providing a service for countless fish on the reef,
including those that could happily eat them.
In return for their bravery, they get a meal that comes to them.
Coral reefs, built by the tiniest of creatures,
occupy less than half of one per cent of the ocean's floor.
Yet they support a quarter of all marine species.
Marine creatures, all without backbones,
from corals, to cuttlefish, to crabs,
make up the majority of life in the oceans.
But they have also had a surprisingly important impact
beyond the marine world.
Their fossilised bodies, shells and skeletons
form the limestone and chalk that now covers huge tracts
of Asia, Europe, and the Americas.
They may be small, but over their two billion-year history,
they have, literally, changed the world.
To capture some of the sequences in this episode,
the Life team had to take underwater filming into uncharted territory.
One shoot meant spending weeks diving under two metres of ice,
another involved laying the foundation for a new coral reef
in the tropics.
A sunken ship can make an ideal location for corals to grow.
So, with this in mind,
the Life team set themselves a challenge...
to make their very own shipwreck.
After months of searching,
they find a boat in the Bahamas that might be suitable.
But there's a lot to do before it can be sunk to the sea bed.
Apparently it's been here for eight years on the jetty.
All this fibreglass insulation has all got to be removed
and it's all hands to the pump, Ian.
We've got to get a pair of overalls.
The team have to put away their cameras and get their hands dirty.
I've got a 50-tonne boat,
and I'm trying to clean it with a paint scraper!
Thoroughly cleaning the boat
increases the chance that coral will grow on it,
and ensures it won't pollute the sea.
It's like Changing Rooms, isn't it?
After eight years rusting on the jetty,
there's no guarantee that she'll even float.
Well, we actually... we do have some holes.
To get this ship ready in time for the inspectors is a massive task,
and they've been doing a brilliant job.
We're cutting panels through to let the water rush through
when she starts sinking.
That's the last bit, OK, guys. Vamos!
Shipshape at last.
Well, she's clean, she's been inspected, and she's ready to be sunk.
What can go wrong now?
-'Here's the very latest with Dean...'
As the tail end of Hurricane Dean sweeps through,
the team are forced to put their plans on hold.
-'..winds of 150 miles an hour...'
Luck doesn't seem to be on their side.
But, two days later, good weather returns.
Here comes a crane, finally!
Fingers crossed, arms crossed, legs crossed, everything's crossed!
They urgently need to get the boat into the water,
or they won't reach the chosen wreck site before nightfall.
She's safely in the water, so far, so good.
Her final voyage...
Cameras are mounted around the deck,
to film her sinking below the surface.
We've spent a week preparing for this, and finally the afternoon's arrived.
But the light's going, so we have to move.
A last few holes are cut. It's time to pump in water,
and to abandon ship.
The crew dive in,
ready to film her descent, while the support boat moves away to a safe distance.
There's nothing more to do, but wait.
Here we go, look at this!
She gently comes to rest, and the right way up, just as they'd hoped.
Now, it's time to let nature take its course.
The crew will be back over the next two years to see how life takes hold.
At this site, the wreck has a very good chance
that it'll be colonised by coral.
With luck and time, it'll eventually become a full-scale reef.
Meanwhile, at the other end of the world,
the Life team's challenge is very different.
Here in Antarctica, just getting underwater will be tough.
In order to work in such a demanding location,
the team needed the help of the National Science Foundation,
at the McMurdo polar research station.
Everyone here has to be able to cope out on the ice
if there's an emergency.
So the Life team joins research scientists for survival training,
to prepare them for any situation.
Even one like this.
A colleague lost in a whiteout is very serious.
This training might mean the difference between rescue, or not.
This is supposed to simulate a whiteout,
we've been looking for someone who's been lost, and you wear
the bucket so you cannot see at all, and it does actually work.
It's just... You can't see anything in this.
With the training over, it's time to travel over the ice to the dive site,
ready to go beneath the frozen surface of the Ross Sea.
But to get through this ice, thick enough to land a jet on,
needs the help of McMurdo's specialist drilling team.
Once the hole is drilled,
a specially designed hut is slid into position.
This will be the team's base for the next four weeks of diving.
Look, this is all we need, all we need to go in here...
the door to another world.
All the equipment the team need for the shots
must come in and out of this hole.
This is as far south as it's possible to dive.
The ice, eight feet thick, it's here for 11½ months of the year,
and we're goin go down underneath and have a look,
to see just what it's like underneath there.
Doug and the team venture below the ice.
The under-ice landscape is both surprising and spectacular,
but it's the animals they've come to film.
So, specialist time-lapse cameras are moved into position.
The animals move very slowly at these freezing temperatures.
But by using time-lapse to speed up the action 500 times,
the team hopes to reveal the behaviour of these creatures for the first time.
With air and warmth running out for this dive,
Doug makes his way back to the lifeline,
and the surface.
Oh! That is amazing. It is so beautiful down there.
All sorts of colours, beautiful stalactites made of ice crystals
hanging down from the top of the ice.
All sorts of things on the bottom, starfish, urchins,
just an amazing profusion of life.
Really lovely, totally unique, unlike anywhere else.
However, after an hour underwater it is getting pretty cold,
so can you give me a hand up, please? Thanks.
There would be another month here, and over 100 dives,
before the sequence was eventually completed.
Subtitles by Red Bee Media Ltd
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