BIRDSONG
In a far corner of Southeast 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.
Corals build themselves homes of limestone
in the warm, clear, shallow seas of the tropics.
Their reefs occupy less than one tenth of 1%
of the ocean floor.
Yet they're home to a quarter of all known marine species.
They are complex, infinitely-varied structures,
providing all kinds of homes for their many residents.
From penthouse suites...
..to backstreet dens.
Here, on Australia's Great Barrier Reef,
a coral grouper lives by hunting for small fish.
But how do you get a meal here
when you're too big to squeeze into crevices?
And the grouper also has a rival.
One which is highly intelligent...
..and seeks the same kind of prey.
An octopus.
It can reach into really narrow cracks.
Even so, its prey often escapes.
What if they could work together?
The grouper turns pale...
..and tries to attract the octopus' attention.
It performs a headstand.
Not only is the grouper signalling to the octopus,
it's indicating where the prey is hiding.
The octopus reaches inside.
The fish take fright...
..and swim straight into the grouper's jaws.
Sometimes the octopus gets the reward,
sometimes the grouper does.
Two very different species have discovered that teamwork
can bring success in Reef City.
Teamwork, in fact, is the very foundation of life on the reef.
The corals themselves also rely on a partnership.
But one of a much more intimate kind.
Corals are colonies of anemone-like animals - polyps.
Some as small as grains of sand.
Living inside the tissues of each polyp
are minute plant-like cells,
invisible to the naked eye.
By day, flourishing in tropical sunshine,
the plant-like cells provide the polyps with up to 90% of their food.
And when it's dark, the polyps continue to feed
by using their tentacles to grab edible particles drifting by.
The polyps also extract calcium carbonate from the seawater
and use it to build a stony housing for themselves.
Coral colonies can continue to grow for centuries, possibly millennia.
And they can build structures that can reach the size of a house.
The biggest of their cities is the Great Barrier Reef.
It runs for over 1,400 miles
along the coast of north-east Australia.
Activity in coral reefs, wherever they are, never ceases.
At dawn, the day shift begins,
accompanied by a chorus of submarine song,
created by fish, shrimps and other inhabitants of the reef.
HARMONIOUS CHORUS
HOOTING
Every resident in this city has its role.
Scavengers, like the sea cucumber, recycle the waste of others.
These parrot fish bite off chunks of coral
and crunch it to extract the contents.
And then excrete the rest as sand.
Green turtles, here in Borneo,
pay regular visits to a particular patch of coral.
This little female is up early and one step ahead of the others.
Turtles travel long distances along the reef in order to get here.
This is their destination - Turtle Rock.
Generations of visiting turtles have worn a hollow in its top.
This is home to blennies and surgeonfish.
They clean the visitors, picking off any algae,
parasites and dead skin that they can find.
Other clients are close behind.
A queue is forming.
The big males barge their way in.
And the smaller female is forced out.
She takes a quick turn around the block...
..while the others are squabbling.
And then she sneaks back in.
So the cleaners get a nutritious meal
and their customers are freed of their parasites
and other encumbrances.
And it's now thought that a spot of pampering at a cleaning station
may even reduce stress.
The lands of the Middle East are so hot and dry
that much of their surface is almost lifeless.
But here in the Red Sea, coral reefs flourish wonderfully.
The waters offshore are almost as rich in life as a rainforest.
A family of bottlenose dolphins are resting on the reef
after a night's feeding offshore.
HIGH-PITCHED WHISTLING
Whilst the adults and their babies sleep...
..the adolescents set off to explore.
And they appear to make up games.
You might call this one Catch the Coral.
CLICKING AND WHISTLING
The rules aren't entirely clear.
HIGH-PITCHED TRILL
They pick up different bits of broken coral...
..and drop them.
Some fall fast.
Others sink more slowly.
And some seem to be descending in a way that pleases everybody.
RAPID CLICKING
Only the most successful city residents
can afford the luxury of playing games.
But such games do have their value.
They may help the youngsters develop the coordination and agility
that they will need when they start hunting in the open sea.
Every reef has a sharply-defined boundary.
WAVES CRASH
Its city walls.
On the outer side is the drop-off.
These ramparts protect the city from the ocean waves.
But twice a day, the walls are covered by the incoming tide.
In the Bahamas, the rush of the water
creates a truly strange phenomenon.
Seamen once told tales of a giant sea monster
lurking here, deep inside the reef,
that would drag sailors to their doom.
Today, we know it is, in fact, a whirlpool,
created by the incoming tide rushing over deep coral caves.
These currents bring in fresh supplies
of microscopic food to the reef from the open ocean.
And in the Maldives, on the biggest tides,
one particular coral lagoon becomes so flooded with plankton...
..that it attracts hundreds of ocean giants.
Manta rays.
With three-metre wingspans.
With their huge, slot-like mouths wide open...
..they filter out the plankton.
And the mantas create a vortex of their own
that further concentrates their food.
This behaviour has been called the manta cyclone.
Then the tide changes and the supply of food is cut off.
The mantas leave the lagoon.
Life on the sheltered side of the reef is tranquil and peaceful.
In these suburbs, any creature wishing to escape
from the bustling crowds of Reef City
can find plenty of space.
On the other hand, there is nowhere to hide.
That, at night, makes it a dangerous place.
Nocturnal predators, such as this lionfish, patrol the reef edge.
The hunter has become the hunted.
A bobbit.
A giant carnivorous worm with jaws as sharp as daggers.
It has an ancestry that stretches back more than 400 million years.
It's a metre long.
It tastes for scent...
..and feels for movement.
Dawn. And with light, the reef becomes a safer place.
Little bream return to foraging.
The bobbit might still be hungry...
..but now, in daylight, the odds have shifted.
The bream can see the bobbit.
Instead of retreating,
they join together to blow away the sand covering the worm.
So taking away its advantage of surprise.
By revealing the bobbit's hiding place,
they will all be able to feed more safely.
But it pays to remember...
..there is a bobbit about.
Out here, on the sand flats, there is safe accommodation for some.
CLICKING
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.
CLICKING
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...
..and the shell is tucked in.
The female lays a safe nursery at last.
CLICKING
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.
Reef creatures go to great lengths
to give their young a head start in life.
And nowhere more so than on the remotest reefs in the world.
French Polynesia - the very heart of the South Pacific.
Protected by their isolation,
some of the reefs here are still virtually pristine.
This marbled grouper has made it in the city and reached adulthood.
Now it's the time to mate.
To find a partner, he must head to
the most dangerous part of this reef - the drop-off.
Patrolled by grey reef sharks.
Hundreds of them.
They seem to be resting.
For now.
Thousands of other groupers have gathered on the seabed below.
The females are almost bursting with eggs.
But to mate with one,
he must first get through the crowd of other waiting males.
And they all have the same thing on their minds.
They jostle to get as close as possible to a female.
This male may have secured pole position,
but the female won't release her eggs for him to fertilise
until conditions are just right.
Now the sharks begin to close in,
sensing that the critical moment is approaching.
The tide is beginning to turn.
This could be the moment to spawn.
Suddenly, the females rush up towards the surface,
releasing their clouds of eggs as they go.
The males pursue them,
simultaneously releasing their sperm.
It's an act the groupers seem prepared to die for.
It's now...
or never.
Their fertilised eggs will now be swept away...
..from the many hungry mouths of the reef-dwellers.
Most of the billions of eggs that cloud the sea will be eaten.
But a few, a tiny but crucial minority,
will find another reef and make it their home.
But today's coral reefs are facing a new threat.
The seas are warming.
A rise in temperature of one or two degrees, for just a few weeks,
can be enough to cause the coral polyps
to eject their plant-like cells.
When that happens, the corals lose both their colour
and their main source of food.
If the high temperatures are sustained, coral -
bleached in this way - is likely to die.
In recent years, it's thought that half the world's coral reefs
have been affected by bleaching.
Including, since 2016, around two thirds of the shallow water corals
on Australia's Great Barrier Reef.
These once-crowded submarine cities...
..are reduced to bleak ruins...
..and many of their inhabitants left homeless.
Some scientists predict that by the end of the century,
coral reef cities as we know them could be a thing of the past.
Is there any future for these most precious of ocean treasures?
Well, that ultimately depends on how fast they heat up,
and how warm the seas become.
And there is a glimmer of hope,
because of the way that corals reproduce themselves.
On one special night of the year, the full moon triggers
an extraordinary event.
The spawning of the coral.
With extraordinary synchrony, entire reefs reproduce.
Billions of fertilised eggs drift away...
..carried by the ocean currents.
And it's not just the corals that spawn.
So do many of the other residents of the reef.
A whole range of young are swept through the oceans,
ready to settle on a vacant site and bring back into existence
the complex community that is a coral reef.
We may not know what the future holds for our seas...
..but coral reefs can regenerate.
As long as some reefs survive...
some hope can remain.
French Polynesia.
Over 4,000 miles from the nearest major landmass.
Protected from fishing pressures, it's home to perhaps the last
great gathering of marbled groupers.
We're finally here.
It's taken quite a few days.
We can't actually say where here is, because we've been asked not
to mention the exact spot, because it's a protected area.
The team face a huge filming challenge, as the groupers
normally spawn just once a year and it lasts less than an hour.
The team's adviser is biologist Yvonne Sadovy, who's been
studying fish spawning aggregations for over 30 years.
Here is as close to the situation it has been for thousands of years.
It's pretty much the only place on the planet that I'm aware of
where these groupers come together in the numbers that we see here.
I really hope that they're there,
because if they're not, then we've come all this way for nothing.
We've got our timing right, haven't we, Denis?
Yes. Confident? Yeah.
..three, four.
They're not disappointed.
Denis has filmed groupers gathering here for 15 years,
and has got to know their behaviour.
The grouper aren't the only ones to be gathering.
It's the highest density of grey reef sharks anywhere on the planet.
This is what a pristine reef should look like.
Lots and lots of predators.
With the groupers here,
now the challenge is predicting exactly when they'll spawn.
To judge if they're close, Yvonne reviews the day's footage.
From what we've seen here, the male's courting the female.
When you can slow these things down, this is where you begin to see
some of this detail. Yeah. If you were a betting woman,
would you bet on tomorrow or the second?
I would say tomorrow or the next day.
I mean, I think they're pretty close.
With only six divers, the team can't stay underwater all the time.
So, based on experience, they decide to dive on the full moon's
changing tides - thought to help co-ordinate grouper spawning.
But this year, the strongest tidal change is at night.
We're just kitting up for the first night dive.
During the day, we've been seeing sharks around this area,
but we don't know how they're going to react at night.
Are you nervous?
Reef sharks hunt at night, so the divers wear chainmail suits
and helmets as a precaution.
One, two, three, four.
Divers, comms check, comms check.
RADIO CRACKLES
Receiving you loud and clear.
RADIO CRACKLES
No sign of grouper spawning, but in the dark,
the shark's attitude has changed.
They're hunting.
Using electroreception to hunt, sharks are tuned to even
the smallest electrical stimulus - including the cameras.
HE GASPS
MUFFLED SPEECH
With the current building, now is the worst possible time to lose
contact with the surface.
We're in the middle of nowhere. We have no idea what's
going on down there. All we can hear is broken communication.
RADIO CRACKLES
With no sign of spawning, they end the dive.
HE SIGHS
The team decide to save their energy for the next change of the tide.
At dawn, they prepare for what they hope will be the spawning
event they've all been waiting for.
Bad news, it's... What?
But after just 20 minutes, the divers are back on the surface
with some devastating news for the crew.
All the groupers have gone?
It happened yesterday. We missed it.
Maybe they spawned last night. I don't...
Oh, God, I... I don't know.
The team were too late.
So my guess would be that we missed spawning by a few hours and possibly
it's that they just reached some kind of critical mass
or critical density and spawned a little bit early.
The following year, Yvonne returns to French Polynesia,
hoping to learn more about the grouper.
This year, Laurent Ballesta
and his experienced crew take on the challenge.
With many more divers, they have a new approach,
and they plan to have Laurent on watch in the water constantly.
And this year, after hundreds of hours of continual monitoring,
they have captured the defining moment of this astonishing event.
The mass spawning of the grouper.
We have the privilege to be able to watch this process
actually occurring and that, that's a real thrill.
They're really important for the biology of the animals.
Thanks to a combined team effort from both crews,
and Yvonne's research over many years,
we could at last tell the marbled grouper's remarkable story.
Next time, we head into the vastness of the open ocean.
To survive in this blue wilderness, some are fast,
others use deadly strategies,
and a few rely on the closeness of their families.
To find out more about our oceans
with this free poster, call...
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..and follow the links to the Open University.