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MUSIC

SIRENS

In the far corner of South East Asia lies the Coral Triangle,

a cluster of the

richest 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 specializes 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 colors and patterns.

And that, apparently, hypnotizes 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 mesmerizing 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 one percent of the ocean floor.

Yet they're home to a quarter of all known marine species.

They are complex, infinitely varying 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's attention.

It performs a headstand.

Not only is the grouper signaling 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 northeast

Australia.

Activity in coral reefs, wherever they are, never ceases.

At dawn, the day shift begins, accompanied by the chorus of

submarine song, created by fish, shrimps and other inhabitants of the reef.

Every resident in this city has its role. Scavengers,

like the sea cucumber, recycle the waste of others. These

parrotfish 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 certain fish. 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.

And some animals come to the reefs simply for rest and relaxation.

The family of bottlenose dolphins are resting

on the reef after a night's feeding offshore.

Whilst the adults and their babies sleep, the adolescents set off to explore.

And they appear to make up games.

We might call this one Catch the Coral.

The rules aren't entirely clear.

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.

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.

It's 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-meter 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.

In the Maldives, on the biggest tides, one particular coral

lagoon becomes so flooded with plankton that it attracts

hundreds of ocean giants.

This behavior 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 meter long.

It tastes for scent and

feels for movement.

It will bite, IT WH as well.

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.

The family of Saddleback clownfish have found an excellent home.

The tentacles of this carpeted anemone can kill, but the clownfish are immune to the

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.

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 fertilizes 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 gray 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 just want 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 fertilize 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.

The fertilized eggs will now be swept away from

the many hungry moths 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 color 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 fertilized 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.

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 advisor is biologist Yvonne Sadovich, who's been

studying fish spawning aggregations for over 30 years. Here,

it is 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 to tell

you right, haven't we, Denis? Confident? Yeah. So for sure

the groupers will be here, and they will be here because

they arrived at the full moon.

They're not disappointed.

Denis has filmed groupers gathering here for 15 years and

has got to know their behaviour. There are hundreds and

hundreds, thousands, but it's difficult to say how many. But

they're still coming in. You see them coming from the lagoon

and gathering into the channel. The groupers

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.

What we've seen here, the males courting the female, when

you can slow these things down, this is when you begin to

see some of this detail. If you were a betting woman, would

you bet on tomorrow? 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 coordinate groupers' spawning. But

this year, the strongest tidal change is at night. We're

just getting 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? We

have to be careful because of the current deep at night with

this heavy device in the shark's shoe. 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.

Receiving it out and clear.

No sign of groupers spawning, but in the dark, the shark's

attitude has changed. They're hunting. Using electro

-reception to hunt, sharks are tuned to even the

smallest electrical stimulus, including the cameras.

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.

With no sign of spawning, they end the dive.

It was a lot of shock. I struggled a little tonight for

swimming. I don't know why. Maybe there was a strong counter

current, but I struggled a little.

But at night, it's like no activity for the groupers. They

really like don't move, they kind of hide and all on the

coral. 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. But after just 20 minutes, the divers are back

on the surface with some devastating news for the crew.

Missed it. Maybe they spawned last night. I don't know.

The team were too late.

So my guess would be that we've 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 Ballester and his experience 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. It

made sense to do this 24-hour dive, to simply

get immersed in the behaviour of the animals.

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 see the grouper in

action. To be able to watch this process actually occurring.

That's a real thrill. They're really important for the

biology of the animals. I love photographing everything,

because there's this side where we capture the moment. And

I think I've never been as immersed in it as during this 24

-hour dive, which also took me a lot of energy. Thanks to a

combined team effort from both crews and Yvonne's research

over many years, we could at last tell the marble 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.

Thanks for watching!