NARRATOR: Just beneath the surface of the tropical seas
lies a world simply stunning to behold.
Home to the richest and most diverse array of life.
A bustling metropolis ablaze in vibrant color.
It may look like a paradise.
But in fact, it's a Darwinian battleground
where every species is locked
in a never-ending struggle for survival.
Each life depends on the coral reef.
This is how it all starts... a single coral larvae
amidst a soup of microscopic creatures
adrift in the open sea.
If just one of these larvae
comes to rest in a suitable spot,
a whole new reef can be born.
In a few days, the larvae become a polyp,
a small, spineless animal.
Tentacles surrounding a primitive mouth.
Identical polyps bud off.
The beginnings of a new colony.
As the polyps multiply,
their protective limestone skeleton also grows.
Branching coral can grow six to eight inches in one year.
These amazing creatures
have long fascinated filmmaker Hugh Pearson.
The fantastic thing about coral reefs or corals
is that essentially one large animal
made up of millions and millions of tiny ones.
So when you actually get your head in there
and look on the microscopic level,
there are these tiny, tiny polyps
and a coral so big can be made of thousands of them.
NARRATOR: These immense colonies
provide the infrastructure for a vast community.
25% of all the animals in the ocean
rely on coral reefs for survival.
Martha Holmes has been diving around coral reefs
her whole life.
I was brought up in the Tropics.
So coral reefs were my playground.
I learned to swim when I was 4, 5, just on the cusp,
and I put a mask and a snorkel on
and they were my playground.
Diving for me is so second nature.
It's just like I was walking down a street.
I just happen to be swimming along a reef.
I'm that comfortable in the water.
NARRATOR: These colorful Christmas tree worms
actually live inside the coral skeleton.
Many species never leave the reef's inner chambers.
Others venture outside only to feed.
For others still, the reef is a source of food
and a place to hide from larger predators.
As in any crowded city,
neighbors must learn to co-exist in tight quarters,
and many species have grown dependent on one another
and not just for food.
Occasionally, even animals
that spend most of their time in the open ocean
come in for some spot cleaning.
Coral reefs are home to an astonishing number of fish.
More than 4,000 different species
from some of the smallest to some of the largest.
Currents carry nutrients
from the deep ocean to the reef shallows,
attracting transients like these whale sharks
that gorge on microscopic plankton.
PEARSON: I've been diving for 20 years now.
I actually learned to dive on the Great Barrier Reef.
Coral reefs are just fantastic and amazing areas of the ocean.
The color and the variety is just astounding.
And then you see it one dive and think, "Wow. That's incredible."
You might go back a different time of day or at night
or a different time of year
and you see a whole other cast of characters.
NARRATOR: Coral reefs primarily grow
in the clear, warm shallows of the Tropics.
It's here that they can get the sunlight they need
to support the millions of microscopic algae
living within their flesh.
And 98% of the coral's food comes from its algae.
Without these tiny plants, the reef could not exist.
As with any other plant,
algae need just the right amount of light.
Not too much, not too little.
So the coral regulates the amount of incoming sunshine
with pigments.
Visible only in ultraviolet light.
PEARSON: Just before we started filming the coral program,
we saw some pictures of corals
which had been taken under U.V. light.
And they were just the most bizarre
and beautiful sort of images and colors.
We really wanted to try and capture that for the series.
So what we had to do
is we got the most powerful U.V. lights we could.
We literally flicked it on,
and, bang, the color that was coming out was incredible.
NARRATOR: Most coral polyps spend their days
protected inside their stony castles.
But even then, they're not safe
from the plucking snouts of butterfly fish.
It's only at night that the corals unfurl their tentacles
and burst to full glory.
But this isn't just a beautiful display.
It's a hunting party.
Corals eat plankton, and their tentacles are armed
with stinging cells which fire on contact.
Once a catch is made,
the tentacles stuff the prey into the polyp's mouth.
Coral is constantly growing.
So it's inevitable that two individuals
will bump into each other.
When corals cross paths, there's no room for negotiation.
Only a fight to the death.
Border disputes between coral are incredibly violent.
The coral on the right
detects the presence of an encroaching neighbor
and prepares for battle.
The strategy is straightforward.
The polyps extrude their guts and a deadly weapon.
Poisonous stinging cells strike to kill.
PEARSON: It's a brutal world, even on a coral reef.
The corals are having a go at each other
just the same as the other animals.
When Martha came back with that footage
of the corals attacking each other,
I remember we were watching it in the office.
We were all gobsmacked
'cause we'd all dived on coral reefs.
They're beautiful, but they're static.
Yet to see these animals, which is what they are,
just attacking each other and exuding their guts
and doing these weird, wonderful,
and horrible things to each other,
it just blew us all away.
And I think the scientists as well.
NARRATOR: Coral is also subject to attack from outside forces,
and this enemy is on the move,
skulking through the coral beds in search of its next victim.
This sinister-looking character is a crown-of-thorns starfish.
Poisonous and aggressive,
it, too, unleashes its digestive organs
and devours the coral polyps hidden inside.
Fortunately,
the coral has allies bolstering its front lines.
Small crabs living amongst the branches
vigorously defend their home.
The crabs attack the starfish on its vulnerable underside.
For the crown-of-thorns, the risk is not worth the reward.
The starfish retreats,
and the coral is left unharmed.
Less easily deterred
are these hump-head parrot fish.
Whenever they drop in to feed, the reef itself is in danger.
The parrot fish are after algae,
but they're voracious and indiscriminate eaters
and can chew through anything that gets in their way,
including coral.
PEARSON: When you're underwater, you can hear them.
This crunch, crunch scraping away.
They're actually eating the coral,
much like a lot of animals, like wildebeests, graze the grass.
NARRATOR: The swallowed coral emerges later as fine sand.
Parrot fish on a single reef can produce five tons of it a year.
But it doesn't go to waste.
It's that sand that forms tropical islands
supporting new life.
Life which will once again turn to the ocean for food.
On the reef, competition for living space continues,
but other battles are also brewing
between predators and prey.
Over millions of years,
the coral reef has been the backdrop
for an evolutionary arms race between predators and prey.
To survive in this competitive environment,
animals need specialized tactics
for attacking or evading the enemy.
Jacks are one of the top predators on the reef.
Size may be an advantage,
but their real weapon is speed.
These jacks are after silversides
who try to repel the attack
by gathering in tight shoals.
Like tiny specks of light flashing through the water,
their highly reflective scales
can easily confound would-be attackers.
But the jacks counter that tactic
by herding the silversides against the reef.
Here, the jacks have a better chance
of separating individuals from the shoal
and picking off the defenseless silversides
with lightning strikes.
For the reef's smaller inhabitants,
it's much safer to hide than run.
Holed up inside the tunnels of a sponge,
these tiny shrimp are no bigger than grains of rice.
They make up one of the reef's most surprising enclaves.
A highly sophisticated society similar to a beehive.
All members of the colony are the offspring of one female,
the queen,
and she is the only one to produce eggs.
HOLMES: What was really exciting
about filming the shrimp in the sponge
was that a eusocial animal that nobody had filmed before...
It's a eusocial animal living underwater.
The science was very, very new.
The scientist was incredibly excited to see his shrimp
that close up in that detail.
NARRATOR: As with bees,
individuals are specialized for particular tasks.
The sentries are armed
with unusually large and powerful claws.
They're always on guard,
ready to attack any and all intruders.
To a polychaete worm,
the sponge makes an excellent hunting ground.
In the labyrinth of tunnels,
it can mount a surprise attack from almost anywhere.
But once the alarm is raised,
the worm loses its advantage and retreats.
For these shrimp, the sponge provides everything they need.
It's a safe home with enough food inside
that they never need to venture out.
Quarters like these are clearly worth defending.
Just as shrimp protect their home,
others defend their hunting grounds.
Glassfish are tempting prey for the red-mouthed grouper.
Its strategy is to slowly swim among them
until it's no longer seen as a threat.
But this grouper has some competition.
The glassfish have also attracted the attention
of a lion-fish.
A venomous ambush predator,
it waits until just the right moment to strike.
Ignoring its rival's poisonous spines,
the grouper tries to drive the lion-fish away.
But the unrelenting lion-fish refuses to comply.
This standoff can continue for hours.
Covering the top of a coral reef
is an organism at the bottom of the food chain.
Algae.
Many reef dwellers forage exclusively
on this thin layer of green.
These powder-blue tangs
are trying to defend their feeding grounds.
On a patch of reef like this, where the algae is bountiful,
there's stiff competition.
When a school of convict tangs bully their way in to graze,
there's little the others can do to stop them.
The powder-blues do their best to shut out the intruders.
But there are just too many of the convicts.
In a matter of minutes, the algae patch is stripped bare.
Still, the blue tangs keep fighting
until, finally, the raiders move on.
When night falls on the coral reef,
a different and very strange set of creatures
crawl out of the crevices.
Near the coral reef,
a remarkable creature is getting ready to feed.
By day, this basket starfish sleeps curled up
inside a sponge.
But at night, it emerges,
spreading its arms into the oncoming current
to fish for plankton.
For a while, the reef is unnaturally quiet
as fish retire for the night.
But before long, marble rays come out to hunt,
using electroreceptors to search for prey buried in the sand.
Their activity attracts sharks.
White-tips.
These predators can hunt any time of day,
but it's at night, when vision is limited,
that they have a distinct advantage
over the competition.
Using electroreception and their extraordinary sense of smell,
they can track prey in pitch darkness.
Tonight, the white-tips are looking for fish
concealed within the reef.
Martha Holmes has always been struck
by the white-tip's hunting prowess.
HOLMES: When white-tip reef sharks hunt in packs at night
and there are a lot of sharks around,
you wouldn't want to be a fish
or anything hiding under a reef or in a crevice at that point.
They really are bullyboys out to get as much food as they can.
NARRATOR: There's nowhere to hide from these killers.
It's a perfect example of a feeding frenzy.
Night after night, reef animals are subject to raids like this.
But life on a coral reef isn't all about eating.
It's also about sex.
It's breeding season on the coral reef,
and the inhabitants of this underwater city
have many different strategies
to ensure the survival of their species.
One of the more curious rituals
is performed by the banded pipefish.
PEARSON: The banded pipefish are one of my favorite animals.
They're just so cute and so cool.
They're related to sea horses,
and you can see that in their snout.
They're almost like straightened-out sea horses.
NARRATOR: Every morning,
the female leaves her sleeping site to find her partner.
They remain together for about 10 minutes,
reaffirming the bond essential to their partnership.
Then they begin to dance.
After about two hours of courting,
they rise from the seabed in tandem,
their bodies entwined around each other.
As the male rubs himself against the female,
she releases her eggs.
And very quickly, he gathers them up.
The eggs are now stuck to his body,
and the female pats them down to make sure they stay put.
Once the eggs are secure, the female leaves her partner.
But she'll be back in the morning
for another round of synchronized swimming,
reinforcing their bond as their offspring develop.
PEARSON: They do their thing first thing in the morning,
so we'd have to set out
at about 5:00 in the morning, before sunrise.
We'd dive down there, and you've only got about half an hour
from when the sun comes up to find them,
to find the male, find the female.
It's a lovely thing to watch.
And, you know, very sort of intimate.
There just seemed to be emotion with those animals.
NARRATOR: Ten days after the eggs are fertilized,
the male casts the hatchlings into the ocean.
Now the young pipefish
are completely independent of their parents.
Since the male took charge of incubating the eggs,
the female can immediately start producing the next batch.
This division of labor allows pipefish to breed
every 10 days,
doubling the number of offspring they would otherwise produce.
This flamboyant character is a cuttlefish,
though he's not really a fish at all
but a mollusk related to octopuses and squid.
This one is trying to impress and seduce a larger female
with his colorful display.
Eventually, it works.
But the big event, the transfer of sperm,
is all over very quickly.
[Whales singing]
On a much grander scale
is the mating ritual of the humpback whale
in which it is thought that the males sing to the females
for breeding rights.
Humpbacks are not true reef dwellers.
Just visitors.
Females come here to give birth after a yearlong pregnancy.
The warm tropical waters provide a safe haven
for the mothers to nurse their calves
before they return to the cold, nutrient-rich waters
where they feed.
PEARSON: The mothers don't eat for about six months,
and yet they have to have a calf,
feed that calf something like 180 gallons of milk a day
for about four months,
then swim back to their feeding grounds.
All without having a bite to eat.
It's a pretty amazing story.
NARRATOR: The females may still be focused on their young,
but the males are here to mate,
and the competition is fierce.
Scientists theorize that males sing
to assert their dominance and desirability.
The louder and longer the song,
the bigger and stronger the singer
and, hence, the more attractive.
[Whale singing]
As the humpback's songs reverberate
throughout tropical waters,
there is another drama playing out.
The corals themselves are preparing to breed.
For ages, the mystery of coral reproduction
had scientists perplexed.
How could creatures fixed to the ocean floor reproduce sexually?
Then, just 30 years ago, researchers unlocked the secret.
Synchronized spawning.
A few days after the full moon, when tidal currents are weakest,
the coral is ready to reproduce.
Each species spawns on a particular night
at a precise time,
maximizing their chances for success.
HOLMES: To film this, we brought in a team of scientists
who know very well which species go off on which night,
and they can literally dive down
and say, "These eggs are just waiting to go,
waiting for the clock to tick."
The moment 8:00 comes, or whatever time their time is,
all the coral heads all over the reef of that species will go.
It's an extraordinary phenomenon to think
that all of these corals right across the barrier reef
are timing down to the hour.
NARRATOR: Some corals are male
and release clouds of sperm into the water.
Nearby, a female will release her eggs.
Other species of coral are both male and female.
They release buoyant packages of eggs
wrapped in sperm
that break apart once they reach the surface.
PEARSON: The coral spawn is a fantastic moment
in the marine calendar.
It's almost like tiny little polystyrene balls
all sort of floating up.
It's quite ethereal, really, because it's very quiet.
It's weird, upside-down snow, really.
NARRATOR: The fertilized eggs float to the surface
and drift away.
Their future uncertain.
Most won't survive,
but a destined few will lay the foundation
for coral reefs to come.
For many creatures who live here,
life is just beginning.
But for others, it could be in danger.
When the stormy season comes,
the animals and the reef itself are at risk.
Lobsters sense a change in water temperature
and disturbances in the sand caused by rising ocean swells.
Under cover of darkness,
they leave their burrows
to outrace the oncoming storm.
The journey will leave them vulnerable,
but reaching the safety of deeper water
is their only chance of survival.
It's a pilgrimage they make every year.
Lobsters from all over the reef join in the mass migration,
conserving energy by tucking into each other's slipstreams.
By daybreak, they reach the edge of the ocean shelf
and climb down into deeper waters.
Here is where they'll stay until the storm season is over.
Storms are an inconvenience,
but hurricanes can be devastating.
When a hurricane strikes,
it can threaten the very structure of a coral reef.
An entire reef, hundreds of years in the making,
can be smashed to pieces in a matter of hours.
But the greatest danger to the reefs today
comes from manmade threats like blast fishing,
pollution, and climate change.
HOLMES: To mankind as a whole,
if you want to consider us as guardians of this planet,
it is essential that we don't screw up the oceans
and let the reefs die
because of something that we do or some way that we live.
PEARSON: The oceans are such a wealth of life,
such a wealth of mystery and diversity and beauty.
You don't have to have a vivid imagination
to see what the oceans will be like if we keep going.
NARRATOR: But it's not too late to change course.
Coral is a resilient animal,
and as long as there is new life looking for a foothold,
there is still hope for the future.
PEARSON: To film the corals,
we had a number of film crews working with coral scientists
on the Great Barrier Reef.
Some were working on the reef at night
to get the wide-angle shots of the coral spawning.
Mike, do you need light balance away from the reef?
I will, yes.
I was very keen on making the coral reef film
that we had a really moody,
different feel for the reef at night.
We had to do that by establishing the lighting.
What I wanted to achieve was a feeling
that it was as natural as possible.
Just so want to get the best images.
Lots and lots of tiny little eggs wafting around.
Working on a reef at night is quite an operation.
I mean, you've got somebody manhandling the lights.
We had two cameramen in the water.
You just sit on the boat and pray everything is going fine.
And then, sure enough, it did.