In all the seas of the world, the warm waters of the tropics
contain the richest and most colourful communities.
Coral reefs.
They may seem like underwater paradise,
but they are perpetual battlegrounds for space.
Even the corals have to fight for it.
In this crowded frenetic community, every individual needs its place,
its own way of surviving.
None of these creatures would be here if it were not for the coral.
A coral larva drifts in the open sea,
floating in a soup of young reef animals.
If just one of these coral larvae survives in a suitable spot,
a new reef will be founded.
In just a few days, the larva changes form and becomes a polyp,
similar to a sea anemone.
Identical copies bud off and, gradually, a colony develops.
Each polyp surrounds itself with a hard skeleton,
and, from this solid base, begins to grow.
It increases in length by an impressive 15cms a year.
This branching coral is only two years old.
A mature reef can be thousands.
Corals provide the foundations
on which the entire reef community relies.
Some organisms,
like the Christmas tree worms, actually live within the coral.
Others climb out, away from the reef,
to filter their food from the water.
As the community grows,
intimate relationships form,
and different creatures become dependant on one another.
Even animals that spend much of their time travelling in the ocean
return to the reef for a clean.
Coral reefs can be home to astounding numbers of fish.
Here swim the smallest and the largest fish in the sea.
Whale sharks are only visitors.
When currents bring nutrient—rich water up from the deep,
they come here to feed.
All these animals are here
because of the coral.
This extraordinary complex maze is built, layer upon layer,
by millions and millions of individual animals —
polyps.
Each polyp's flesh is supported by a limestone skeleton.
Below the gut...
..is the place where most of the growth occurs.
Here, the living tissue deposits an intricate lattice of limestone.
Beneath that, the limestone skeleton is bare,
having been vacated by the living coral tissues.
This is the hard structure that forms the foundation of the reef.
A single reef can extend for many miles.
Coral reefs are only found in the clear, warm,
shallow waters of the tropics.
Sunlight is vital to them, even though they are animals,
because inside their flesh live millions of tiny single—cell algae,
plants.
And all plants need sunlight to photosynthesise sugars.
98% of the food the corals consume is produced by the algae.
Without them,
the reef would not exist.
Like any plant, algae need just the right amount of light.
Not too much, not too little.
The corals regulate that with pigments that we can only see
when they are illuminated by ultraviolet light.
Most corals, for protection,
spend the day withdrawn into their stony fortresses.
Even then they are not safe from the jaws of these butterfly fish.
At night, the corals take in water,
expand their tentacles and emerge to feed.
They collect plankton.
Each tentacle has stinging cells, which fire on contact.
Once the prey is caught, it's passed down to the polyp's mouth.
It is at night, when the polyps are extended,
that they add to the limestone foundations beneath them.
Inevitably, the corals begin to overgrow each other,
and that means trouble.
When neighbours get too close,
they detect one another's presence chemically.
The aggressor on the right prepares for battle.
The polyps extrude their guts
and simply digest their rivals alive.
A no—man's—land — a band of white skeleton —
is the only evidence of the night's border dispute.
Some corals are targeted by yet more deadly predators.
Predators that can crawl in search of their victims.
Crown of thorns starfish —
poisonous, invincible eating machines.
They also extrude their gut and digest coral wholesale.
But some corals have help.
Small crabs living within their branches resist these onslaughts
and defend their home.
From beneath, they launch an attack
on the vulnerable underside of the starfish.
Even the crown of thorns will retreat
from such a determined attack.
This coral is left unharmed.
Humphead parrotfish, nearly a metre and a half in length.
Their jaws are so powerful they can bite through rock.
When they descend to feed, the reef itself is under threat.
They are indiscriminate feeders,
taking both rock and coral alike in their quest for algae.
These fish play a large part in the erosion of the reef.
The rock and coral emerges later as a fine sand.
On a single reef, they can produce tonnes of it every year.
This soft sand forms the tropical beaches
that we find so alluring.
Over time, the sand builds up to form an island,
which is then colonised by animals and plants.
Trees take root.
Birds arrive.
The guano from thousands of terns which have chosen to nest here
enriches the sandy soil, which then can support more plants.
But these terns, like other seabirds,
depend on the ocean for their food.
Below water, there is not only competition for living space,
but a continual contest between predators and prey.
It's the arms race between them that over millions of years
has produced today's extraordinary diversity of form.
Jacks are one of the key predators on the reef.
Their weapon is speed.
They seek silversides, whose defence is to congregate
in confusing shoals of shimmering silver.
The jacks deal with that by herding the silversides onto the reef.
Here, the jacks have a better chance
of separating individual fish from the shoal.
The jacks can now catch the isolated individuals
with lightning attacks.
It's far safer to be hidden on the reef itself,
within the tunnels of a sponge, for example.
These tiny shrimp are no bigger than grains of rice.
These shrimps are unique.
It's recently been discovered
that they have a highly—sophisticated social system,
similar to that of bees.
All members of the colony are the offspring of one female.
She is the queen and the only one to produce eggs.
As in a colony of bees,
different individuals are specialised for particular tasks.
Some are guards and are armed
with particularly large and powerful claws.
They are on watch at all times, ready to tackle intruders.
A polychaete worm. For it, a sponge is an excellent hunting ground.
In such a maze of tunnels, attack can come at any time
from any quarter.
Once the guards are alerted, the worm loses its advantage.
Better to retreat intact than risk serious injury.
The sponge not only makes a safe home for the shrimps,
it also supplies them with food, so they never need venture outside.
An establishment that provides all their needs
is clearly well worth defending.
Just as shrimps guard their home,
other animals defend their hunting grounds.
Glassfish make tempting prey for the redmouth grouper.
Its strategy is to swim slowly amongst them
till they no longer see it as a threat.
There are other fish here, too.
Lionfish are ambush predators,
taking their time and watching for the right moment.
But there isn't room here for two predators.
The grouper, braving the lionfish's poisonous spines,
tries to evict its rival.
But lionfish are persistent.
This grouper spent many hours simply defending his hunting patch.
Some animals prefer to avoid conflict whenever possible.
These harlequin shrimp, having captured a starfish,
are taking it back to a safe house
beyond the reach of competitors and danger.
The problem is that starfish have minds of their own
and five large sticky arms.
By the time the shrimps have prised off one arm,
another has reattached itself.
Only by manoeuvring the starfish onto its back
can they have any hope of gaining the advantage.
Even so, getting it back home
is a major undertaking.
The starfish is now a living larder.
If the shrimps can hang onto it, it will feed them for days to come.
The top of the reef is usually covered
by a thin layer of green algae, another living larder.
And many fish depend on it.
Powder blue tangs defend their right
to graze on a particular patch.
But, for a larder as well stocked as this, there is always competition.
When a shoal of convict tangs decide to graze,
little can stop them.
The powder blue tangs try to keep them off...
..but they are overwhelmed by sheer numbers.
The territory is stripped of algae in minutes.
The blue tangs appear to be fighting a losing battle.
But, eventually, they begin to get the upper hand.
They persist with their attacks
until the marauders are well on their way.
When night falls, some very strange creatures creep out of crevices
and crawl over the reef.
This moving bush is an animal—
a basket star —
which spreads out its arms to catch the night's plankton.
The reef becomes uncannily tranquil.
Fish retire, hiding themselves where they can.
Marbled rays come out to hunt for prey buried in the sand...
..using electro—receptors to scan the seabed.
Their activity attracts sharks.
White tips.
At night, when vision is of little use, sharks have a real advantage.
They can still use both smell and electro—reception to track fish.
These sharks are also hunting for fish concealed within the reef.
Their slender shape
enables them to squeeze through surprisingly narrow gaps.
There is nowhere to hide.
Few animals are safe during these feeding frenzies.
Night after night, the reef animals are subjected to these raids.
But life on the reef is not just about food,
it's also about sex.
There are many different breeding strategies,
but each is aimed at maximising the number of young that will survive.
Every afternoon for two months, brown surgeonfish can be seen
streaming across reefs in the Red Sea.
They all head for the same place,
usually some prominent feature.
Here, they wait for the light to fade.
Suddenly, females within the group
make a dash away from the reef to release their eggs.
They're followed by the quickest and closest males,
all of whom are striving to fertilise the eggs.
Inevitably, others come here to feast on such easy food.
As the surgeonfish spawn,
fusiliers move in above to eat the nutritious eggs.
These are just the first of many predators which will feed
on the eggs and developing larvae
as they drift during the next few weeks.
Other fish are less casual about their eggs.
Banded pipefish stay close to a small chosen area on the reef.
Every morning at sunrise, the female leaves her sleeping site
and swims to find her partner.
For ten minutes or so, they remain together,
reaffirming the bond that is essential to their partnership.
They swim together around his territory
in a simple greeting dance.
Throughout the summer, when the female's eggs are ripe,
courtship begins in earnest in the early morning.
It takes time. After about two hours, they rise off the seabed,
entwining their two bodies.
The male rubs himself against the female,
stimulating her to release her eggs.
And now, swiftly, the male takes them.
The eggs, now stuck to his belly,
are patted down to ensure that they stay there.
The female then leaves him,
but, every morning, returns for a session of synchronised swimming
and so ensures that their bond is maintained.
Ten days later, under the cover of darkness, the male shakes his body
and the young pipefish are born.
Only now are they independent of their parents.
Since the male takes charge of the eggs as soon as they are laid,
the female can start producing the next batch.
Without his help, the pair could only breed every 20 days,
rather than every ten.
By sharing the work, they double
the number of young they can produce in any one year.
A flamboyant cuttlefish.
Unlike most cuttlefish, this one spends much of its time walking,
rather than jetting across the seabed.
This is a male.
He is using his colourful display
to try and seduce the larger female, who seems unimpressed.
Eventually, she concedes.
The final event, the transfer of sperm, is very quick.
(WHALE SONG)
A singing male humpback whale.
Humpbacks are only visitors to the reef.
After a pregnancy that lasted a whole year,
the females come here to give birth and suckle their newly—born young.
Their investment in their single offspring is considerable,
for each female will continue to nurse it
for a further six to 12 months.
But the males are here to mate.
The lone males sing to establish their relative seniority.
The louder and longer the song, the bigger and stronger the singer.
The better the song, the larger the male,
the more mating opportunities he will get.
All these different mating strategies have the same aim —
to ensure that the greatest possible number of offspring
will live long enough to breed themselves.
Corals also reproduce sexually,
but being fixed to the seabed, they can't move to find a mate.
Somehow, they must synchronise their sexual activity.
They do so using the rising water temperatures of spring
and the phases of the moon.
A few days after the full moon in late spring,
when tidal currents are at their weakest,
the corals of the Great Barrier Reef are ready to spawn.
Some corals are male and release clouds of sperm.
Nearby, a female will be releasing eggs.
Other species of coral are both male and female.
These release packages of eggs
already pre—wrapped in sperm.
Bundles of eggs and sperm float to the surface
to mix with others from further along the reef.
Each kind of coral times its release
to a certain hour on a certain night.
That maximises the chances for cross—fertilisation.
The fertilised eggs
drift away from the reef.
The stormy season brings real danger to the animals of the reef.
Lobsters in the Caribbean sense a change in the water.
The temperature drops and powerful swells disturb the sand.
Under the cover of darkness,
they emerge to run before the storm
and risk crossing the exposed sand flats
to seek shelter in deeper water.
Every year they make this journey.
From all over the reef, lobsters come to join the march.
They conserve energy by travelling in one another's slipstream.
And there is the added benefit
of safety in numbers.
By daybreak, they've reached the edge of the deep reef
and down they go.
For the rest of the stormy season,
they will remain in the shelter of deep water,
out of harm's way.
Sometimes, during the stormy season, a hurricane builds.
Then the very structure of the reef itself is under threat.
An entire reef can be destroyed by just one big storm.
Hundreds of years of growth gone in a few hours.
Out in the ocean, new life continues to develop.
In time, coral larvae will return to colonise the rubble
and a new reef will grow on the wasteland.