Original subtitles

In all the seas of the world the warm waters of the tropics contain the richest

and the most colourful communities

Coral reefs

They may seem like underwater paradise,

but they are perpetual battle grounds... for space

Even the corals have to fight for it

In this crowded, frenetic community every individual has to find its own place,

its own way of surviving

But 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 settles in a suitable spot

and survives 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 fifteen centimetres 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 are formed

and different creatures become increasingly dependent on one another

Even animals that spend much of their time

travelling in the open ocean return to the reef for a clean

Corals 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

and 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-celled algae - plants

And all plants need sunlight to photosynthesise sugars

Ninety eight per cent of the food that corals consume is produced by the algae

Without them the reef would not exist

Like any other 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

But 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 batteries of stinging cells, which fire on contact

Once the prey is caught, it's passed down to the polyp's mouth

It's 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

and 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 they swallow 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 on the reef 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 and their defence is to congregate

in confusing shoals of shimmering silver

The Jacks try to 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 that they never need venture outside

And establishment that provides for 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

until 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 with starfish is that they have minds of their own

and five large sticky arms

By the time the shrimps have prised off 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 spread 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 immediately followed by the quickest and closest of the 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 in the ocean during the next few weeks

Other fish are less casual about their eggs

Banded pipe fish 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 and after about two hours

they rise together off the seabed entwining their 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

she will return for a session of synchronised swimming

and so ensure that their bond is maintained

Ten days later, under the cover of darkness,

the male shakes his body and the young pipe fish 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 immediately producing the next batch

Without his help, the pair could only breed every twenty days,

rather than every ten

So, by sharing the work, they're doubling 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

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 twelve 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

and 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 of 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 ocean 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 their energy by travelling in one another's slipstream

And there is the added benefit f 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

and 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