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In all the seas of the world,
the warm waters of the tropics

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contain the richest
and most colourful communities.

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Coral reefs.

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They may seem
like underwater paradise,

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but they are perpetual
battlegrounds for space.

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Even the corals
have to fight for it.

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In this crowded frenetic community,
every individual needs its place,

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its own way of surviving.

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None of these creatures would be
here if it were not for the coral.

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A coral larva
drifts in the open sea,

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floating in a soup
of young reef animals.

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If just one of these coral larvae
survives in a suitable spot,

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a new reef will be founded.

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In just a few days, the larva
changes form and becomes a polyp,

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similar to a sea anemone.

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Identical copies bud off
and, gradually, a colony develops.

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Each polyp surrounds itself
with a hard skeleton,

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and, from this solid base,
begins to grow.

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It increases in length
by an impressive 15cms a year.

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This branching coral
is only two years old.

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A mature reef can be thousands.

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Corals provide the foundations

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on which the entire
reef community relies.

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Some organisms,

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like the Christmas tree worms,
actually live within the coral.

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Others climb out,
away from the reef,

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to filter their food
from the water.

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As the community grows,

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intimate relationships form,

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and different creatures
become dependant on one another.

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Even animals that spend much
of their time travelling in the ocean

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return to the reef for a clean.

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Coral reefs can be home
to astounding numbers of fish.

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Here swim the smallest
and the largest fish in the sea.

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Whale sharks are only visitors.

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When currents bring nutrient—rich
water up from the deep,

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they come here to feed.

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All these animals are here

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because of the coral.

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This extraordinary complex maze
is built, layer upon layer,

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by millions and millions
of individual animals —

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polyps.

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Each polyp's flesh is supported
by a limestone skeleton.

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Below the gut...

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..is the place
where most of the growth occurs.

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Here, the living tissue deposits
an intricate lattice of limestone.

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Beneath that,
the limestone skeleton is bare,

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having been vacated
by the living coral tissues.

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This is the hard structure
that forms the foundation of the reef.

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A single reef can extend
for many miles.

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Coral reefs are only found
in the clear, warm,

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shallow waters of the tropics.

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Sunlight is vital to them,
even though they are animals,

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because inside their flesh live
millions of tiny single—cell algae,

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plants.

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And all plants need sunlight
to photosynthesise sugars.

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98% of the food the corals consume
is produced by the algae.

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Without them,

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the reef would not exist.

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Like any plant, algae need
just the right amount of light.

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Not too much, not too little.

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The corals regulate that
with pigments that we can only see

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when they are illuminated
by ultraviolet light.

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Most corals, for protection,

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spend the day withdrawn
into their stony fortresses.

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Even then they are not safe
from the jaws of these butterfly fish.

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At night, the corals take in water,

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expand their tentacles
and emerge to feed.

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They collect plankton.

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Each tentacle has stinging cells,
which fire on contact.

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

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It is at night,
when the polyps are extended,

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that they add to the limestone
foundations beneath them.

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Inevitably, the corals
begin to overgrow each other,

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and that means trouble.

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When neighbours get too close,

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they detect
one another's presence chemically.

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The aggressor on the right
prepares for battle.

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The polyps extrude their guts

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and simply
digest their rivals alive.

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A no—man's—land —
a band of white skeleton —

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is the only evidence
of the night's border dispute.

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Some corals are targeted
by yet more deadly predators.

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Predators that can crawl
in search of their victims.

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Crown of thorns starfish —

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poisonous,
invincible eating machines.

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They also extrude their gut
and digest coral wholesale.

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But some corals have help.

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Small crabs living within their
branches resist these onslaughts

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and defend their home.

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From beneath, they launch an attack

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on the vulnerable underside
of the starfish.

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Even the crown of thorns
will retreat

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from such a determined attack.

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This coral is left unharmed.

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Humphead parrotfish,
nearly a metre and a half in length.

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Their jaws are so powerful
they can bite through rock.

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When they descend to feed,
the reef itself is under threat.

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They are indiscriminate feeders,

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taking both rock and coral alike
in their quest for algae.

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These fish play a large part
in the erosion of the reef.

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The rock and coral emerges later
as a fine sand.

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On a single reef, they can produce
tonnes of it every year.

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This soft sand
forms the tropical beaches

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that we find so alluring.

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Over time, the sand builds up
to form an island,

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which is then colonised
by animals and plants.

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Trees take root.

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Birds arrive.

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The guano from thousands of terns
which have chosen to nest here

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enriches the sandy soil,
which then can support more plants.

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But these terns,
like other seabirds,

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depend on the ocean for their food.

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Below water, there is not only
competition for living space,

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but a continual contest
between predators and prey.

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It's the arms race between them
that over millions of years

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has produced today's
extraordinary diversity of form.

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Jacks are one of the key predators
on the reef.

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Their weapon is speed.

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They seek silversides,
whose defence is to congregate

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in confusing shoals
of shimmering silver.

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The jacks deal with that by herding
the silversides onto the reef.

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Here, the jacks
have a better chance

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of separating individual fish
from the shoal.

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The jacks can now catch
the isolated individuals

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with lightning attacks.

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It's far safer to be hidden
on the reef itself,

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within the tunnels of a sponge,
for example.

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These tiny shrimp are no bigger
than grains of rice.

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These shrimps are unique.

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It's recently been discovered

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that they have
a highly—sophisticated social system,

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similar to that of bees.

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All members of the colony
are the offspring of one female.

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She is the queen
and the only one to produce eggs.

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As in a colony of bees,

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different individuals are
specialised for particular tasks.

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Some are guards and are armed

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with particularly
large and powerful claws.

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They are on watch at all times,
ready to tackle intruders.

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A polychaete worm. For it, a sponge
is an excellent hunting ground.

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In such a maze of tunnels,
attack can come at any time

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from any quarter.

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Once the guards are alerted,
the worm loses its advantage.

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Better to retreat intact
than risk serious injury.

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The sponge not only makes
a safe home for the shrimps,

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it also supplies them with food,
so they never need venture outside.

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An establishment that provides
all their needs

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is clearly well worth defending.

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Just as shrimps guard their home,

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other animals defend
their hunting grounds.

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Glassfish make tempting prey
for the redmouth grouper.

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Its strategy is to swim
slowly amongst them

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till they no longer see it
as a threat.

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There are other fish here, too.

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Lionfish are ambush predators,

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taking their time
and watching for the right moment.

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But there isn't room here
for two predators.

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The grouper, braving the lionfish's
poisonous spines,

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tries to evict its rival.

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But lionfish are persistent.

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This grouper spent many hours
simply defending his hunting patch.

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Some animals prefer to avoid
conflict whenever possible.

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These harlequin shrimp,
having captured a starfish,

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are taking it back to a safe house

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beyond the reach
of competitors and danger.

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The problem is that starfish
have minds of their own

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and five large sticky arms.

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By the time the shrimps
have prised off one arm,

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another has reattached itself.

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Only by manoeuvring
the starfish onto its back

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can they have any hope
of gaining the advantage.

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Even so, getting it back home

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is a major undertaking.

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The starfish
is now a living larder.

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If the shrimps can hang onto it,
it will feed them for days to come.

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The top of the reef
is usually covered

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by a thin layer of green algae,
another living larder.

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And many fish depend on it.

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Powder blue tangs
defend their right

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to graze on a particular patch.

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But, for a larder as well stocked
as this, there is always competition.

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When a shoal of convict tangs
decide to graze,

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little can stop them.

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The powder blue tangs
try to keep them off...

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..but they are overwhelmed
by sheer numbers.

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The territory is stripped of algae
in minutes.

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The blue tangs appear
to be fighting a losing battle.

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But, eventually,
they begin to get the upper hand.

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They persist with their attacks

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until the marauders
are well on their way.

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When night falls, some very strange
creatures creep out of crevices

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and crawl over the reef.

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This moving bush is an animal—

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a basket star —

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which spreads out its arms
to catch the night's plankton.

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The reef becomes
uncannily tranquil.

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Fish retire, hiding themselves
where they can.

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Marbled rays come out to hunt
for prey buried in the sand...

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..using electro—receptors
to scan the seabed.

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Their activity attracts sharks.

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White tips.

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At night, when vision is of little use,
sharks have a real advantage.

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They can still use both smell
and electro—reception to track fish.

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These sharks are also hunting
for fish concealed within the reef.

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Their slender shape

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enables them to squeeze
through surprisingly narrow gaps.

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There is nowhere to hide.

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Few animals are safe
during these feeding frenzies.

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Night after night, the reef animals
are subjected to these raids.

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But life on the reef
is not just about food,

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it's also about sex.

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There are many different
breeding strategies,

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but each is aimed at maximising
the number of young that will survive.

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Every afternoon for two months,
brown surgeonfish can be seen

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streaming across reefs
in the Red Sea.

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They all head for the same place,

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usually some prominent feature.

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Here, they wait
for the light to fade.

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Suddenly, females within the group

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make a dash away from the reef
to release their eggs.

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They're followed
by the quickest and closest males,

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all of whom are striving
to fertilise the eggs.

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Inevitably, others come here
to feast on such easy food.

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As the surgeonfish spawn,

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fusiliers move in above
to eat the nutritious eggs.

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These are just the first
of many predators which will feed

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on the eggs and developing larvae

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as they drift
during the next few weeks.

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Other fish are less casual
about their eggs.

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Banded pipefish stay close
to a small chosen area on the reef.

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Every morning at sunrise,
the female leaves her sleeping site

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and swims to find her partner.

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For ten minutes or so,
they remain together,

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reaffirming the bond that is
essential to their partnership.

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They swim together
around his territory

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in a simple greeting dance.

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Throughout the summer,
when the female's eggs are ripe,

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courtship begins in earnest
in the early morning.

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It takes time. After about two hours,
they rise off the seabed,

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entwining their two bodies.

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The male rubs himself
against the female,

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stimulating her
to release her eggs.

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And now, swiftly,
the male takes them.

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The eggs, now stuck to his belly,

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00:35:20,080 --> 00:35:23,790
are patted down
to ensure that they stay there.

247
00:35:44,240 --> 00:35:46,914
The female then leaves him,

248
00:35:47,080 --> 00:35:51,438
but, every morning, returns for
a session of synchronised swimming

249
00:35:51,600 --> 00:35:55,514
and so ensures
that their bond is maintained.

250
00:35:56,920 --> 00:36:02,598
Ten days later, under the cover of
darkness, the male shakes his body

251
00:36:02,760 --> 00:36:05,912
and the young pipefish are born.

252
00:36:12,760 --> 00:36:17,277
Only now are they
independent of their parents.

253
00:36:17,440 --> 00:36:23,311
Since the male takes charge
of the eggs as soon as they are laid,

254
00:36:23,480 --> 00:36:26,439
the female can start
producing the next batch.

255
00:36:26,600 --> 00:36:30,514
Without his help, the pair could
only breed every 20 days,

256
00:36:30,680 --> 00:36:32,273
rather than every ten.

257
00:36:32,440 --> 00:36:34,432
By sharing the work, they double

258
00:36:34,600 --> 00:36:38,310
the number of young
they can produce in any one year.

259
00:36:49,640 --> 00:36:52,109
A flamboyant cuttlefish.

260
00:36:55,160 --> 00:37:00,519
Unlike most cuttlefish, this one
spends much of its time walking,

261
00:37:00,680 --> 00:37:03,400
rather than jetting
across the seabed.

262
00:37:03,560 --> 00:37:05,552
This is a male.

263
00:37:10,400 --> 00:37:13,074
He is using his colourful display

264
00:37:13,240 --> 00:37:19,237
to try and seduce the larger female,
who seems unimpressed.

265
00:37:29,000 --> 00:37:31,435
Eventually, she concedes.

266
00:37:35,440 --> 00:37:41,038
The final event, the transfer
of sperm, is very quick.

267
00:37:46,400 --> 00:37:48,392
(WHALE SONG)

268
00:38:01,080 --> 00:38:04,152
A singing male humpback whale.

269
00:38:14,800 --> 00:38:17,713
Humpbacks are only visitors
to the reef.

270
00:38:17,880 --> 00:38:21,271
After a pregnancy
that lasted a whole year,

271
00:38:21,440 --> 00:38:26,435
the females come here to give birth
and suckle their newly—born young.

272
00:38:26,600 --> 00:38:30,640
Their investment in their single
offspring is considerable,

273
00:38:30,800 --> 00:38:33,474
for each female
will continue to nurse it

274
00:38:33,640 --> 00:38:35,757
for a further six to 12 months.

275
00:38:35,920 --> 00:38:39,231
But the males are here to mate.

276
00:38:46,720 --> 00:38:52,318
The lone males sing to establish
their relative seniority.

277
00:38:56,440 --> 00:39:02,072
The louder and longer the song,
the bigger and stronger the singer.

278
00:39:34,520 --> 00:39:37,752
The better the song,
the larger the male,

279
00:39:37,920 --> 00:39:41,516
the more mating opportunities
he will get.

280
00:39:44,720 --> 00:39:49,158
All these different mating
strategies have the same aim —

281
00:39:49,320 --> 00:39:53,200
to ensure that the greatest
possible number of offspring

282
00:39:53,360 --> 00:39:56,353
will live long enough
to breed themselves.

283
00:40:01,840 --> 00:40:04,275
Corals also reproduce sexually,

284
00:40:04,440 --> 00:40:09,640
but being fixed to the seabed,
they can't move to find a mate.

285
00:40:09,800 --> 00:40:14,397
Somehow, they must synchronise
their sexual activity.

286
00:40:14,560 --> 00:40:18,349
They do so using the rising
water temperatures of spring

287
00:40:18,520 --> 00:40:20,318
and the phases of the moon.

288
00:40:31,720 --> 00:40:35,475
A few days after the full moon
in late spring,

289
00:40:35,640 --> 00:40:38,394
when tidal currents
are at their weakest,

290
00:40:38,560 --> 00:40:43,032
the corals of the Great Barrier Reef
are ready to spawn.

291
00:40:49,800 --> 00:40:54,829
Some corals are male
and release clouds of sperm.

292
00:40:55,000 --> 00:40:59,233
Nearby, a female
will be releasing eggs.

293
00:41:15,000 --> 00:41:19,279
Other species of coral
are both male and female.

294
00:41:22,200 --> 00:41:26,035
These release packages of eggs

295
00:41:26,200 --> 00:41:29,159
already pre—wrapped in sperm.

296
00:42:20,920 --> 00:42:24,072
Bundles of eggs and sperm
float to the surface

297
00:42:24,240 --> 00:42:28,439
to mix with others
from further along the reef.

298
00:42:31,960 --> 00:42:35,795
Each kind of coral
times its release

299
00:42:35,960 --> 00:42:38,714
to a certain hour
on a certain night.

300
00:42:38,880 --> 00:42:42,760
That maximises the chances
for cross—fertilisation.

301
00:42:49,640 --> 00:42:51,438
The fertilised eggs

302
00:42:51,600 --> 00:42:54,240
drift away from the reef.

303
00:43:23,680 --> 00:43:28,800
The stormy season brings real
danger to the animals of the reef.

304
00:43:35,480 --> 00:43:40,760
Lobsters in the Caribbean
sense a change in the water.

305
00:43:40,920 --> 00:43:45,073
The temperature drops and powerful
swells disturb the sand.

306
00:43:47,440 --> 00:43:49,796
Under the cover of darkness,

307
00:43:49,960 --> 00:43:52,998
they emerge to run before the storm

308
00:43:53,160 --> 00:43:56,756
and risk crossing
the exposed sand flats

309
00:43:56,920 --> 00:43:59,879
to seek shelter in deeper water.

310
00:44:04,800 --> 00:44:07,634
Every year they make this journey.

311
00:44:14,520 --> 00:44:18,833
From all over the reef,
lobsters come to join the march.

312
00:44:19,000 --> 00:44:24,200
They conserve energy by travelling
in one another's slipstream.

313
00:44:31,960 --> 00:44:34,191
And there is the added benefit

314
00:44:34,360 --> 00:44:36,352
of safety in numbers.

315
00:44:49,040 --> 00:44:52,750
By daybreak, they've reached
the edge of the deep reef

316
00:44:52,920 --> 00:44:54,513
and down they go.

317
00:44:54,680 --> 00:44:57,115
For the rest of the stormy season,

318
00:44:57,280 --> 00:45:00,591
they will remain in the shelter
of deep water,

319
00:45:00,760 --> 00:45:02,638
out of harm's way.

320
00:45:19,400 --> 00:45:23,553
Sometimes, during the stormy
season, a hurricane builds.

321
00:45:23,720 --> 00:45:28,636
Then the very structure
of the reef itself is under threat.

322
00:46:55,480 --> 00:47:00,999
An entire reef can be destroyed
by just one big storm.

323
00:47:01,160 --> 00:47:06,155
Hundreds of years of growth
gone in a few hours.

324
00:47:22,640 --> 00:47:27,032
Out in the ocean,
new life continues to develop.

325
00:47:27,200 --> 00:47:32,480
In time, coral larvae will return
to colonise the rubble

326
00:47:32,640 --> 00:47:36,520
and a new reef will grow
on the wasteland.