On the 25th December 1643,
the Captain of a ship, homeward bound from the East Indies,
sighted land just south of Java.
He had found the coral-encrusted tip of an extinct volcano,
which he called Christmas Island.
Had he landed, he would have found no ordinary tropical paradise.
Because this island, now Australian, is ruled by crabs... hundreds of millions of them.
The sea SHORE is the place where you would expect to find crabs.
Crabs are crustaceans, and basically sea creatures,
like this amphibious Grapsus.
But here, crabs are not confined to the coast, for Christmas Island is unique for its LAND crabs.
The Indian Ocean brings year-round warmth and high humidity,
which have fostered a lush covering of forest.
This jungle is ideal for land crustaceans -
like these blue crabs.
It's their hard, mineralised shell that has enabled these creatures to explore life on land.
It's strong enough to support them, and impermeable enough to stop them drying out.
Covered with sensitive bristles, it is jointed like a suit of medieval armour.
The first of five pairs of legs are armed with powerful claws,
and the shell around the body encloses the crab's gill chambers.
They have to be kept fully charged with water.
Although the claws are packed with muscle,
they're still capable of being used with all the delicacy of forceps.
Crabs even have built-in cutlery to manipulate food into the mouth.
The armour makes crabs look fierce.
But their eyes betray the fact that inside the armour are creatures of some sensitivity.
Out of water they might find it hard to keep their eyes clean, for they have no eyelids like us.
Instead they have special brushes.
Blue land crabs need water to breathe, and so can only live near pools and streams.
Other kinds are not so restricted.
These are red crabs.
There are 130 million of them, and they dominate the forest.
When it's dry, they take refuge in moist burrows, from which they emerge to feed.
Touching and tasting with the legs, the red crabs are the gardeners of the forest.
It's they who decide which seeds survive, and which don't.
They also chew the leaf litter.
Elsewhere in the world, fallen vegetation is broken down by a range of organisms.
But here it is chiefly crabs.
Their droppings are fertiliser, returning nutrients to the plants,
and, as they burrow, they till the soil.
With over 4,000 crabs to the acre,
the forest floor is kept clear of vegetation, leaving little for others...
..except these Christmas Island thrushes.
Even forest crabs enjoy a drink.
But they do so less often than the blue crabs.
Their gill chambers are like lungs,
which need to be kept moist, but do not have to be constantly recharged with water.
Red crabs have a simpler way of keeping their vision clear.
The built-in eye cups are filled, drop by drop, to make effective eye washes.
16 kinds of land crabs live on Christmas Island,
including this large hermit.
It collects a shell from the shore, then lumbers inland with it as portable protection.
But, in the forest, are the biggest of all land crabs.
These mighty robber crabs have prodigious strength.
They've got claws like wire cutters,
and they make light work of tearing this frigate bird to pieces.
The robber crab's ancestors were marine hermits.
Today they no longer pick up discarded mollusc shells,
but have themselves become massively armoured,
and are the most terrestrial of all crabs.
Guided by an astonishing sense of smell,
they scuttle long distances to scavenge fruit and carcases.
Robbers are the hyenas of the crab world.
THUNDER
The November monsoons bring torrential rain to Christmas Island,
and this triggers one of Nature's most spectacular events.
All over the island, red crabs are on the move.
They appear from everywhere, and they seem to know where they're going.
They pour out from the forest,
channelled by gullies and other natural pathways.
They're heading for the nearest shore... to spawn.
It's an exhausting business, especially if there's little to drink,
So the legions march, mostly while it's cool, in the early morning and evening.
For them, the journey is essential.
Unable to raise their young on land, they MUST trek several kilometres each year to their ancestral home...
..the sea.
The journey is a perilous one,
for the exodus from the forest along the traditional routes brings them into an alien world.
TRAIN HOOTS
Phosphate used to be mined on the island,
and a million crabs are crushed by the rail and road traffic each year.
The carnage accounts for only one per cent,
and the dead are quickly scavenged.
Occasionally the crabs hit back, as their powerful claws puncture the tyres that kill them.
By early December, the residents of Christmas Island wake up to find crabs on their doorsteps.
To some it may seem like a nightmare, but most people here carry on regardless,
and work round the crustaceans which you can't even eat.
The migrating creatures turn up everywhere,
and they certainly have no regard for fair play.
This golf course has special rules
for when crabs get in on the game!
The large males are first to reach their destination.
After two weeks, the travel-weary crabs quench their thirst,
and allow the waves to cleanse their bodies of dirt and grime.
But, like human bathers, these air-breathing land crabs must be cautious when taking a dip.
They're hopeless swimmers and easily drown.
Perhaps the taste of the salt and the feel of the waves
reassures them that they have, at last, reached the right place to spawn.
Like people, they can suffer from heat stroke, which CAN be fatal.
The full force of the tropical sun sends them scuttling back into the shade.
Once they've freshened up, the males retire to the terraces above the shore
to prepare for the arrival of the females.
This is where the business of breeding begins in earnest.
To start with, the males excavate burrows to accommodate their mates.
Now, these otherwise peaceful crabs
become fiercely territorial - especially in defence of burrows.
Most arguments are mainly bravado and bluff.
Certainly the full force of the claws is rarely brought to bear on an opponent.
The female often takes the initiative by stepping into the male's burrow.
The mating then proceeds amidst a confusing tangle of legs.
The male then takes over, manoeuvring his mate into position.
He soothes her by tapping a gentle tattoo on her back.
If she's in the mood to mate, she'll go limp.
If she continues to struggle, the male will reject her.
This is the prelude to proper coupling.
The males have specially modified legs, with which the sperm is introduced into the female.
After the sexual embrace, the male abandons her to team up with others returning to the forest.
His mate has the burrow to herself.
Soon she lays 100,000 eggs into a pouch formed by her lowered abdomen and tiny, special limbs.
Here she'll wait for about 12 days, while the embryos develop,
before making HER rendezvous with the sea.
MATING CALL
Meanwhile, the birds start to breed.
Christmas Island is renowned for its seabirds.
Several, like this frigate, are unique to the island.
As Christmas Day approaches,
the calls of our egg-laden female signal that she's ready to spawn.
CLICKING CALL
She's joined by millions of others, each bearing bunches of ripe eggs.
Beaches are few and far between,
so these crabs have to use a treacherous stretch of coast,
the remnants of an ancient coral reef.
Here, the corrosive effects of rain and sea have created these limestone pinnacles,
which are razor-sharp and deeply honeycombed.
Once the crabs have negotiated the labyrinth of rock,
they find themselves in a magical setting for the spawning.
After days underground, the females must surely relish the cool salt spray.
The spawning crabs are governed by the lunar cycle.
Their arrival MUST coincide with the last quarter of the waning moon, at high tide just before dawn.
Nightfall - and the females await the very peak of the tide,
into which they will release their eggs.
Spawning is not without its dangers, for those that lose their grip will either drown,
or be dashed on the rocks below.
Meanwhile, in the relative safety of a nearby beach,
other crabs are congregating, each bearing 100,000 eggs,
and all waiting for the tide to reach its highest point.
This is the climax of the migration.
Suddenly, as if by some command,
they throw caution to the wind and scramble for the surf.
This is where they'll consign their eggs to the sea.
The scene is not without poignancy.
Although the ocean will give life to the mother-crab's offspring, SHE may be drowned.
Claws held high to shake free her eggs, it's a dangerous moment,
as she's likely to be knocked over and swept away.
As the females spawn, the water runs brown with eggs.
By dawn most females are heading home. Only a few remain.
Bundles of eggs which fell onto the sand provide easy pickings for loitering crabs.
Some arrived too late to spawn, and must wait for the next night.
The odd one decides to go ahead anyway.
Some females may have taken part in this spectacular spawning for as many as 10 consecutive years.
For others, it's their last.
But the sea holds the promise of new life.
These tiny planktonic crab larvae are called zoea.
The odds are greatly stacked against their survival.
As the tide sweeps them out, they run the gauntlet of many voracious fish.
Those that manage to cross the fringing reef are then at the mercy of the ocean currents.
Where they go, is a mystery.
Most years they completely vanish, presumably swept far from their island home.
But in about one year in six, the ocean currents bring back a crop of young crabs.
After a month at sea, the larvae are still swimming, using their tiny tails like shrimps.
But now they are creatures of TWO worlds, anxious to clamber ashore.
In a January dawn,
the water's edge teems with millions of the little creatures.
Shortly, they'll moult, casting off for ever their ability to live in the sea,
and turn into miniature crabs.
In exceptional years, a seething carpet of crablets overwhelms the coastal terraces
and the visiting beachcombers.
For local children, the sudden influx of young crabs provides them with new playthings.
But the fresh arrivals can expect no quarter from hungry adult crabs.
Their parental concern stopped when their eggs were consigned to the sea.
In a few days, the moment to moult arrives.
Each becomes transformed into a miniature land crab - the size of an ant.
With the energy of new life, they scamper into the forest.
The tiny crabs retrace the journey made by their parents.
Then they mysteriously disappear for 12 months or so.
In three or four years' time, the survivors will emerge from the forest
to join the great spawning migration of the crabs of Christmas Island.
Subtitles by Janet Waters. 1988