The waters that cover most of the planet are in constant movement.
As the moon circles around the spinning earth,
so the pull of its gravity causes the oceans to rise and fall,
and twice every day, the sea surges up and down the coasts of the continents.
In the Bay of Fundy in North America,
the shape of the coast and the slope of the seabed
produces the highest tides of all, rising 50 feet.
Living in this in-between world, which is neither sea nor land,
demands very special talents.
This is a battle ground.
In many places, the sea is forcing the land to retreat, cutting back its cliffs
and leaving islands and towers as markers of the territory that the land has lost
The debris is swept away
and strewn on beaches farther down the coast as sand and gravel.
In some places, the land is advancing.
In the tropics, mangroves are moving out into the sea, gathering mud
and building new territory for land-living creatures.
Even in the mouths of rivers,
where fresh water laden with sediment mingles with the salt water of the sea,
new land is being created of a sort.
I'm in an estuary in the west of England.
You might think that this mud is not the most attractive stuff in which to live.
Certainly, animals that do live in it have to face some severe problems.
Part of their time they're out of water like this,
part of the time they're underwater.
The saltiness of the water, too, varies.
Fresh water comes down from the land, the tides bring in salt water.
And then there's the nature of this extraordinarily sticky mud itself.
It is so glutinous that little oxygen gets into it
but the rewards for enduring these unpromising conditions are high.
Edible particles deposited on the surface of the mud
are cautiously sucked up by the searching siphon of Scrobicularia,
a mollusc whose main body, enclosed in a shell, hides in the mud for safety.
A tiny crustacean, Corophium, half an inch long,
grazes on the bacteria which proliferate in millions,
breaking down rotting organic matter in the mud.
Ragworms live in burrows
and will tackle Corophium, algae, bacteria, almost anything that's around.
The puddles are flecked with floating mucus.
It is produced by spire shells, no bigger than grains of wheat.
The mucus attracts bacteria, and the spire shells eat the lot.
The peacock worm fans out its tentacles from the top of its tube
to gather food particles before they settle.
Beating threads on each filament of the fan
transport the catch down to the mouth at the centre.
While it feeds, it also disgorges a cement of mud and mucus
and builds up the margin of its tube.
The cockle lies with its shell agape,
filtering the water by sucking it in through one siphon...
...and blowing it out through another.
Mussels use the same technique, collecting within their shells
substantial quantities of the abundant and nutritious drifting particles.
When the tide goes out, they clamp their shells tightly together
to keep in their moisture and to keep out attackers,
but some creatures know how to deal with that.
Each oyster-catcher has its favourite technique for dealing with mussels.
It is usually the same as that used by its parents
though a bird needs years of practice before it becomes really expert.
Some hunt in the shallow waters for mussels that have not yet shut their shells.
Others carry unattached shells away from the main flock
so they've got a little privacy.
They skilfully place the mussel in such a position that they can cut it open along its hinge.
Other individual birds resort to brute force.
They hammer their way in through the shell itself.
As the tide retreats still further, spire shells are exposed,
as many as 35,000 buried within a single square yard.
All these mud feeders together constitute a rich prize,
and there are abundant claimants.
Sandpipers, on migration, depend on them,
but at all times of the year, wading birds come to the estuaries to feed.
The godwit, equipped with long legs and a long bill,
can wade in water several inches deep
and collect food before it can be reached by other birds.
The curlew prefers to work out of water.
Its long bill enables it to probe deep into the mud for a worm,
and serves equally well as a pair of forceps.
The dunlin is a smaller bird and goes for smaller prey:
Ragworms and insect larvae.
It feels for its food with its short bill.
The ringed plover, with a very short bill,
can only collect food from the surface and locates it by sight.
It works alone so that its prey won't be disturbed by pattering feet
and withdraw before being spotted.
The scything action of the avocet
collects creatures that live in the liquid mud.
Their bills are very sensitive. As soon as they close on something edible,
the bird can juggle it up into its mouth.
The quantities of food taken by wading birds from estuaries is enormous.
Some species consume every day about a third of their own weight in food.
In a year, a single oyster-catcher
can consume the flesh over half a ton of cockles,
and many an estuary supports tens of thousands of wading birds,
so these places are rich indeed.
As the river brings down more and more particles of mud,
so the flats grow bigger and higher,
and on their surface they develop a slimy skin,
and that's formed by microscopic plants, algae.
They start the process of consolidation.
But soon, bigger plants get root, like this glasswort,
and now the process really speeds up.
As the high tide brings in more mud particles, they clog around the stems of the glasswort
and don't swill back to the sea when the tide fall
So with each new tide, the flats grow higher and higher.
Glasswort is a plant of the cold estuaries of Europe.
In the tropics, the colonisers of mud are not small plants but trees:
Mangroves.
This mud is the pulverised remains of rocks eroded from the Himalayas
that has been carried down by the Ganges for 1,000 miles
and dumped on the edge of the Bay of Bengal.
This is the biggest intertidal forest of all, the Sunderbans, 4,000 square miles of it,
and here roam many animals that usually live in dry-land forests.
Axis deer.
Woodpeckers: The Indian golden-banded.
And wild boar.
But mangrove forests also harbour creatures that live nowhere else at all.
The proboscis monkey eats almost nothing but mangrove leaves.
It developed that specialism on the island of Borneo,
and has never spread overseas, trapped by its own specialised requirements.
Mangroves themselves are distributed widely through the tropics,
for they have evolved from many different plant families
and today there are some 40 different species of them.
The flowers of this pioneering mangrove are pollinated by the wind.
The seed doesn't immediately leave the parent tree.
It starts to grow while it is still attached,
producing a green shoot a foot long with a sharp end to it.
If it falls when the tide is in,
it floats horizontally in the buoyant salt water
and may be carried for miles before being stranded.
If the tide is out, it stabs the mud and stays in that position when the tide returns.
It puts out rootlets from the bottom and leaves from the top,
and within a few days, it's firmly established.
Just as in cold-water estuaries,
there's a lot of organic matter in this mud.
Because it's so sticky, it isn't stirred up, so there's little oxygen in it,
and the process of rotting produces within the mud itself
an acid, smelly, poisonous chemical: Hydrogen sulphide.
So these roots don't go down far into the mud.
Instead, they support the trees by their sheer number.
But what about the other things that normal roots do for normal trees,
like gathering nutrients and water and oxygen?
Well, these roots deal with the nutrient problem like this.
It has this cluster of very fine roots
which don't go more than an inch or so below the surface of the mud,
but it is on the surface of the mud that the bulk of the nutrients are found.
As for water, there's plenty of it here, but it's salty.
Some mangroves have a special membrane around the cells in the root hairs
which filters off the salt.
Others absorb the salt but then excrete it from the leaves,
or concentrate it in the leaf and then the leaves are shed.
And oxygen, well, there are several different solutions to that problem.
This mangrove has pores actually in these prop roots
which absorb the oxygen directly.
This one has roots which actually grow upwards,
so keeping pace with the rising surface of the accumulating mud.
It's not only plants in the mangrove swamps that have difficulty in getting oxygen.
So do animals, and this time, low tide, is a period of particular difficulty.
Many molluscs, like cockles and mussels elsewhere,
shut their shells to keep what moisture they have
and wait for the food-and-oxygen-bearing water to return.
For them, it's a period of inactivity, but for other creatures, it's just the opposite.
The mudskipper, of course, is a fish.
There are several different kinds.
This one lives near high-water mark,
and is the sort that spends most time out of water.
It has to keep its skin moist for it absorbs oxygen through it.
It also keeps its mouth full of water swilling over its gills.
It feeds on the little crabs that graze on the mud
And having got one, it needs another mouthful of water.
A second kind lives close to low-water mark,
so it is only out of water for an hour or so each day.
It sifts the liquid mud for small crustaceans and worms.
In between these two kinds lives the largest of the three.
It is a vegetarian, collecting algae and other microscopic plants from the mud.
And it, too, nips back every now and then for a wet.
It guards its grazing rights with vigour,
building walls around its territory.
And when neighbours meet, there's trouble.
On clear mud, their territories form a patchwork of walled ponds.
These flats are very flat, so when a male starts to advertise for a mate,
he has to be a bit of a gymnast.
When a female is enticed into his private pond,
he can continue his courtship at close quarters
in a more conventionally fish fashion,
with flexed fins, waggling tail and enormous excitement.
They'll spawn in a burrow at the bottom of the pond.
This crab is too big to be intimidated by mudskippers,
even when it does wander through their territories.
Its scissoring mouthparts not only sort out its food but help it to breathe.
On top of its shell, there is a puddle of water,
and as its mouthparts move,
they circulate this into a gill chamber within the shell,
out again and up to the reservoir on the top.
Eventually, the oxygen in the water is exhausted
and the crab has to return to the sea, tip it off and get a fresh supply.
Close by the edge of the sea, the tiny soldier crabs feed with frantic haste.
No one else will steal their mud, but they have to eat an enormous quantity
to extract the few particles necessary to keep alive.
They have to work at it pretty well non-stop and have no time to waste.
High up, beyond the reach of all but the highest tides,
lives the large mangrove crab.
It keeps moist by boring its hole as much as six feet deep to reach water.
The lure that tempts it out is a newly fallen mangrove leaf.
And quickly back to safety.
Among the air-absorbing roots of the mangroves, fiddler crabs are busy.
The females collect mud with both pincers,
working with the same frantic speed as the soldier crabs.
The males need to munch just as much mud as the females,
but work with one hand only,
for one of their claws is so big that it is useless for feeding.
They use it instead to wave at passing females.
But it is also a weapon to brandish at rivals.
A less well-equipped male
gets a nasty hammering even before he can get out of his hole.
The claw is long enough to reach down into the burrow
to give his opponent a tweak where he's least expecting it.
The purpose of the wave is to encourage a female to follow a male into his burrow.
Is it possible perhaps just to take a moment or so off from munching mud?
At low tide, there's lots for birds to eat on the mangrove mud,
just as there is on estuaries elsewhere.
Terns hawk for fish that are easier to catch now in the shallowing waters.
Kingfishers pounce on the fiddler crabs.
Great white heron stalk and stab.
The returning tide signals "all change" for everyone.
This African mangrove snail crops the algae growing on the mud,
but it mustn't stay there when the tide comes in, for it would be attacked by fish.
It takes refuge up in the trees.
Its speediest climb is barely faster than the rise of the tide,
so it has to set off in good time.
Its internal alarm clock tells it when it should do so.
The soldier crabs are so well adapted to their life scavenging on the exposed mud
that they have become breathers of air, and without it they will drown.
As the tide advances, each constructs a little igloo
which traps a bubble of air with which the crab can breathe while the tide is in.
The mudskippers' territorial walls built with such labour are breached by the incoming wavelets.
Higher up, the mudskippers shelter in burrows.
The incoming tide brings new creatures into the swamps.
Shoals of fish arrive, searching for morsels deposited by the river while the tide was out.
In the swamps of South-East Asia, archer fish feed on insects that have fallen on the surface.
Uniquely, they also have a way of collecting insects from above the water.
There is a groove in the roof of their mouth,
so that a sudden thrust of the tongue produces a spurt of droplets like a water pistol.
When there is a crowd, a marksman can't be sure of getting his prize.
So in company, it may be better to try a direct assault.
The larger fish are themselves food for otters,
but these hunters have broad appetites
and will enthusiastically tackle snails, crabs and even mussels.
They are great travellers, swimming for many miles up into fresh water or down into the sea
and even out to offshore islands,
and they have an enormous appetite for play.
The largest of all living reptiles is found among mangroves:
The estuarine crocodile, a monster that grows to 23 feet long.
Like its ancestors that lived when dinosaurs dominated the earth,
it's an ocean-going creature,
and, as a consequence, it's the most widely distributed of all crocodiles
living from the Bay of Bengal through northern Australia to the Pacific,
even reaching isolated mangrove swamps on the islands of Fiji.
As the mangroves establish themselves farther out into the sea,
the mudflats they've built grow higher and higher.
Rainwater washes them clean of salt,
and eventually they become dry fertile forest, beyond the reach of the sea.
The banks of mud and sand that the rivers lay down around their mouths,
even when they are not big enough to rise above water,
protect the land against the attacks of the sea, for tall waves can't travel across shallow water.
But if a current sweeping down the coast carries away the sediment
and scours the sea floor clean,
then waves arrive at the coast full of power.
Where the land dips steeply into the sea,
the territory between the tides is not miles across but condensed into a narrow band.
The creatures that live here, like all intertidal creatures, are threatened by two dangers.
At the high-water mark, there are physical problems of being dried out,
and at the low-water mark, there are biological problems
of animals that creep up from the sea to prey upon the intertidal creatures.
The interplay of those two sets of problems
produces a series of horizontal bands along the coast,
each dominated by the particular species
which best deals with the problems at that particular level.
Such bands can be seen on coasts all over the world,
but here on the north-west coast of America, they are strikingly clear.
The bottom band of all is only fully exposed
when the moon and the sun are in such an alignment that they pull together
and the tide withdraws a long way from the edge of the dry land.
Organisms here only tolerate a brief exposure to the air
and are unable to prevent themselves from being dried out.
The sea urchin, in water, gnaws away at encrusting algae.
But out of water, it can do nothing but simply hang on to the rocks.
Nearby, giant sea anemones droop their tentacles,
and many withdraw them, for in air there is nothing to feed on.
Sea squirts can only filter for their food spasmodically.
Starfish are meat-eaters, and this species feeds on mussels.
It envelops them with its adhesive arms, wrenches apart their shells,
and feeds on the flesh within.
Below low-water mark, they kill any mussel that tries to establish itsel
But like many of these low-level creatures, they can't feed out of water.
So higher up, where the rocks are exposed to air for longer, conditions favour the mussels,
and they form a dense band,
cropped at the lower edge by starfish, but beyond their reach higher up.
The massed mussels provide shelter for lots of other creatures:
Small starfish, too small to tackle a mussel,
worms and crustaceans, winkles and other molluscs.
The mussels hold on to the rocks with bundles of threads,
but can't withstand the pull of the roughest waves
and in winter storms, sheets of them may be ripped away.
In more exposed places where the waves beat with a particular ferocity,
mussels give way to goose-necked barnacles which clasp the rock with a long fleshy foot.
They feed by holding out stiff, fan-like arms which catch particles from the waves,
not when they crash in, but as their waters flow gently back.
On the most exposed promontories, the mussels are ousted by a plant:
An odd-looking alga known as a sea palm
which lives only on these north-western coasts of North America.
The crown of leaves at the top of its rubbery stem
enables the sea palm to harness the power of the waves and use it to attack the mussels.
The plants, perhaps surprisingly, are annual.
In the spring, an individual plant may achieve the difficult feat
of getting hold of an individual mussel in the mussel bed, as this one has done.
When it's mature, it will produce spores,
but only when it's out of water as it is now.
So instead of the spores being distributed widely as those of other plants are...
...the spores of the sea palm trickle down the grooves in these leaves
and into the mussel bed here.
When the first storms of the autumn come,
they may catch underneath the fronds of this plant and rip it up.
But the holdfast grips the mussels so firmly that the mussels come away with it,
revealing the bare rock,
and that means that the offspring of other nearby plants
can get a hold on the bare rock.
So by the sacrifice of one palm growing on a mussel one year,
next year there will be a whole grove of palms growing firmly on the bedrock.
But mussels do require a certain amount of immersion every day
if they are not to dry out and die,
and this line marks exactly that.
Above it, no mussel can live.
The creatures that can are these: Barnacles.
Clamped tightly to the rocks, they conserve very effectively the moisture within their shells.
They collect the minute quantities of food they require to grow and reproduce
from the relatively infrequent submersions at high tide,
which in some cases may only occur for an hour once a month.
So each level on a rocky shore is dominated by the organisms
that best deal with the precise combination of pounding by the waves,
exposure to the air, and attack by deep-water predators.
None, in the long run, can claim permanent occupation,
for the attacks of the waves are unceasing.
With unfailing accuracy, the sea picks out the softer parts of the rocks
and cuts its way into them.
Water at great pressure is driven into joints and cracks
until it penetrates a cliff and forms a blowhole.
On the southernmost tip of Australia, storms of great ferocity sweeping up from the south,
with the full force of the Antarctic gales behind them,
beat away at sandstone cliffs which have lines of weakness that run horizontally and vertically,
so the rock is cut away in huge blocks.
The sea, having demolished the cliffs, then works on the debris.
During storms, it picks up the boulders and hurls them at the cliff face.
At calmer times, it rolls the rocks over the seabe and casts them up on shingle banks.
Every movement chips and grinds the fragments until they are reduced to sand grains,
and now even a gentle current can pick them up and carry them for miles down the coast,
eventually to abandon them in banks and strands
in the lee of islands or in sheltered bays.
Every wave of every tide stirs up the surface of the sand,
so plants find it impossible to get any grip on it as they can on rocky shores or mudflats.
So a beach like this looks as lifeless as any part of the margins of the land.
But if the sand grains are not too small and compacted,
then each will retain around it a thin film of moisture even when the tide is out,
and in that microscopic space, animals can live.
These translucent boulders are, in fact, sand grains,
and the tiny snake-like animal a worm that could sit on a pinhead.
All these inhabitants of the sand
are, necessarily, adept at writhing, gliding and crawling
as they search for the few edible fragments trapped between grains, or pursue one another.
This one is only a temporary lodger in the sand. It is the larva of a mollusc.
A hydra lives here. It's like the one that's common in freshwater ponds,
but it has one elongated tentacle with which it anchors itself.
A nematode worm produces glue from a gland on its tail
which helps it to maintain its position.
This is another larva that at the beginning of its life floats in the se
but settles down into the sand to continue its development.
It builds a tiny tube of mucus which it carries about with it
and clings to with bristles on its flanks.
When it grows up, it does the same thing on a larger scale, above the sand.
It's a worm called the sand mason.
Now it not only builds a tube, but it adds long tassels to the top.
These slow down the water so that suspended food particles fall
and can be gathered by the waving tentacles.
The tubes need constant renewal,
and this is how the sand mason does it, speeded up 125 times.
Although plants can't grow on these perpetually moving sands,
those dislodged from the rocky parts of the coast by waves are washed up here,
and there are plenty of creatures on the beach waiting for them.
These are sand-hoppers.
They hide below the surface to avoid being baked and dried out by the sun,
but now there is food to be had.
On many beaches, their numbers are astronomic.
There can be as many as 25,000 of them in one square yard of beach sand.
The sand-hoppers favour rotting vegetation.
Rotting flesh attracts crabs.
The remains of a squid is a banquet for ghost crabs.
Occasionally, when there is a chance, it may be better to cut off a length
and haul it away to consume it in the privacy of a burrow.
The crabs and the shrimps live close to the high-tide mark.
The incoming waters bring with them another team of scavengers.
This periscope on a South African beach belongs to a mollusc: A plough snail.
It inflates its plough-like foot by pumping in water,
and it uses it not so much as a ploughshare as a surfboard.
The waters pick it up and wash it swiftly inshore, together with its potential food...
...a stranded jellyfish.
The plough snails detect its presence from the taste of decay in the surrounding water
and advance on it with great speed.
To avoid being swept up the beach and being stranded, they eat fast,
and then, while there is some food left, they burrow into the sand.
There they wait for the tide to turn
so that they can ride back on their surfboards to deeper water and safety.
Very few sea creatures venture above the limit of the highest tide and survive.
One group of animals is compelled to do so by the nature of their ancestry,
and on this one beach in Costa Rica, they stage an astonishing invasion.
Turtles.
They are Ridleys, the smallest of the sea-going turtles, only a couple of feet long.
Turtles are descended from land-living reptiles,
and, like all reptiles, they lay eggs that only develop and hatch in air.
Every year, adult females, having mated at sea, must move onto dry land.
They arrive at a rate of up to 5,000 an hour.
They use only one or two of the thousands of beaches that seem to be suitable.
What is more, they only choose to do so on just a few nights in the year
between August and November.
Efficient though their flippers are in water,
they are barely strong enough to lift the turtle clear of the sand.
It has to drag itself up the beach.
This mass breeding may be an advantage to the turtle.
Since it only occurs on a few nights a year,
their eggs can't support a large permanent population of predators,
as they might if the turtles were to lay over several months.
Yet, even so, for reasons that we still don't understand,
less than one in a hundred of the eggs produces a hatchling which reaches the sea.
Each female lays a hundred or so.
That done, she carefully fills in the hole.
A few coatimundi and vultures come down from the forest to plunder,
but they make little impact on the millions of eggs that are laid.
Next night, many thousands more Ridleys arrive.
On other beaches, more secretly, other very different turtles are laying.
This is the largest of all the marine turtles.
This magnificent creature is the giant leatherback turtle.
And it's a most mysterious animal.
It's a solitary wanderer of the oceans.
Individuals turn up almost anywhere in the tropics but they go much farther than that.
They've been recorded as far south as Argentina,
and as far north as the British Isles and North America.
It's a creature of mystery, because although we know what it feeds on,
which is sea urchins and fish and, oddly enough, jellyfish, we know little else about it.
We don't know how long they live. We don't know how the male finds females.
We don't know how females navigate to find nesting sites like this one.
Indeed we didn't know where the main nesting sites were until 25 years ago.
Then it was discovered that some nested on the Suriname coast of South America
and some nested here, on the east coast of Malaysia.
Of course, the people here have always known about the turtles
and have always plundered those eggs.
Today, however, there are more people than ever here,
and the eggs are plundered more seriously,
so undoubtedly, this huge and extraordinary creature is in danger.
But maybe the leatherback turtle has other breeding grounds that we don't know about.
Maybe it goes to small, tiny coral islands in the emptiness of the ocean
to find beaches far away from man.
That, indeed, is where we ourselves will be going in the next programme.