There is a force sufficiently powerful to move the oceans
of this world.
It is a force not of this Earth.
The Moon is large enough to generate gravity
and with sufficient force to pull on the Earth 230,000 miles away.
As the Moon orbits the Earth,
its gravity sweeps across the face of our planet.
Its power drags a great bulge of oceanic water in its wake...
..the rising tide.
The River Amazon in Brazil.
On some special days, the gravitational forces
of the Moon and the sun combine, to extraordinary effect.
A growing tidal wave from the ocean is being forced 200 miles inland.
This is a tidal bore.
Fortunately, tidal bores are rare,
but the Moon does create strong tides out in the world's oceans
on every day of the year.
The Bay of Fundy in Nova Scotia.
The tides here are the largest in the world,
and have a profound effect on marine life,
creating a rich feeding ground.
A feast that attracts some of the largest diners on the planet...
..humpback whales.
But they are not the biggest threat to the herring.
These are finback whales.
At 70 tonnes, they are the second largest animal on Earth,
but so streamlined they are the fastest of the great whales.
This combination of speed and immense size
makes the finback a voracious hunter of schooling fish.
The Bay of Fundy can attract so many fish
that, during the summer,
as many as 500 of these magnificent whales hunt here every day.
The feeding is best where the tides run strongly.
So the whales move further into the bay,
following tidal rips and searching for fish.
Their movements are closely watched by flocks of Cory shearwaters.
As the whales dive down towards the fish, more and more birds gather,
anxious to pick up scraps.
The flowing tide
may provide a feast, but, before long, it will turn.
In just six hours, 100 billion tonnes of water
will flow out of the bay,
the sea level falling by as much as 15 metres
and exposing vast tracts of mud and sand.
At first sight, a barren place, entirely devoid of life.
In fact, the damp sand is packed with microscopic life,
the meiofauna, feeding in a sandy underworld,
quite unaffected by the departure of the sea.
But life is not all roses in this miniature world.
A sand bubbler crab in Northern Australia.
It hunts meiofauna.
Just a centimetre across,
the sand bubbler works at breakneck speed,
filtering out the meiofauna and kicking aside the waste.
The crab will clean
every grain of sand within a metre of its burrow.
Endless practice for the best backheel in the natural world.
The crabs work fast
because they can only sieve when the sand is damp.
Remarkably, they work the entire surface of the beach
within a couple of hours of the tide retreating.
Then they simply return to their burrows and await the next tide.
Underwater, the falling tide is the cue
for some bizarre activity.
These slow—moving clams use their muscular feet
to bury themselves under the sand.
If they fail to get under cover,
the tide will leave them exposed to the air and they will perish.
But once underground, they can wait,
safe beneath the beach,
And not a moment too soon.
June in south east Alaska.
In just four hours, a vast beach is exposed by the falling tide.
The bears are hungry.
At this time of year, the pickings on land are few and far between.
But any food here has long since buried itself deep under the sand.
To a hungry adult bear, that is no barrier.
They smell the clams through the sand and simply dig them out.
For such large animals,
they show quite extraordinary dexterity
at opening the unfortunate shellfish.
Cubs try their luck, too, none too successfully.
But for the adults, the shellfish feast
lasts as long as the tide remains out.
Table Mountain in South Africa.
Every day, the retreating waves leave flotsam on the beach.
And this creature is scenting the currents
for the odour of rotting fish.
The tide carries the scent far into the surf zone.
Responding to the smell, snails emerge from the sand.
This is a race against the tide.
The snails need to find their meal
before the tide leaves it beyond reach.
But snails are slow and the tides fall rapidly.
These, however, are no ordinary snails.
They can surf!
They ride the waves up the beach
but, all too soon, the tide leaves the fish beyond the surf zone.
Without the sea, there's a danger the snails will lose the scent,
but as long as the sand remains damp,
they can still follow a faint trail to the food.
Once there,
they tuck in with macabre relish.
Good things come to those who wait.
Soon, the heat of the sun forces them to retreat into the sand
to await the return of the next tide.
February in Britain.
The falling tide is eagerly awaited by these knot.
As the water retreats, countless small invertebrates
seek shelter under the mud.
And with good reason.
Waders are specialists at probing in the mud,
their variety of beak shapes
designed for reaching different invertebrates —
keys that unlock the safety of the tidal flats.
But within a few hours, the tide will turn again.
Soon, the waders are out of their depth.
The creatures of the mud are safe once more.
Underwater, incoming tides can create a strong current,
and flounder are experts at hitching a tidal lift.
They are shaped rather like a kite,
a perfect design for gliding on the tide.
In Newfoundland on the east coast of Canada,
large numbers of flounder ride the currents up into the shallows.
They've come to hunt invertebrates
that will emerge now that the water is back.
The pickings in the shallows can be very good.
The activity has not gone unnoticed.
But ospreys can't dive deeply.
As long as the water is more than a metre deep, the flounder is safe.
Going too far inshore
can be a risky business.
This fish buries itself completely in the sand
at any sign of danger.
But when the tide floods in again, as long as the coast is clear,
these sand lancet will re—emerge.
After a wait of six hours under the sand,
they are desperate for food.
Unlike flounder, they head out to sea,
looking for shallow, open water
where the tidal currents will concentrate their food — plankton.
In untold thousands, they stream towards the best feeding grounds,
where they simply pick up
tiny planktonic creatures from the water.
But, if they swim too far off—shore in search of food,
they risk meeting large predators that live in deeper water.
Dogfish — small sharks.
The sand lancet have strayed out of their safe depth.
The effect of the turning tide
can be totally different on a rocky shore.
Here, on the coast of Vancouver Island in Canada,
the sun bakes the exposed rock.
It's almost impossible to dig underground when the sea retreats,
so these mussels and barnacles are fully exposed to the sun's heat,
literally cooking in their own shells.
And the seaweed simply dry to a crisp.
It can be a wait of many hours before the water returns.
Throughout each month, the size and strength of the tide changes.
The biggest tides of all happen
when the gravities of the sun and Moon pull in unison.
That happens immediately after the new moon...
..and again after the full moon.
These are called the spring tides.
They revealvast tracts of seabed that would normally be covered.
For these racoons, it's a chance to look for a seafood feast.
A mother ventures forth with her kits.
With the spring tide, they've come further down
than smaller tides would normally allow.
Searching with their sensitive paws,
they look for suitable prey.
With the extreme low tide, they could find something special.
And what could be better than a red rock crab?
That is, if it weren't for the risk of a painful pinch.
With crabs, there's no substitute for experience.
The mother makes an expert's catch.
But the kits learn fast.
And for those that don't, begging is always worth a try.
All too soon, the returning tide will cover the racoon's table.
For the invertebrates, it's a welcome relief,
but in rough weather, they are exposed
to the worst of the waves.
Even when there are no waves,
the incoming tide can create considerable forces underwater.
The gaps between these islands on the east coast of Vancouver Island
channel the tidal flow.
As the tide keeps rising, gradually the water flows faster,
and soon these 30—metre—long bull kelp plants
bend to the current.
They are sufficiently flexible to cope without too much damage,
but there are some spots where the currents are especially powerful.
This is the Nakwakto Rapids.
At the turn of the tide, water from almost 700 miles of coastal fjords
will have to empty through a gap half a mile wide.
Within a few minutes, the current is already picking up speed,
until water roars by at over 17 miles an hour.
Tidal currents are not always damaging.
Here, in the Poor Knight Islands of New Zealand,
weak tides run through rock arches, making it an ideal resting place
for stingrays.
These rays congregate here in huge numbers every March.
They've come to breed.
The arches funnel the current, which the rays can ride
with the minimum of effort, so saving energy.
Nearby, out in open water,
a school of two spotted demoiselle fish are feeding on plankton,
and the current is perfect for sweeping their food past them.
Once the current starts to weaken,
there is insufficient food
to warrant the risks of swimming out in the open.
So, the demoiselles head off to find shelter en masse.
Safety is in numbers.
More and more demoiselles pour towards the cave entrances
that riddle the Poor Knight Islands.
Safe at last. Inside the cave, they are less exposed
to attack from predators.
Thousands of demoiselles and blue mao mao
wait for the return of the current, when, once again,
they will head out into the open to feed.
The Moon's gravitational pull is weaker
nearer to the Earth's equator.
So, the more equatorial the location, the smaller the tides.
Out in the Caribbean Sea, the tidal movements are slight.
Even so, they are sufficient
to push free swimming plankton in their path.
These are thimble jellyfish.
They swim towards sunlight,
and invisible boundaries formed by the tidal motion
help to herd them together, until they gather in immense swarms.
They put the tropical sunshine to good use.
Their brown colour comes from algae
that live inside the jellyfish's body
and photosynthesise energy from the sun.
In open water they are fairly safe,
but the tide is sweeping the whole swarm gently towards the Bahamas,
where hungry mouths are waiting.
Although the sea level doesn't change much,
the tides are still pushing an enormous volume of ocean water
through the small gaps between the island cays.
Underwater, the tidal currents race past soft corals...
..and on over the sandy banks themselves.
It is an immense area of coral sand
that is only just submerged at high tide.
This incoming tide is bringing in a fresh supply of oceanic plankton.
Razorfish gather at the best spots
to catch the pick of the microscopic feast.
Although there's plenty here for these small fish to eat,
gathering in one place
makes it easy for their predators to find them.
A nurse shark is little threat.
(CHIRPS AND CLICKS) But this sound...
heralds a quite different danger.
A bottlenose dolphin.
It's using its sonar to locate razorfish beneath the sand.
Once it finds a suitable target, it simply digs out its prey.
The buried fish have no defence against this attack.
They simply have to wait and hope they aren't found out.
This dolphin appears to have a razorfish craving.
Well, she is pregnant.
Success at last.
The incoming tide sweeps on towards America,
flooding across vast flat plains of seagrass.
They are so shallow that, at low tide,
all large fish are forced to retreat into deep water channels,
like these nurse shark...
..and stingrays.
Both predators hunt crustaceans on the seagrass beds,
but until the tide brings enough water,
they will have to wait.
So now, for this tulip snail, it appears safe
to patrol the shallows in search of a meal.
But is it?
This is a rather bigger snail.
At five kilograms in weight, the giant horse conch
has little to fear from any shark, and it has a taste for tulip snail.
Sensing the approaching danger,
the snail flees.
But, in a world of snail paces,
the conch is something of a Ferrari.
It calls for desperate measures.
Exhausted by the effort of its last—ditch attempt,
the tulip snail is slowly gunned down.
The tide has to rise for another hour
before the big predators can feed.
But out on the flats, the scent of dying snail wafts away on the tide.
It's a scent these hermit crabs are particularly partial to.
It's vital that the crabs have the best possible protection
from the heavy teeth of the waiting sharks and rays.
For that, they need the shell with the perfect fit.
Today, there is new real estate on offer —
and competition in this housing market is fierce.
The action becomes even more desperate
when the shell of the devoured snail is ready for release.
This crab simply can't wait any longer.
But it's a decidedly risky acquisition.
The risk paid off handsomely.
The new shell is both lighter and stronger than the old home.
It's not a moment too soon, because the tide is flooding the plains.
At last, the predators are free to start their foraging.
Both stingrays and shark have an electrical sense
which they use to search for buried invertebrates.
They can sense minute movements beneath the sand.
Finding a promising signal, this ray digs out its meal.
An unprotected hermit crab would have no chance.
Within a few hours, the tide ebbs out once more
and all the predators are forced to leave.
They will have to wait until the next high tide
before making another feeding foray.
At certain times of the year, called the equinox,
spring tides are exceptionally large and rise higher than normal.
Predators can reach the shallowest fringes of the seagrass flats.
These two—metre—long tarpon are going further inshore still.
They are heading for the mangroves.
These flooded forests cover huge areas of the coastal shallows.
Extraordinarily, the roots of the mangrove can live in salt water,
and they make a perfect nursery for small fish.
Silversides and snapper find sanctuary in the maze of roots.
Big predators seldom find a way in here.
And now the tide is falling once more.
The water starts losing what little oxygen it contained,
and quickly becomes stagnant.
Most predators have abandoned the mangroves,
but the tarpon are still here, trapped by the falling tide.
Dissolved oxygen is fast running out,
but they have a vital survival technique...
..they can breathe air.
Pumped up with fresh oxygen,
they can easily out—manoeuvre the dozy silversides.
The tide has turned again.
And this is no ordinary tide.
Since it is the equinox, the tide is rising fast,
but now, out to sea,
a hurricane is on its way, forcing the tide yet higher.
The passing storm leaves large areas of the coast flooded.
Low—lying islands, like the Bahamas,
are particularly prone to the storm flooding.
The sun's power here is immense.
As the tide recedes and the remaining flood water evaporates,
a remarkable transformation takes place.
The mud is coated with a magical world of salt.
Any remaining water is extremely salty.
Very few creatures can survive here...
except brine shrimp.
And, on the water's edge —
brine flies.
Both are the favourite food of an extraordinary animal.
The Caribbean flamingo.
Remarkably, they actually seek out such briny places.
They are the best spots for them to find their food.
They also provide the protection
the flamingos need to raise their young.
Nesting sites like this are surrounded by corrosive brine.
It's a formidable barrier
to any predator seeking to dine on flamingo chicks.
The flamingos take the precaution of building raised nests
just in case of further flooding.
Strangely, it's actually the power of the storm tides
that gives the flamingos food and a perfect habitat in which to breed.
The breeding of many animals in the ocean
is closely co—ordinated with the tidal cycles.
The half moon in November. It's the time of small tides.
Christmas Island in the Pacific.
Strange happenings are afoot.
It's one of only a few nights each year
when female Christmas Island crabs
risk heading down towards the sea.
They number in tens of thousands
and all of them are laden with hundreds of eggs.
They have to shed them into the ocean
if the eggs are to develop into baby crabs.
But these are land crabs
and they can neither swim nor breathe underwater.
There's a great risk of drowning.
That is why they pick the smallest tides, to minimise the danger.
The eggs will develop far offshore.
And, in exactly one month's time,
a swarm of baby crabs will return, again choosing the perfect tide.
Whether it's the daily or monthly cycle,
tides are the rhythm of the ocean —
its pulsing clock.
For every tide brings opportunity
to marine life somewhere in the world.
Now, a spring tide is flooding the shallows,
and hunters are on the prowl.
A small group of bottlenose dolphin
are working their way inshore
to start a quite extraordinary hunting campaign.
After one successful pass, the dolphin move off to start again.
One animal peels off from the group and swims in a circle,
stirring up the mud and driving the mullet towards the other dolphins.
It's a remarkable team effort and it's extremely effective.
The dolphin will feed like this
for as long as the tide grants them access to the shallows.
Eventually,
the falling tide will force the dolphin to leave the flats
and the mullet will be safe once more, until the next high tide.
Because, in the ocean, every turn of the tide
spells the difference between life and death somewhere.