Original subtitles

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 pull together 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 tons, the second-largest animal on Earth,

but so beautifully streamlined

that it is 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's 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 tons of water will flow out of the bay,

the sea level falling by as much as 15 meters

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 centimeter across,

the sand bubbler works at breakneck speed,

passing sand grains into its mouth,

filtering out all the meiofauna,

and kicking aside the waste.

The crab will clean every grain of sand

within a meter of its burrow, endless practice for

the best back heel 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 just a couple of hours of the tide retreating.

Then they simply return to their burrows

and await the next tide.

Let's begin.

My dog is one of us.

You and me, girl.

Home protection? Hunting.

Out here, most folks let things go.

I guess I'm not most folks.

I want to live.

Are you prepared to kill?

Yes.

One of us is not leaving this island.

You ready?

Underwater,

a 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 deep down in the sand,

safe beneath the beach.

And not a moment too soon.

June in Southeast Alaska, and in just four hours,

a vast, sandy 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 somewhere on the beach,

and this creature is scenting the currents

for the odor 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 their 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 that 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.

Before long, 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

are seeking shelter under the mud.

And with good reason.

Waders are specialists at probing in the mud,

their wide variety of beak shapes

designed for reaching different invertebrate dinners,

keys that unlock the safety of the tidal flats.

But within a few hours, the tide will turn again.

And soon, the waders are out of their depth,

and the creatures of the mud are safe once more.

Underwater, incoming tides can create a strong current,

and flounders are experts at hitching a tidal lift.

They're 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 meter deep,

the flounder will be 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 reemerge.

After a wait of six hours under the sand,

they're desperate for food,

and unlike flounder, they head out to sea.

They're looking for shallow open water,

where the tidal currents will concentrate their food,

plankton.

In their 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 offshore in search of food,

they risk running into large predators

that live out 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 virtually impossible to dig underground

when the sea retreats,

so these mussels and barnacles

are fully exposed to the heat of the sun,

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 reveal vast tracts of sea bed

that would normally be covered.

For these raccoons,

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 the beach

than smaller tides would normally allow.

Searching with their extraordinarily 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 large 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 raccoon's table.

For all 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 small 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 giant,

30-meter long bull kelp plants bend to the current.

They're 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 of less than 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 a damaging force.

Here in the Poor Knights Islands

in New Zealand,

weak tides run through rock arches,

making 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 a 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 about out here in the open,

so the demoiselle 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 far less exposed to attack from predators.

Thousands of demoiselles and blue maomao

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,

and out here in the Caribbean Sea,

the tidal movements are slight.

Even so, they're sufficient to push

free-swimming plankton in their path.

These are thimble jellyfish.

They swim towards the 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 color comes from algae

that live inside the jellyfish's body

and photosynthesize energy from the sun.

In open water, they're 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 water from the ocean

through the small gaps between the island cays.

Underwater, the tidal currents race past soft corals...

...and on over the sandy banks themselves.

It's 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,

and 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.

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 defense against this attack.

They will 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're 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 flooding tide brings enough water

for them to swim in, 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 kind of 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 still 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 that 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 a 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, and it's not a moment too soon,

because the tide is flowing in strongly,

flooding the plains.

At last, the predators are free to start their foraging.

Both the stingrays and sharks

have a highly developed 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 even higher than normal.

Now predators can reach the very shallowest fringes

of the seagrass flats,

and these two-meter-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 trees 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 these 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 outmaneuver 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 by the sea.

And 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 floodwater 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 favorite food of an extraordinary animal...

...the Caribbean flamingo.

Remarkably, they actually seek out such briny places.

They're the best spots for them to find their food.

And 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 predators

seeking to dine on flamingo chicks.

The flamingos take the precaution

of building raised mud nests

just in case of further flooding.

Strangely, it's actually the power of the storm tides

that gives the flamingos both their food

and a perfect habitat in which to breed.

A 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.

Around the island,

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,

which is why they pick the smallest tides of the month

to minimize the danger.

The eggs will develop far offshore,

and in exactly one month's time,

a great 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 rapidly in a circle,

stirring up the mud and driving the mullet

towards the other waiting 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.