Original subtitles

NARRATOR: Caught between land and sea,

at the heart of a secret, subtropical shoreline,

is a forest like no other.

Mangroves.

Where changing tides rule

over one of the most abundant ecosystems on the planet,

and one of the most hostile.

It is home to extraordinary creatures,

fighting a daily battle for survival

in one of nature's last wildernesses.

South Africa's eastern coast.

A subtropical realm where the warm Indian Ocean

meets a wild shoreline.

From offshore reefs

that teem with life, to the inland lakes and rivers,

this is a pristine natural kingdom.

A place rich enough in food and diversity

to support giants...

from the ancient predators of the ocean,

to the huge hunters of the lakes and estuaries.

From the air, the oceans and lakes

look like separate worlds.

But both are intimately bound together by a third ecosystem

few humans ever explore.

The mangrove forest: the living heart of this coast.

It's an incredible machine

poised at the meeting place of fresh and salt water.

Sheltering in and under its branches,

a variety of secretive forest dwellers,

each one uniquely adapted to life in this maze

of waterways and mud flats.

But the mangrove's presence is felt far beyond these estuaries.

The forest is also a nursery

for fish that thrive on offshore reefs,

an undersea wonder sustained in part by the mangroves.

And almost all of this variety and richness depends

on the toil of an army of unsung heroes.

High in the canopy,

a family of vervet monkeys begins its day.

It could be a forest anywhere in eastern Africa,

except for one remarkable detail.

This forest is flooded.

Not that the monkeys seem to care.

Their attention is elsewhere.

Each one of these mangrove flowers

hides a sweet drop of nectar.

And for this family,

that's free, concentrated energy.

Satisfying their sugar craving might seem like a hedonistic way

to spend a morning, but the monkeys' sweet tooth

is also helping the forest.

Pollen dusted around a greedy snout

will be transferred to the next flower and pollinate the tree.

A sweet deal for both monkeys and mangroves.

They're so focused on their foraging

that they barely notice that the forest is starting to change.

The first signs are easy to miss.

The transformation starts at a snail's pace.

These are mangrove snails.

At high tide they cluster high above the water,

out of the reach of fish below.

But now they are inching downwards.

It seems a fatal move.

The water below is full of fish that would eagerly snap them up.

But the snails creep on, undeterred.

Somehow they have sensed

that the sea is beginning to drain away,

and they will be able to forage on the mud below.

Just how the snails predict the tides is unknown,

one of the mysteries of this secret world.

The ocean is retreating.

Inch by inch,

the secrets of the mangrove forest are revealed.

Two species of tree dominate here,

white and black mangroves,

but both share the same incredible survival skills.

Many of the world's trees survive extreme conditions.

But for almost all of them, salt water is a death sentence.

Too much salt around their roots prevents plants

from absorbing water.

Salt also creates a toxic build-up

around the root,

slowly poisoning the tree.

But the mangroves have an elegant solution.

If salt around the roots is dangerous,

then remove it as far from the roots as possible.

Salt is drawn up and away and deposited in the leaves,

safely away from the main workings of the tree.

But salt is only half the challenge.

This mud is also life-threatening.

Like animals, plants need oxygen

to break down stored food to grow.

But the water that saturates this mud

has very little oxygen in it.

The solution: a snorkel.

Aerial roots grow up above the ground,

allowing the forest to breathe.

This is a living factory working around the clock,

shunting salt up from the floor to the leaves above

and sucking air down into the roots.

But the snorkel isn't only for inhaling oxygen.

Mangroves have to vent carbon dioxide,

and so the roots also act as chimneys,

keeping the whole factory going even when the tide rises.

Now, though, the chimneys are high and dry

and the factory floor is coming alive.

The monkeys are messy eaters.

What doesn't get swallowed gets dropped.

Unwittingly they are helping the forest to regenerate.

Discarded seed pods fall onto recently exposed mud.

A black mangrove seed pod detaches from the parent tree,

spearing into the mud.

This fruit has already germinated,

and now it has somewhere to grow.

If the tide doesn't carry it away,

it will extend roots from one end and shoots from the other,

and the forest will gain a new tree.

But the odds are stacked against the vulnerable seeds,

because in this factory,

the workers of the morning shift are arriving.

They have lain hidden while the tide was high.

Now, the first worker cautiously appears.

The red-claw mangrove crab,

the unsung hero of an entire ecosystem.

They move slowly at first,

but this worker is racing the tide.

In a few hours all this potential food

will be swept away.

Time isn't the crab's only enemy.

The heat is building fast.

To stay alive in this cauldron, the crabs have had to adapt.

They keep their respiratory organs covered

in a thin film of water, continually passing water

through their gill cavity as if they were underwater.

It looks orderly from a distance,

but with so much food on offer, arguments are inevitable.

When two crabs eye the same morsel,

it's every crab for itself.

These are high-stake battles.

The strongest crabs will get to carry away the best pieces

of plant matter,

and the smaller crabs will have to make do

with leaner scraps.

The morning's forage has gone well for the monkeys.

Time for the youngsters to burn off some excess energy.

Down below, though, there's no time for play.

A vast clean-up operation is underway.

What seemed to be a lifeless stretch of mud is transforming

into a busy industrial operation.

The crabs are not alone.

The retreat of the sea has turned the mangroves

into a more recognizable kind of forest,

and a more traditional forest dweller makes a shy approach.

Standing just 16 inches at the shoulder,

the red duiker is still tall enough to reach some

of the mangrove's lowest-hanging fruit and leaves,

and by now there are also rich pickings on the ground.

There are leopards in this region, so it treads lightly.

But here in the forest,

the duiker will not be disturbed.

For now, it is safe.

The tugs-of-war between the red claws are intense,

but for one character of the forest floor,

it's not about how tough you are,

but how tough you look.

It's late morning.

The mud is drying as the forest becomes unbearably hot.

The perfect landscape for a tiny gunslinger.

This is the fiddler crab.

The naturalists who named it thought the males

with their outsized claws looked as if

they were playing a fiddle.

They might be small, just a few centimeters across,

but they're feisty.

And when it comes to shows of force,

the claw definitely calls the tune.

They've left their burrows near the water's edge

to join the red claws scavenging for food on the mud.

But there's more going on here than just feeding.

Some of these males have come out to find a mate.

Others are defending their territory.

And in both scenarios,

the claw plays a starring role.

Female fiddler crabs are less flamboyant,

having two equal-sized claws and more subdued coloring

than their mates.

Now they watch as the wooing starts.

As sales pitches go, it's straightforward:

"I've got a huge claw and my own burrow."

He's not exaggerating.

Fiddler crabs are small, but their burrows

are comparatively vast,

sometimes up to 2 feet long.

To sweeten the deal, males also build small decorations

over the entrance to their burrow

which serve as enticement to females

and also help them to identify their new home.

But in a crowded neighborhood like this,

competition for homes can be fierce.

A male who lets his guard down can find his burrow invaded

by a rival.

Brandishing his claw is a way of keeping his territory clear

of potential usurpers.

It looks threatening,

but the posturing is also a strategy

to avoid physical conflict.

If things do escalate, the males will try

to intimidate each other into backing down

by squaring up and seeing who has the bigger claw.

(birds and insects chirping)

Noon is approaching,

and the red claws are starting to feel the heat.

Not so the fiddler crabs.

In winter they stay buried in their burrows,

but now, as the temperature soars,

these rugged little show-offs stay out to see and be seen.

It's thanks to a superb adaptation

that allows them to be equally comfortable

on dry land as well as in the water.

They have gills for breathing in the wet,

but they also have a primitive lung,

which means they can spend months out of water.

The heat is too much for the regular workers.

The winners take their trophies and head for their burrows.

Much of the morning's debris has been cleared away

by the red claws, but their work is more

than just gathering food.

By dragging the debris underground,

they have radically sped up the rate of decomposition.

Composting that would normally take weeks takes days.

They're also aerating the soil with thousands of burrows

they have excavated,

filling this landscape with air pockets.

Stirred, aerated and pumped full of compost,

this soil has been perfectly prepared by the crabs

for the next mangrove sapling to take root,

and for the next chapter of the forest's story.

High noon.

The forest slows down in the oppressive heat.

It's a time for conserving energy.

The vervet monkeys have come down to the shade for a siesta.

Only a baby monkey fueled with pure nectar

still has the energy to play.

For the dominant male of this group,

a fork in a tree provides an ideal armchair.

It's dramatically cooler down here on the forest floor

than up in the canopy and now, safely out of the sun,

there's time for some personal grooming.

Vervets are omnivores, and any ticks or lice they find

on themselves are small packages of protein.

They might look lazy, but in this forest,

nothing is wasted.

Low tide.

The forest floor has become a hostile environment,

too dry for fish, too muddy for land dwellers.

But for one unique mangrove dweller,

it's pretty much perfect.

The mudskipper is an eccentric little amphibious fish

that lives comfortably in both water and on land.

And, just sometimes, in the air.

Moving around on its pectoral fins,

the mudskipper can survive out of water

thanks to a special breathing adaptation.

Not only can they breathe air through their mouths,

but they can also absorb it through their skin.

But this only works when the skin and membranes are wet,

so this fish will take a regular mud bath

to keep everything slick and in working order.

And if all else fails,

the mudskipper has an enlarged gill chamber,

which allows it to seal a bubble of air inside its body

for extended visits out of water.

Mudskippers are well named.

The "mud" part needs no explanation.

To see the "skipper" part, however,

you have to wait

until one of them feels threatened.

And then...

That speed also comes in useful for hunting.

The fly has quick reflexes...

but it's no match for a lunging mudskipper.

Along the water's edge,

the others are waiting for their own meals to fly too close.

The chances are good

they'll see their prey long before it sees them.

Large eyes mounted on top of their heads

give mudskippers a panoramic view of their surroundings.

They don't have tear ducts to keep their eyes moist,

but there are other ways to wet an eyeball.

The mudskippers rule this small muddy kingdom for now,

but soon they'll have to surrender it once more.

The snails on the tree trunks have turned

and are inching their way upwards.

The tide is coming in again.

For the snails, it's a journey of just a few inches,

but it's ruled by gigantic, planet-sized forces.

The ocean is being heaved up by an irresistible force:

the moon.

Where the earth faces the moon,

the oceans are pulled slightly towards it,

creating a bulge of water we call high tide.

On the other side of the planet,

the moon's pull is at its weakest.

Here oceans bulge in the opposite direction,

creating two synchronized high tides, which rise twice daily

as the moon revolves around the earth.

A tidal pattern

that is fine-tuned by the sun's gravity,

global rotation, and the shape of the coastline,

creating spring and neap tides.

The rising water is good news for the red claws.

The seawater will bring relief from desiccation.

But while the water might be welcome

on their sensitive breathing organs,

it will also soon be a threat to their burrows.

Above the tide line, emergency renovations have taken place.

The red claws are now sealed inside their dry burrows,

where they'll sit out the high water.

The incoming flood is also causing major domestic problems

for some of the mudskippers.

It seems strange that water should be a threat to a fish,

but mudskippers are no ordinary fish.

Raising a family down in the mud, with very little oxygen,

makes special demands on the mudskipper.

A devoted father,

he has done most of the parenting.

It was he who dug the burrow

and who tended the eggs for the week they took to hatch.

But now he faces a force beyond his control: the sea.

His burrow is flooded with oxygen-poor water.

His young are in danger of suffocating.

And so he is creating an air-pocket inside his home,

turning himself into a mobile oxygen tank

by taking big gulps of air, before heading down

to save his family, one breath at a time.

The Indian Ocean is on the move,

surging over the reefs and pushing inland.

Salt water floods into the estuary,

and at last it reaches the mangrove forest.

The red claws and fiddlers are deep in their burrows.

Along the banks, the mudskippers lie waiting

for the transformation of the forest.

The flooded forest appears deserted.

But as the tide rises,

the mangrove is welcoming a new cast of characters.

Thousands of tiny fish, recently hatched,

have been swept in from the sea.

And suddenly the mangrove transforms from a forest

into a nursery.

These juvenile fish have survived a perilous journey,

hatching out at sea

and then cautiously trekking upriver.

But now that they're here, these warm waters

and the labyrinth of roots

are a sanctuary.

Safe from the ocean's predators, they can feast

on the nutrient-rich forest floor

until they are mature enough to run the gauntlet

back to the ocean.

They're also small enough to benefit

from the cocktail of bacteria

that are helping decompose the mangrove's waste.

For a fish of this size,

even tiny microbes make a good sized snack.

The fish aren't alone.

Shrimp thrive here,

protected from the currents and predators of the ocean.

Of course, the old rules still apply here.

The fish are still the hunter, the shrimps still the hunted.

But at least in these calm backwaters

they have a fighting chance to reach maturity.

The success of the mangrove forest in nurturing shrimp is,

unfortunately, a double-edged sword.

Being one of the planet's

most biologically productive ecosystems

makes it a prime target for the one predator

no animal or plant can hide from-- humans.

Every year, more and more mangrove forests

are being hacked down to make way

for shrimp farms and similar aquaculture projects.

Here, at least, the human footprint is relatively faint.

Out in the lake, a young fish huddles

against what looks like another mangrove shoot.

But for adult fish,

this isn't a sanctuary.

It's a death trap.

These delicate curves are fish traps,

perfected a thousand years ago by ancestors

of the local Tembe-Thonga communities in this region.

They're elegant and beautifully efficient.

As the tide retreats and adult fish begin

to swim back downstream towards the sea,

they naturally follow the large network of guiding fences.

It's a wall no more threatening than the bank of a river

or a growth of reeds, but it funnels them directly

into the final corral,

where fishermen can spear them at their leisure.

It's not an indiscriminate harvest.

These juveniles are small enough to escape,

meaning that this is a largely sustainable technology.

Even the posts themselves can become a living part

of this landscape.

Mangrove timber used in the traps can take root,

and the forest extends by a few more trees.

Importantly, the traps don't only provide food for humans.

They also create a new habitat in which smaller species thrive.

Like a man-made miniature mangrove forest,

these posts provide cover and even food

for a variety of fish and crustaceans.

These baby parrot fish follow the tide upstream

and graze in the palisades that make up the traps.

Nearby, a group of much more flamboyant babies.

These are lion fish,

their bright pattern announcing loud and clear

that they are venomous and definitely not on the menu.

They are great hunters,

fanning out their wide pectoral fins to corner their prey.

The sun's rays illuminate this underwater world.

Mid-afternoon light that is too bright

for one secretive creature.

The floral moray eel is a night hunter.

It waits out the day, coiled up in a crevice

where it can remain undisturbed.

Later, as darkness falls,

it will come snaking out of its lair in search of prey.

For now, though, the baby fish are safe.

Only a handful of the fish in this system

end up in the traps.

The rest have safely made their way downstream,

flushed out into the estuary and then into the ocean,

where they were born.

Their destination is the reefs, underwater islands

of abundant life, where brightly colored tropical fish

dart between blooming anemones.

A hawksbill turtle pauses on its oceanic journey

to graze on sponges.

The reefs are fertile ground

for a variety of marine plants,

providing abundant food for the animals that live here.

For adult parrot fish, the reef itself is a tasty treat.

It's an undersea garden,

far more vibrant than the somber mangrove forests upstream.

And yet most of this spectacle

owes its existence to the mangrove.

In fact, some species of tropical fish

are twice as plentiful on reefs that border mangrove forests,

a testament to their value both as nurseries

and giant factories producing nutrients.

Out on the open water, a paradise for birds.

Flamingos thrive here,

sifting for shrimps and small fish.

They look less pink than their relatives,

the lesser flamingo.

But when they take to the air,

a beautiful flash of famous flamingo pink.

The pinkness of flamingos is a byproduct of their diet.

Shrimp contain an organic pigment called a carotenoid.

But not all the birds in these skies

are as benign as the flamingos.

High tide has pushed the vervets back up

into their treetop haunt again.

Now, in the heat of the late afternoon,

the monkeys have to move carefully in the top canopy.

A young vervet

wandering too far from the adults

or distracted by a flower

would be easy pickings for the largest raptors

that haunt these skies.

At dusk, smaller aerial predators

return to the mangroves to roost.

These are yellow-billed kites.

They're secretive animals,

and the dense, tangled branches provide an ideal hiding place.

But privacy isn't the only reason the kite migrates here.

It's an opportunistic hunter and scavenger.

When low tide returns, the mangroves will provide

a wealth of options,

from fish and crabs to insects.

As the light fades, it's time for a new phase in the life

of the mangrove forest and the lake system that it feeds.

Hippos begin to stir.

These great beasts have been lying in the water all day,

keeping cool and socializing.

Now, they leave their watery haunts

to spend their nights alone,

foraging in the grassland that surrounds the lakes.

They're mostly herbivorous,

and from a distance they seem slow and lazy.

But looks can be deceptive.

Africa is home to a variety of dangerous predators,

but hippos are the continent's most feared killer,

responsible for nearly 2,000 human deaths every year.

As dusk falls over the mangroves,

the landscape changes once again.

Soon the snails will signal the turn of another tide

and migrate slowly downwards.

The moon is tugging at the earth's oceans,

starting to drain the water out of the mangroves.

The bright face of the eternal clock that runs this factory.

The monkeys are settling down for the night.

Tomorrow will be a long, hot day of foraging,

and they need their rest.

But the forest floor beneath never rests.

Tonight, only a few seed pods and flowers

will drop into the mud.

But when they do, announcing themselves

with a soft vibration on the mud,

the red-claw mangrove crabs will stir in their burrows,

and the night shift will go to work.

Mangroves are an enigma, incredibly resilient

and also surprisingly fragile.

Forever in flux between high tide and low tide,

land and sea, salt and fresh water,

they are the foundation

for a dazzling variety of life on this coast.

And keeping the entire system working,

the red-claw mangrove crab,

the unsung hero of the mangroves.

A tireless worker helping to maintain and sustain

this unique forest, ultimately feeding

the incredible diversity in the ocean beyond.

And a living reminder that our own survival depends

on our connectedness

with the great life-giving machine

we know as planet Earth.

Captioned by Media Access Group at WGBH, access.wgbh.org