Original subtitles

Our planet's continents are fringed by shallow seas.

Rarely more than 200 metres deep, they lie on the continental shelves,

which may stretch sometimes for hundreds of miles

before the sea floor drops into deeper, darker waters.

Altogether, they constitute a mere 8% of the world's oceans,

but they contain the vast majority of its marine life.

A male humpback whale sings to attract a mate.

The whales have just returned to their breeding grounds

in the shallow seas of the tropics.

The calf is no more than a few weeks old.

Despite being three metres long and weighing nearly a ton,

he is nonetheless vulnerable.

But his mother watches over him and, as he begins to tire,

she supports him close to the surface so that he can breathe more easily.

These shallows around the equator are excellent nurseries.

They're warm, calm and contain very few predators.

The playful calf is now drinking 500 litres of milk a day.

But his mother must starve. There is nothing for her to eat here.

Like many tropical shallow seas, these crystal-clear waters are virtually lifeless.

They receive year-round sunlight,

but they lack the nutrients essential for the growth of plankton.

The mother will be trapped here for the next five months

until her calf is strong enough to make the journey to the feeding grounds near the poles.

Coral reefs are oases in a watery desert.

Most tropical shallows are barren,

but these coral havens contain one quarter of all the marine life on our planet.

Reefs are the work of polyps, tiny colonial animals like minute sea anemones.

Yet the Great Barrier Reef is so big it can be seen from the moon.

It's actually 2, 000 separate reefs that together form a barrier

stretching for over 1, 000 miles along Australia's northeastern coast.

Despite its vast size, this reef does not contain

the greatest variety of marine life on the planet.

For that, one must travel north to Indonesia.

There are individual reefs in Indonesia that contain almost as many kinds of fish

as live in the whole of the Caribbean.

There are also 10 times the number of coral species.

Corals thrive in these waters with the help of microscopic plants, algae,

that grow within the tissues of the polyps.

And the polyps feed by snaring passing morsels with their tentacles.

At night, the algae are inactive, but then the polyps put out even more tentacles.

So coral, in effect, feeds around the clock.

This well-balanced alliance brings benefits to both polyps and algae,

and between them, they turn the barren seas into rich gardens.

The Indonesian reefs contain such a variety of life because they lie at a giant crossroads.

This is the meeting place for different seas, the Indian Ocean and the Pacific.

Here, everything demands a closer look.

On the surface of this sea fan, there are two polyps that are not polyps.

They're pygmy seahorses, the world's smallest,

less than two centimetres high.

They are males, settling a territorial dispute by head-butting.

An electric flash?

No, the display of a file clam.

Perhaps this extraordinary pulsation of the clam's fleshy mantle

is a warning to frighten away nibbling fish, but no one really knows.

And there are snakes here, too. Lots of them.

These are banded sea kraits.

They lay their eggs on land but they hunt here in the water.

They're too slow to catch fish in a straight chase,

so they seek prey that is hiding in the nooks and crannies of the coral.

Their bite is highly venomous and paralyses their victims.

And on this reef, the snakes do not hunt alone.

Shoals of yellow goatfish and trevally are seeking similar prey,

and they attract the snakes' attention.

As one group of hunters searches the reef, they are joined by the other.

At least 30 snakes have now joined the caravan.

The big fish scare the prey into cracks and there the snakes can catch them.

And anything fleeing from the kraits

will swim straight into the mouths of the waiting trevally.

There's nowhere to hide.

As the raiders scour the reef, more and more snakes join the hunt.

This cooperation between snakes and fish, spectacular though it is,

has only recently been observed,

for it only happens on the most remote reefs in Indonesia.

Perhaps such hunting alliances were once a common sight,

but today no more than 6% of Indonesia's reefs are in their pristine state.

Beyond the coral stretches a world of shifting sand.

Out there, with nowhere to hide, survival is not easy and camouflage can be crucial.

If this wasn't moving, you might think it was a shell or a rock.

In fact, it's an octopus.

A gurnard. Its huge pectoral fins disguise its shape.

And they can also help in clearing away sand when searching for food.

The jawfish hides underground.

The wonderpus octopus, on the other hand, has such a powerful bite

that it has a special warning display to tell others to keep out of its way.

Here and there, plants manage to take root and they are cropped by green turtles.

Seagrasses are the only flowering plants that have managed to grow in the sea.

Although they put out a few ribbon-like leaves,

they produce very extensive networks of fleshy stems, rhizomes,

that are buried in the sand.

At their lushest, they can transform the seabed into an underwater meadow.

The largest expanse grows in the shallow waters of Shark Bay in Western Australia.

These vast aquatic grasslands stretch for 1, 500 square miles.

And like terrestrial prairies,

they support herds of grazers, dugongs.

Dugongs are the largest herbivores in the sea.

They can be three metres long and weigh half a ton,

and they eat nothing but seagrass, mostly the fleshy rhizomes,

which they excavate with their mobile lips.

A herd can clear a patch of seagrass the size of a football pitch in a single day.

Food is not evenly distributed in the tropical shallow seas and it can take a lot of finding.

But bottlenose dolphins are inquisitive, energetic and very intelligent

and they have discovered a shoal of bait fish.

Together they ride a wave, using it to carry them into the shallows

and there it will be easier to make the catch.

In Western Australia, these dolphins have taken on an even tougher challenge.

The fish have taken refuge close to the beach where the water is only a few centimetres deep.

Tail-slapping is a method dolphins often use to stun their prey,

but it doesn't seem to work here.

The fish are tantalisingly close, but they're still out of reach.

So the dolphins try another technique.

Vigorously pumping their tails, they work up some speed

and then they hydroplane.

Their momentum carries them right through the shallowest waters and onto the fish.

Now they're in real danger of being stranded.

But fortune favours the brave.

Younger dolphins lie alongside, watching.

But, so far, only eight individuals here have mastered this daring technique.

Although most life in tropical waters is concentrated around the coral reefs

and the seagrass meadows, there are some spectacular exceptions.

The desert of Bahrain seems a very unlikely place

to find a crowded, bustling colony of seabirds.

But every year, 100, 000 Socotra Cormorants gather here to breed.

It's swelteringly hot and only vigorous panting can prevent the birds from fatal overheating.

This hardly seems a good place to rear young.

But at least there are no land-based predators here.

The only source of trouble is likely to be the neighbours.

So each nest is built just beyond pecking reach.

But what about food?

There's only bare sand and the warm shallow sea beyond.

Neither seem likely to produce enough nourishment to support birdlife on this scale.

The answer is blowing in the wind.

Sand whipped up by shamals, offshore winds, blows into the seas of the Arabian Gulf.

With the grains come nutrients, which act as fertiliser,

and they transform the shallow sea into a rich fishing ground.

So, paradoxically, it's the roasted sands of Arabia

that prevent the Gulf from being another desert in the sea.

The whale calf is now five months old.

He's almost doubled in size and his days in his tropical nursery are coming to an end.

It has been a warm and safe place in which to grow up,

but there's nothing to eat here for his mother.

She has been living off her fat reserves for the last eight months

and she's close to starving.

She must leave now while she still has enough energy

to guide and protect her calf on the long voyage ahead.

All across the tropics, humpbacks are heading away from the equator

towards the rich temperate seas of both the southern and the northern hemispheres.

These are colder, rougher and more dangerous waters.

Mother and calf must stay close.

They can send sound signals to one another above the roar of the ocean

by slapping fins on the surface.

In winter, the temperate seas are lashed by violent storms.

The turbulence stirs the water and draws nutrients up from the depths.

But nutrients alone cannot support life.

There must also be sunlight.

In the spring, as the sun daily climbs higher in the sky, the algae start to grow.

Blooms the size of the Amazon rainforest turn the seas green.

Individually, the algae are tiny,

but together they produce three quarters of all the oxygen in our atmosphere.

They're eaten by an array of bewildering creatures.

Salps appear in the plankton soup.

Individuals link together to form chains which can stretch for 15 metres.

Pumping water through their bodies, they strain out algae and other edible particles.

Comb jellies cruise through the water.

They, too, flourish in this seasonal soup

and for short periods they appear in astounding numbers.

Krill, shrimp-like creatures.

By weight, they're the most abundant animals on the planet.

A single swarm can contain two million tons of them.

And that is a lot of fish food.

The shallow temperate seas support the greatest concentrations of fish on our planet.

Huge shoals migrate from their overwintering grounds in the depths

to feed in these rich waters.

It's these shoals that support most of the world's sea mammals.

Sea lions have all the agility and speed needed to collect what they want

and seemingly delight in doing so.

Dusky dolphin, often in pods 200 strong,

work together to reap the harvest.

They break up the shoals into smaller, more manageable balls

and all the hunters benefit.

By midsummer, the surface nutrients have all been absorbed.

The algae die and the food chain collapses.

In a few special places, however,

the temperate seas sustain these levels of life throughout the summer.

Along the coast of California, ocean currents carry a constant supply of nutrients

up from the depths to the surface layers.

These upwellings fertilise forests of giant kelp that thrive in the summer sunshine.

The algal towers are as high as a three-storey house

and they can grow by half a metre a day.

Life in the kelp is as full of drama as in any other forest,

but the cast is less familiar.

An army of sea urchins is mounting an attack.

The urchin plague strikes at the kelps' holdfasts, their crucial attachments to the rock.

Holdfasts are extremely tough, but each urchin has five teeth

which are self-sharpening and are replaced every few months.

Urchins fell vast areas of kelp forest,

creating clearings known as urchin barrens.

Yet "barrens" is a poor description.

Millions of invertebrates invade the seabed.

The most fearsome predator here is a giant.

The sunflower starfish is a metre across,

with an appetite for brittle stars.

It uses its feet to taste for prey.

When its actions are speeded up, it becomes clear

that the predator's fondness for the brittle stars

is almost matched by the brittle stars' ability to get out of the way.

Sand dollars, flat sea urchins, cluster together as a defence.

But it doesn't seem to work against the sunflower starfish.

The predator extrudes its stomach and wraps it around its victims,

liquefying their soft parts.

Nothing is left of them except their white skeletons.

The Californian upwellings are seasonal and relatively small.

But in southern Africa, they're so big they create seas rich enough

to support colonies of over a million seals.

The Benguela Current sweeps along the western coastline of southern Africa,

driving nutrient-rich waters up to the surface.

And then, at the southern tip of Africa,

it meets the Agulhas Current arriving from the east.

The result? Even richer waters.

The seals here thrive on a diet of fish and squid.

In temperate seas, there may actually be more squid than fish.

These are chokka squid and they lay their egg capsules

in sandy shallows bathed by the warmer Agulhas Current.

Each capsule contains a hundred tiny squid.

Within a few days, they develop spots of pigment

which, when they're adult, they will use to communicate with one another.

With females continuing to lay eggs and males still preoccupied with repelling rivals,

the squid drop their guard.

Stingray.

Short-tail stingray can be up to two metres across.

They're the largest of all the stingrays and they have appetites to match.

Another predator is on the prowl.

The aptly named ragged-tooth shark.

Raggies grow to three metres long but they share these waters with a shark twice their size.

The great white.

The largest predatory fish on the planet.

Each dawn, Cape fur seals leave their colony to go fishing.

To reach the open sea, they must cross a narrow strip of water

and that is patrolled by great whites.

Each seal is indeed swimming for its life.

The shark relies on surprise.

The great white's turn-of-speed is powered by a high metabolism.

They only thrive in cold temperate seas,

for only these waters contain sufficient food necessary to fuel such a ravenous predator.

As you travel towards the poles, North or South,

the colder, stormier seas can become even richer.

Midway between South Africa and the South Pole

lies the isolated island of Marion.

The island sits in the infamous Roaring Forties

where incessant gale-force winds draw nutrients up from the depths,

ensuring plenty of food for King penguins.

The Kings are returning from a three-day fishing trip with food for their chicks.

But first they must cross a crowded beach,

threading their way between gigantic and bad-tempered elephant seals.

The 200, 000 penguins breeding here are testament to the richness of the fishing.

King chicks are dependent on their mothers for over a year

and this puts a great deal of pressure on the parents.

Being flightless, the returning penguins must cross the open beach on foot.

Fur seals that have come to the beach to breed are waiting for them.

Fur seals normally live on krill,

but these have now acquired an unexpected taste for blubber-rich penguins.

Penguins may be featherweights by comparison,

but they have razor-sharp bills and a feisty character.

The seal could easily lose an eye.

The only safe way to grab a penguin is from behind

and the birds are well aware of that.

Both animals are clumsy on this terrain but the penguin has the more to lose.

Made it.

Two out of three penguins survive the seal attacks and succeed in reaching their ever-hungry chicks.

The humpbacks are nearing the end of their epic journeys.

After two months and thousands of miles,

they're entering the polar seas, both in the north and the south.

In the far north, winter is over at last

and the ice is starting to melt.

The Aleutian Island chain, running west from Alaska,

is the gateway to the Bering Sea.

With the retreating ice, rough weather and ferocious currents stir up these shallow seas.

Add sunshine and the mix is spectacularly productive.

Five million shearwaters have flown almost 10,000 miles from Australia to get here.

In all, 80 million seabirds come here for the summer,

the greatest concentration to be found anywhere on Earth.

The humpbacks have finally arrived.

The giant shearwater flocks hunt the krill swarms,

sometimes diving to depths of 40 metres to reach them.

A large humpback eats three tons of krill a day.

The polar seas in summer are the most productive on the planet

and the whales gorge themselves round the clock.

The fat reserves they lay down now will keep them alive during the year to come.

But it may not always be this way.

Fish and krill stocks are declining so rapidly

that spectacles like this may soon be part of history.

Once the mother and calf have reached their feeding grounds, they will separate.

With luck, the calf will make the epic journey across the oceans

from equator to pole another 70 times,

cruising back and forth between the shallow seas

where life proliferates so abundantly on our planet.