Our planet's continents are fringed by shallow seas.
Rarely more than two hundred meters deep they lie on the continental shelves
which may stretch sometimes for hundreds of miles
before the sea floor drops into deeper darker waters.
All together they constitute a mere eight percent of the world's oceans
but they contain the vast majority of it's 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 meters 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 five hundred liters 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.
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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 two thousand separate reefs
that together form a barrier stretching for over a thousand miles
along Australia's northeastern coast.
Despite it's 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 ten 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
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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 sea horses, the world's smallest, less than two centimeters 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 crates. 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 paralyzes their victims.
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And on this reef the snakes do not hunt alone.
Shoals of yellow goatfish and trivali are seeking similar prey
And they attract the snakes' attention. As one group of hunters searches the reef
they're joined by the other.
At least thirty 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 crates will swim straight into the mouths of the waiting
trivali.
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 six percent 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. It's huge pectoral fins disguise it's 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
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that it has a special warning display to tell others to keep out of it's way.
Here and there, plants manage to take root
and they're 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 extended networks of fleshy stems, risomes,
that are buried in the sand.
At their lushest they can transform the sea bed
into an underwater meadow.
The largest expanse grows in the shallow waters of Shark Bay, in Western Australia.
These vast aquatic grasslands stretch for fifteen hundred square miles
and, like terrestrial prairies, they support herds of grazers.
Dugongs.
Dugongs are the largest herbivores in the sea.
They can be three meters long and weigh half a ton
and they eat nothing but seagrass, mostly the fleshy risomes,
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
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but bottlenose dolphins are inquisitive, energetic,1 and very intelligent
and they have discovered a shoal of baitfish.
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 ever tougher challenge.
The fish have taken refuge close to the beach
where the water is only a few centimeters deep.
Tail slapping is a method dolphins often use to stun their prey
but it doesn't seem to work here.
The fish are tantalizingly 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 favors 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,
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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, a hundred thousand 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 neighbors
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 bird life 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 fertilizer
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.
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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 can not 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
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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 fifteen meters.
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 two hundred strong
work together to reap the harvest.
They break up the shoals into smaller, more manageable balls, and all the hunters benefit.
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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 fertilize forests of giant kelp, that thrive in the summer sunshine.
The algal towers are as high as a three story house
and they can grow by half a meter 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 kelp's 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 know 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 meter across, with an appetite for brittle stars.
It uses it's feet to taste for prey.
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When it's 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 defense
But it doesn't seem to work against the sunflower starfish.
The predator extrudes it's stomach, and wraps it around it's 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 chokker 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.
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With females continuing to lay eggs
and males still preoccupied with repelling rivals, the squid drop their guard.
Stingray.
Short tailed stingray can be up to two meters 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 meters 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 it's 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
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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 two hundred thousand 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.
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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 Bearing 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 ten thousand miles from Australia to get here.
In all eighty 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 forty meters 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.
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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 seventy times
cruising back and forth between the shallow seas
where life proliferates so abundantly on our planet.
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