Tidal Seas The Blue Planet


Tidal Seas

David Attenborough narrates a natural history of the oceans. As snails surf the waves in pursuit of a meal, how do the tides create opportunities for marine life?


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Transcript


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There is a force sufficiently powerful to move the oceans of this world.

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It is a force not of this earth.

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The moon is big enough to generate gravity -

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and with sufficient force to pull on the earth 230,000 miles away.

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As the moon orbits the earth,

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its gravity sweeps across the face of our planet.

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Its power drags a great bulge of oceanic water in its wake.

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The rising tide.

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The River Amazon in Brazil.

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On some special days,

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the gravitational forces of the moon and the sun pull together to extraordinary effect.

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A growing tidal wave from the ocean is being forced 200 miles inland.

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This is a "tidal bore".

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Fortunately, tidal bores are rare.

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but the moon does create strong tides

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out in the world's oceans on every day of the year.

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The Bay of Fundy in Nova Scotia.

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The tides here are the largest in the world,

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and have a profound effect on marine life,

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creating a rich feeding ground...

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..a feast that attracts some of the largest diners on the planet.

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Humpback whales.

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But they are not the biggest threat to the herring.

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These are finback whales, at 70 tons, the second-largest animal on earth -

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but so beautifully streamlined

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that it is the fastest of the great whales.

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This combination of speed and immense size

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makes the finback a voracious hunter of schooling fish.

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The Bay of Fundy can attract so many fish

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that during the summer as many as 500 of these magnificent whales hunt here every day.

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The feeding is best where the tides run strongly.

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So the whales move further into the bay,

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following tidal rips and searching for fish.

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Their movements are closely watched by flocks of Cory shearwaters.

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As the whales dive down towards the fish, more and more birds gather, anxious to pick up scraps.

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The flowing tide may provide a feast,

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but before long, it will turn.

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In just six hours, 100 BILLION tons of water

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will flow out of the bay -

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the sea level falling by as much as 15 metres

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and exposing vast tracts of mud and sand...

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..at first sight, a barren place entirely devoid of life.

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In fact, the damp sand is packed with microscopic life, the meiofauna,

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feeding in a sandy underworld

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quite unaffected by the departure of the sea.

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But life is not all roses in this miniature world.

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A sand bubbler crab in northern Australia. It hunts meiofauna.

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Just a centimetre across, the sand bubbler works at breakneck speed,

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passing sand grains into its mouth, filtering out all the meiofauna and kicking aside the waste.

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The crab will clean every grain of sand within a metre of its burrow.

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Endless practice for the best back-heel in the natural world.

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The crabs work fast because they can only sieve when the sand is damp.

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They work the entire surface of the beach

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within just a couple of hours of the tide retreating.

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Then they simply return to their burrows and await the next tide.

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Underwater, the falling tide is the cue for some bizarre activity.

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These slow-moving clams use their muscular feet

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to bury themselves under the sand.

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If they fail to get under cover, the tide will leave them exposed to the air, and they will perish.

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But once underground, they can wait deep down in the sand,

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safe beneath the beach.

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And not a moment too soon.

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June in south-east Alaska,

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and in just four hours, a vast, sandy beach is exposed by the falling tide.

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The bears are hungry.

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At this time of year, the pickings on land are few and far between.

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But any food here has long since buried itself deep under the sand.

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To a hungry adult bear, that is no barrier.

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They smell the clams through the sand and simply dig them out.

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For such large animals,

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they show extraordinary dexterity at opening the unfortunate shellfish

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Cubs try their luck, too,

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none too successfully.

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For the adults, the shellfish feast lasts

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as long as the tide remains out.

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Table Mountain in South Africa.

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Every day, the retreating waves leave flotsam somewhere on the beach.

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This creature is scenting the currents

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for the odour of rotting fish.

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The tide carries the scent far into the surf zone.

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Responding to the smell, snails emerge from the sand.

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This is a race against the tide. The snails need to find their meal

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before the tide leaves it beyond their reach.

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But snails are slow, and the tides fall rapidly.

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These are no ordinary snails.

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They can surf.

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They ride the waves up the beach. But all too soon, the tide leaves the fish beyond the surf zone.

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Without the sea, there's a danger that the snails will lose the scent,

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but as long as the sand remains damp,

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they can still follow a faint trail to the food.

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Once there, they tuck in

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with macabre relish.

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Good things come to those who wait.

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Before long, the heat of the sun forces them to retreat into the sand

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to await the return of the next tide.

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In the water, incoming tides can create a strong current,

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and flounders are experts at hitching a tidal lift.

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They're shaped rather like a kite,

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a perfect design for gliding on the tide.

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In Newfoundland, on the east coast of Canada, large numbers of flounder ride the currents into the shallows.

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They've come to hunt invertebrates that will emerge now that the water is back.

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The pickings in the shallows can be very good.

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The activity has not gone unnoticed.

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But ospreys can't dive deeply.

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As long as the water is over a metre deep, the flounder will be safe.

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Going too far inshore can be a risky business.

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This fish buries itself completely in the sand at any sign of danger.

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But, when the tide floods in again, as long as the coast is clear,

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these sand lancet will re-emerge.

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After a wait of six hours under the sand, they're desperate for food

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and, unlike flounder, they head out to sea.

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They're looking for shallow, open water where the tidal currents will concentrate their food -

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plankton.

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In their untold thousands, they stream towards the best feeding grounds

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where they simply pick up tiny, planktonic creatures from the water.

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But if they swim too far offshore in search of food,

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they risk running into large predators

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that live out in deeper water.

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Dogfish - small sharks.

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The sand lancet HAVE strayed out of their safe depth.

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The effect of the turning tide can be totally different on a rocky shore.

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Here, on the coast of Vancouver Island in Canada,

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the sun bakes the exposed rock.

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It's virtually impossible to dig underground when the sea retreats,

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so these mussels and barnacles are fully exposed to the sun's heat - literally cooking in their shells.

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And the seaweed simply dry to a crisp.

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It can be a wait of many hours before the water returns.

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Throughout each month, the size and the strength of the tide changes.

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The biggest tides of all happen

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when the gravities of the sun and moon pull in unison.

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That's immediately after the new moon

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and again after the full moon.

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These are the "spring tides". They reveal vast tracts of sea-bed that would normally be covered.

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For these raccoons, it's a chance to look for a seafood feast.

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A mother ventures forth with her kits.

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With the spring tide, they've come further down the beach than smaller tides would normally allow.

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Searching with their extraordinarily sensitive paws,

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they look for suitable prey.

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With the extreme low tide, they could find something special.

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What could be better than a red rock crab?

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That is, if it weren't for the risk of a painful pinch.

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With large crabs, there's no substitute for experience.

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The mother makes an expert's catch.

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But the kits learn fast.

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And for those that don't, begging is always worth a try.

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All too soon, the returning tide will cover the raccoons' table.

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For the invertebrates, it's a welcome relief, but in rough weather they are exposed to the worst of the waves.

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Even when there are no waves,

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the incoming tide can create considerable forces underwater.

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The gaps between these small islands on the east coast of Vancouver Island channel the tidal flow.

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As the tide keeps rising, gradually the water flows faster,

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and soon, these giant, 30m-long bull kelp plants bend to the current.

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They're sufficiently flexible to cope without too much damage,

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but there are some spots where the currents are especially powerful.

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This is the Naquatto Rapids.

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At the turn of the tide, water from almost 700 miles of coastal fjords

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will have to empty through a gap of less than half a mile wide.

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Within a few minutes, the current is picking up speed,

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until water roars by at over 17mph.

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Tidal currents are not always a damaging force.

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Here, in the Poor Knight Islands of New Zealand,

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weak tides run through rock arches.

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It's ideal for resting stingrays.

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These rays congregate here in huge numbers every March.

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They've come to breed.

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The arches funnel the current,

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which the rays can ride with the minimum of effort, so saving energy.

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Nearby, out in open water, a school of two-spotted demoiselle fish feed on plankton,

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and the current is perfect for sweeping their food past them.

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Once the current starts to weaken,

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there is insufficient food to warrant the risks of swimming about out here in the open,

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so the demoiselles head off to find shelter en masse. Safety is in numbers.

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More and more demoiselles pour towards the cave entrances that riddle the Poor Knight Islands.

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Safe. In the cave, they are far less exposed to attack from predators.

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Thousands of demoiselles and blue maomao wait for the return of the current,

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when once again they will head out into the open to feed.

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The moon's gravitational pull is weaker nearer to the earth's equator,

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so the more equatorial the location, the smaller the tides.

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And out here in the Caribbean Sea, the tidal movements are slight.

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Even so, they are sufficient to push free-swimming plankton in their path.

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These are thimble jellyfish.

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They swim towards the sunlight, and invisible boundaries formed by the tidal motion help herd them together

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until they gather in immense swarms.

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They put the tropical sunshine to good use.

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Their brown colour is from algae that live inside the jellyfishes' bodies

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and photosynthesise energy from the sun.

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In open water, they're fairly safe.

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But the tide is sweeping the whole swarm gently towards the Bahamas,

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where hungry mouths are waiting.

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Although the sea level doesn't change much,

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the tides are still pushing an enormous volume of water from the ocean

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through the small gaps between the island cays.

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

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..and on over the sandy banks themselves.

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It's an immense area of coral sand

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that is only just submerged at high tide.

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This incoming tide is bringing in a fresh supply of oceanic plankton,

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and razorfish gather at the best spots

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to catch the pick of the microscopic feast.

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Although there's plenty here for these small fish to eat,

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gathering in one place makes it easy for their predators to find them.

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A nurse shark is little threat...

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SQUEAKS AND CHIRRUPS

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..but this SOUND heralds a different danger.

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WHIPLASH CRACK FAINT CREAK

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CHIRPS AND CLICKS

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CLICKS AND BUZZES

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"ZIP" CALL A bottlenose dolphin.

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ZIP! WHEE!

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It's using its sonar to locate razorfish beneath the sand.

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IT BUZZES, CLICKS AND PURRS

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Once it finds a suitable target, it simply digs out its prey.

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THEY SQUEAK SOFTLY

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THEY TWITTER

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"ELECTRIC-ARC" FIZZ

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The buried fish have no defence against this attack.

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They will simply have to wait and hope they aren't found out.

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This dolphin appears to have a razorfish craving.

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Well, she IS pregnant.

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Success at last.

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THEY GIVE CREAKY SQUEAKS

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The incoming tide sweeps on towards America,

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flooding across vast, flat plains of seagrass.

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They're so shallow that at low tide, all large fish are forced to retreat into deep-water channels -

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like these nurse sharks...

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..and stingrays. Both predators hunt crustaceans on the seagrass beds.

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But until the flooding tide brings enough water for them to swim in, they will have to wait.

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So now, for this tulip snail, it appears safe

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to patrol the channels in search of a meal.

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But is it?

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This is a rather bigger kind of snail.

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At 5kg, the giant horse conch has little to fear from any shark -

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and it has a taste for tulip snail.

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Sensing the approaching danger, the snail flees.

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But in a world of snail paces, the conch is something of a Ferarri.

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It calls for desperate measures.

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Exhausted by the effort of its last-ditch attempt,

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the tulip snail is slowly gunned down.

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The tide has to rise for another hour

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before the big predators can feed.

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But out on the flats, the scent of dying snail wafts away on the tide.

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It's a scent these hermit crabs are partial to.

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It's vital that the crabs have the best possible protection

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from the heavy teeth of the waiting sharks and rays.

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For that, they need the shell with a perfect fit.

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Today, there is new real estate on offer, and competition in this housing market is fierce.

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The action becomes even more desperate

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when the shell of the devoured snail is ready for release.

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This crab simply can't wait any longer.

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But it's a decidedly risky acquisition.

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The risk paid off handsomely.

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The new shell is both lighter and stronger than the old home - and it's not a moment too soon,

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because the tide is flowing in strongly, flooding the plains.

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

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Both the stingrays and sharks have a highly-developed electrical sense

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which they use to search for buried invertebrates.

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They can sense minute movements beneath the sand.

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Finding a promising signal, this ray digs out its meal.

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An unprotected hermit crab would have no chance.

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Within a few hours, the tide ebbs out once more

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and all the predators are forced to leave.

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They will have to wait. until the next high tide before making another feeding foray.

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At certain times of the year, at the equinox, spring-tides are exceptionally large,

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and rise even higher than normal

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Now, predators can reach the very shallowest fringes of the seagrass flats,

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and these two-metre-long tarpon are going further inshore still.

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They are heading for the mangroves.

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These flooded forests cover huge areas of the coastal shallows.

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Extraordinarily, the roots of the mangrove trees can live in salt water,

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and they make a perfect nursery for small fish.

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Silversides and snapper find sanctuary in the maze of roots.

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Big predators seldom find a way in here.

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Now the tide is falling once more.

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The water starts losing what little oxygen it contained,

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and quickly becomes stagnant.

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Most predators have abandoned the mangroves,

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but the tarpon are still here, trapped by the falling tide.

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Dissolved oxygen is fast running out.

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They have a vital survival technique.

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They can breathe air.

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Pumped up with fresh oxygen, they easily outmanoeuvre dozy silversides.

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The tide has turned again.

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And this is no ordinary tide.

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Since it is the equinox, the tide is rising fast.

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But now, out to sea, a hurricane is on its way -

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forcing the tide yet higher.

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The passing storm leaves large areas of the coast flooded by the sea.

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And low-lying islands like the Bahamas are particularly prone to the storm flooding.

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The sun's power here is immense.

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As the tide recedes and the remaining floodwater evaporates, a remarkable transformation takes place.

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The mud is coated with a magical world of salt.

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Any remaining water is extremely salty.

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Very few creatures can survive here except brine shrimp...

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..and, on the water's edge,

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brine flies.

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Both are the favourite food of an extraordinary animal.

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The Caribbean flamingo.

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Remarkably, they actually seek out such briny places.

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

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BIRDS HONK

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They also provide the protection the flamingoes need to raise young.

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nesting sites like this are surrounded by corrosive brine.

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It's a formidable barrier

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to any predators seeking to dine on flamingo chicks.

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The flamingoes take the precaution

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of building raised mud nests

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just in case of further flooding.

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ADULTS HONK AND QUACK

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CHICK PIPES

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Strangely, it's actually the power of the storm tides that gives the flamingoes both their food

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and the perfect habitat in which to breed.

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The breeding of many animals in the ocean is closely co-ordinated with the tidal cycles.

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A half-moon in November.

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It's the time of SMALL tides.

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Christmas Island in the Pacific.

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Strange happenings are afoot.

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It's one of only a few nights each year

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when female Christmas Island crabs

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risk heading down towards the sea.

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Around the island, they number in tens of thousands - and all of them are laden with hundreds of eggs.

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They have to shed them into the ocean

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if the eggs are to develop into baby crabs.

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But these are LAND crabs,

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and they can neither swim nor breathe underwater.

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There's a great risk of drowning,

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so they pick the smallest tides of the month to minimise the danger.

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The eggs will develop far offshore,

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and in exactly one month, a swarm of baby crabs will return -

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again, choosing the perfect tide.

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Whether it's the daily or monthly cycle,

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tides are the rhythm of the ocean, its pulsing clock.

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Every tide brings opportunity to marine life somewhere in the world.

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DOLPHINS TWEET

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Now, a spring-tide is flooding the shallows,

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and hunters are on the prowl.

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A small group of bottlenose dolphin are working their way inshore

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to start a quite extraordinary hunting campaign.

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After one successful pass, the dolphin move off to start again.

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One animal peels off from the group and swims rapidly in a circle,

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stirring up the mud and driving the mullet towards the other waiting dolphins.

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It's a remarkable team effort - and it's extremely effective.

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The dolphin will feed like this for as long as the tide grants them access to the shallows.

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Eventually, the falling tide will force the dolphin to leave the flats, and the mullet will be safe again -

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until the next high tide...

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..because, in the ocean,

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every turn of the tide spells the difference between life and death

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somewhere.

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When trying to film the tidal behaviours of marine creatures, timing is of the essence.

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Many events in the sea only take place at certain tidal phases,

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and some, on only one particular tide in the whole year.

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One of the most spectacular displays depends completely on nature's tidal clock.

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It's the spawning of the coral reefs,

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which release their eggs and sperm in synchrony after a spring full moon.

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A crew was there on Australia's Great Barrier Reef to keep the appointment,

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but things don't always go as planned.

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Lizard Island - a backwater behind the centre of the barrier reef.

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A good place to film coral, but the exact details of coral spawning are still not properly understood.

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Only 20 years ago, they discovered that the entire Barrier Reef went off.

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It goes off on three or four nights, the whole reef, all the corals spawn.

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They've been working on it ever since. They still don't know much.

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Because the spawning could happen in several places at once,

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Martha took two cameramen, Peter Scoones, a veteran of coral seas,

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and Mike Pitts who, although familiar with these waters, had never seen coral spawning before.

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-Mike?

-It's working.

-Can you light balance away from the reef?

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So we're not shining on the corals.

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The spawning activity starts normally starts two days after a full moon,

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and the crew were ready with a complex system of underwater lights.

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What I wanted to achieve with the lights was that it was natural,

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certainly not front lit with a light stuck on the front of the camera,

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it had to have more atmosphere.

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As well as directing, Martha is in charge of dive safety and has to wait on the surface.

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Just so want to get the best images. When they are all underwater,

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I'm praying that someone's seen a fantastic coral going off and they've lit it beautifully.

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That first night, the corals showed no signs of spawning.

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Meanwhile, the team was also trying another strategy.

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On shore in a laboratory on nearby Lizard Island,

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cameraman, Steve Downer and director, Hugh Pearson were hoping to film close-ups of coral spawning

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in more controlled conditions.

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We spent four or five nights in the laboratory getting these shots.

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In the end, the coral did spawn and we got some shots of a spawn leaving the corals in a big close-up.

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Although that was successful, out at sea, everyone were still hoping that the big event would finally happen.

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Today is night three of a potential spawning.

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Although Craig has said tonight is going to be the quietest,

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they also know up until an hour before spawning,

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so they can give warning before it happens.

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The third night seemed a little more promising.

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Some of the corals began to release sperm in clouds into the water.

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But this wasn't the spectacle the team wanted.

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Just looks like a foggy cloud coming out of it.

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Lots and lots of tiny eggs wafting around.

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It takes a long time. Once they start it doesn't go in one bang.

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It just goes one, two... four, five...one.

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-You wait five minutes and you might get two.

-Watching a stone getting ready to spawn is...

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Some pleasing images, but only a start,

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and not enough to complete the sequence.

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-We're not balanced for that... That's mine!

-I know.

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Tonight is the big one. Trouble is, the wind's got up.

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When the conditions get rough, the corals don't spawn

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because the sperm and the eggs would mix so much.

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There's a possibility things won't happen tonight.

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But, hey. Positive thinking.

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Martha's fears proved right, and despite the hopeful start

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spawning was sparse on the fourth and fifth night,

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making it difficult to catch the spectacle on camera.

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One little coral, and I thought, "I can get an angle on that."

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I was getting into position and it went poof!

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Come back! I didn't get that.

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She had expected the spawning to be more vigorous.

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There was only one day left to complete the filming.

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The scientists were convinced of spawning that night.

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Elation was dampened by another setback.

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Ten minutes ago we were sitting out on the back deck,

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Peter said his camera was dead.

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We've got an hour or so until the spawning starts.

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I just had a letter from the coral expert

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who said tonight's the night.

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I'm hoping it isn't because we've only got one camera.

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I'm sure Mike will do a great job.

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At the last minute, Peter succeeded in repairing his camera.

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The team's patience was rewarded.

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Large areas of the reef spawn together in a spectacular simultaneous display.

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There's mystery in seeing six branching corals go off at the same time.

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They're not connected to each other,

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yet they know they had to spawn at the same time.

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Nobody has worked that one out yet.

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Timing had been everything. Within a few hours

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the team returned to the boat to review the results.

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It didn't look anything like that.

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There!

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When they're coming through...

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Wow! Well done, everybody.

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It's amazing. Thank you very much. I know it's been a struggle.

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ALL: Whoa!

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Can you rewind that?

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-Spikey bits.

-Whoa!

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David Attenborough narrates a natural history of the oceans. This edition explores a force so powerful that it moves oceans - the tides. All over the world tides create opportunities for marine life: giant sting rays glide on currents for a rest, the fastest snail of all surfs waves in pursuit of dinner, minute crabs play football on the beach, racoons binge and, in a unique sequence, dolphins play a deadly game of catch. For in the ocean, each turn of the tide spells life or death, somewhere.


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