0:00:28 > 0:00:29BIRDSONG
0:00:37 > 0:00:42In a far corner of Southeast Asia lies the Coral Triangle.
0:00:46 > 0:00:50A cluster of the richest coral reefs in the world.
0:00:55 > 0:00:59Undersea cities crammed full of life.
0:01:05 > 0:01:08As in any crowded metropolis,
0:01:08 > 0:01:10there is fierce rivalry for space...
0:01:16 > 0:01:17..for food...
0:01:22 > 0:01:23..and for a partner.
0:01:25 > 0:01:29But the reef is also a place full of opportunity.
0:01:42 > 0:01:44A cuttlefish.
0:02:00 > 0:02:04It specialises in hunting crabs.
0:02:12 > 0:02:15But a large crab is a dangerous quarry.
0:02:24 > 0:02:26It has powerful claws.
0:02:30 > 0:02:33The cuttlefish, however, has a remarkable talent.
0:02:39 > 0:02:42Its skin contains millions of pigment cells,
0:02:42 > 0:02:46with which it can create ever-changing colours and patterns.
0:02:56 > 0:03:00And that, apparently, hypnotises the crab.
0:03:22 > 0:03:26A cuttlefish may be clever, but a shark is bigger.
0:03:28 > 0:03:30And it eats cuttlefish.
0:03:33 > 0:03:35Time to disappear.
0:03:51 > 0:03:53Back to the hunt.
0:03:55 > 0:03:59A new target, but the same mesmerising technique.
0:04:16 > 0:04:19For those that manage to establish themselves
0:04:19 > 0:04:21in these bustling undersea cities,
0:04:21 > 0:04:24there can be great rewards.
0:04:44 > 0:04:47Corals build themselves homes of limestone
0:04:47 > 0:04:51in the warm, clear, shallow seas of the tropics.
0:04:55 > 0:04:58Their reefs occupy less than one tenth of 1%
0:04:58 > 0:05:00of the ocean floor.
0:05:04 > 0:05:09Yet they're home to a quarter of all known marine species.
0:05:13 > 0:05:18They are complex, infinitely-varied structures,
0:05:18 > 0:05:23providing all kinds of homes for their many residents.
0:05:26 > 0:05:28From penthouse suites...
0:05:29 > 0:05:31..to backstreet dens.
0:05:37 > 0:05:40Here, on Australia's Great Barrier Reef,
0:05:40 > 0:05:44a coral grouper lives by hunting for small fish.
0:05:50 > 0:05:52But how do you get a meal here
0:05:52 > 0:05:55when you're too big to squeeze into crevices?
0:06:04 > 0:06:07And the grouper also has a rival.
0:06:12 > 0:06:14One which is highly intelligent...
0:06:19 > 0:06:21..and seeks the same kind of prey.
0:06:24 > 0:06:25An octopus.
0:06:47 > 0:06:50It can reach into really narrow cracks.
0:06:58 > 0:07:02Even so, its prey often escapes.
0:07:14 > 0:07:16What if they could work together?
0:07:32 > 0:07:34The grouper turns pale...
0:07:38 > 0:07:41..and tries to attract the octopus' attention.
0:07:47 > 0:07:49It performs a headstand.
0:07:55 > 0:07:59Not only is the grouper signalling to the octopus,
0:07:59 > 0:08:02it's indicating where the prey is hiding.
0:08:15 > 0:08:17The octopus reaches inside.
0:08:21 > 0:08:23The fish take fright...
0:08:26 > 0:08:30..and swim straight into the grouper's jaws.
0:08:38 > 0:08:42Sometimes the octopus gets the reward,
0:08:42 > 0:08:44sometimes the grouper does.
0:08:50 > 0:08:54Two very different species have discovered that teamwork
0:08:54 > 0:08:58can bring success in Reef City.
0:09:15 > 0:09:19Teamwork, in fact, is the very foundation of life on the reef.
0:09:28 > 0:09:31The corals themselves also rely on a partnership.
0:09:33 > 0:09:36But one of a much more intimate kind.
0:09:45 > 0:09:50Corals are colonies of anemone-like animals - polyps.
0:09:50 > 0:09:53Some as small as grains of sand.
0:09:56 > 0:09:59Living inside the tissues of each polyp
0:09:59 > 0:10:02are minute plant-like cells,
0:10:02 > 0:10:04invisible to the naked eye.
0:10:10 > 0:10:12By day, flourishing in tropical sunshine,
0:10:12 > 0:10:18the plant-like cells provide the polyps with up to 90% of their food.
0:10:31 > 0:10:35And when it's dark, the polyps continue to feed
0:10:35 > 0:10:40by using their tentacles to grab edible particles drifting by.
0:10:48 > 0:10:52The polyps also extract calcium carbonate from the seawater
0:10:52 > 0:10:56and use it to build a stony housing for themselves.
0:10:59 > 0:11:05Coral colonies can continue to grow for centuries, possibly millennia.
0:11:09 > 0:11:13And they can build structures that can reach the size of a house.
0:11:28 > 0:11:32The biggest of their cities is the Great Barrier Reef.
0:11:33 > 0:11:36It runs for over 1,400 miles
0:11:36 > 0:11:39along the coast of north-east Australia.
0:11:50 > 0:11:55Activity in coral reefs, wherever they are, never ceases.
0:11:57 > 0:12:00At dawn, the day shift begins,
0:12:00 > 0:12:04accompanied by a chorus of submarine song,
0:12:04 > 0:12:09created by fish, shrimps and other inhabitants of the reef.
0:12:09 > 0:12:12HARMONIOUS CHORUS
0:12:14 > 0:12:16HOOTING
0:12:23 > 0:12:27Every resident in this city has its role.
0:12:27 > 0:12:32Scavengers, like the sea cucumber, recycle the waste of others.
0:12:34 > 0:12:38These parrot fish bite off chunks of coral
0:12:38 > 0:12:40and crunch it to extract the contents.
0:12:53 > 0:12:56And then excrete the rest as sand.
0:13:12 > 0:13:14Green turtles, here in Borneo,
0:13:14 > 0:13:18pay regular visits to a particular patch of coral.
0:13:24 > 0:13:28This little female is up early and one step ahead of the others.
0:13:48 > 0:13:52Turtles travel long distances along the reef in order to get here.
0:13:56 > 0:14:00This is their destination - Turtle Rock.
0:14:07 > 0:14:11Generations of visiting turtles have worn a hollow in its top.
0:14:24 > 0:14:30This is home to blennies and surgeonfish.
0:14:32 > 0:14:35They clean the visitors, picking off any algae,
0:14:35 > 0:14:37parasites and dead skin that they can find.
0:14:42 > 0:14:45Other clients are close behind.
0:14:53 > 0:14:55A queue is forming.
0:15:11 > 0:15:14The big males barge their way in.
0:15:30 > 0:15:33And the smaller female is forced out.
0:15:42 > 0:15:44She takes a quick turn around the block...
0:15:50 > 0:15:52..while the others are squabbling.
0:16:05 > 0:16:07And then she sneaks back in.
0:16:18 > 0:16:22So the cleaners get a nutritious meal
0:16:22 > 0:16:25and their customers are freed of their parasites
0:16:25 > 0:16:27and other encumbrances.
0:16:32 > 0:16:37And it's now thought that a spot of pampering at a cleaning station
0:16:37 > 0:16:39may even reduce stress.
0:16:55 > 0:16:58The lands of the Middle East are so hot and dry
0:16:58 > 0:17:01that much of their surface is almost lifeless.
0:17:05 > 0:17:09But here in the Red Sea, coral reefs flourish wonderfully.
0:17:15 > 0:17:20The waters offshore are almost as rich in life as a rainforest.
0:17:44 > 0:17:49A family of bottlenose dolphins are resting on the reef
0:17:49 > 0:17:51after a night's feeding offshore.
0:17:51 > 0:17:53HIGH-PITCHED WHISTLING
0:18:05 > 0:18:07HIGH-PITCHED WHISTLING
0:18:11 > 0:18:14Whilst the adults and their babies sleep...
0:18:16 > 0:18:19..the adolescents set off to explore.
0:18:23 > 0:18:25And they appear to make up games.
0:18:35 > 0:18:37You might call this one Catch the Coral.
0:18:37 > 0:18:40CLICKING AND WHISTLING
0:18:43 > 0:18:46The rules aren't entirely clear.
0:18:51 > 0:18:53HIGH-PITCHED TRILL
0:19:03 > 0:19:05They pick up different bits of broken coral...
0:19:08 > 0:19:10..and drop them.
0:19:12 > 0:19:13Some fall fast.
0:19:17 > 0:19:19Others sink more slowly.
0:19:37 > 0:19:41And some seem to be descending in a way that pleases everybody.
0:19:41 > 0:19:42RAPID CLICKING
0:19:57 > 0:19:59Only the most successful city residents
0:19:59 > 0:20:02can afford the luxury of playing games.
0:20:04 > 0:20:07But such games do have their value.
0:20:07 > 0:20:11They may help the youngsters develop the coordination and agility
0:20:11 > 0:20:14that they will need when they start hunting in the open sea.
0:20:27 > 0:20:31Every reef has a sharply-defined boundary.
0:20:31 > 0:20:32WAVES CRASH
0:20:32 > 0:20:35Its city walls.
0:20:40 > 0:20:43On the outer side is the drop-off.
0:20:44 > 0:20:47These ramparts protect the city from the ocean waves.
0:20:51 > 0:20:55But twice a day, the walls are covered by the incoming tide.
0:21:05 > 0:21:08In the Bahamas, the rush of the water
0:21:08 > 0:21:10creates a truly strange phenomenon.
0:21:21 > 0:21:25Seamen once told tales of a giant sea monster
0:21:25 > 0:21:28lurking here, deep inside the reef,
0:21:28 > 0:21:31that would drag sailors to their doom.
0:21:50 > 0:21:53Today, we know it is, in fact, a whirlpool,
0:21:53 > 0:21:58created by the incoming tide rushing over deep coral caves.
0:22:04 > 0:22:06These currents bring in fresh supplies
0:22:06 > 0:22:09of microscopic food to the reef from the open ocean.
0:22:14 > 0:22:16And in the Maldives, on the biggest tides,
0:22:16 > 0:22:21one particular coral lagoon becomes so flooded with plankton...
0:22:24 > 0:22:27..that it attracts hundreds of ocean giants.
0:22:30 > 0:22:31Manta rays.
0:22:33 > 0:22:36With three-metre wingspans.
0:22:45 > 0:22:49With their huge, slot-like mouths wide open...
0:22:50 > 0:22:52..they filter out the plankton.
0:23:01 > 0:23:05And the mantas create a vortex of their own
0:23:05 > 0:23:08that further concentrates their food.
0:23:46 > 0:23:50This behaviour has been called the manta cyclone.
0:24:06 > 0:24:10Then the tide changes and the supply of food is cut off.
0:24:14 > 0:24:16The mantas leave the lagoon.
0:24:26 > 0:24:31Life on the sheltered side of the reef is tranquil and peaceful.
0:24:36 > 0:24:39In these suburbs, any creature wishing to escape
0:24:39 > 0:24:43from the bustling crowds of Reef City
0:24:43 > 0:24:45can find plenty of space.
0:24:49 > 0:24:52On the other hand, there is nowhere to hide.
0:25:00 > 0:25:04That, at night, makes it a dangerous place.
0:25:10 > 0:25:16Nocturnal predators, such as this lionfish, patrol the reef edge.
0:25:47 > 0:25:50The hunter has become the hunted.
0:26:10 > 0:26:11A bobbit.
0:26:13 > 0:26:18A giant carnivorous worm with jaws as sharp as daggers.
0:26:22 > 0:26:27It has an ancestry that stretches back more than 400 million years.
0:26:35 > 0:26:37It's a metre long.
0:26:58 > 0:27:00It tastes for scent...
0:27:07 > 0:27:09..and feels for movement.
0:27:54 > 0:27:58Dawn. And with light, the reef becomes a safer place.
0:28:01 > 0:28:04Little bream return to foraging.
0:28:14 > 0:28:16The bobbit might still be hungry...
0:28:19 > 0:28:22..but now, in daylight, the odds have shifted.
0:28:25 > 0:28:28The bream can see the bobbit.
0:28:32 > 0:28:35Instead of retreating,
0:28:35 > 0:28:39they join together to blow away the sand covering the worm.
0:28:45 > 0:28:49So taking away its advantage of surprise.
0:28:55 > 0:28:57By revealing the bobbit's hiding place,
0:28:57 > 0:29:00they will all be able to feed more safely.
0:29:04 > 0:29:06But it pays to remember...
0:29:15 > 0:29:17..there is a bobbit about.
0:29:30 > 0:29:35Out here, on the sand flats, there is safe accommodation for some.
0:29:37 > 0:29:39CLICKING
0:29:44 > 0:29:49A family of saddleback clownfish have found an excellent home.
0:29:52 > 0:29:56The tentacles of this carpet anemone can kill.
0:29:58 > 0:30:01But the clownfish are immune to the poison.
0:30:07 > 0:30:09So they can shelter from danger.
0:30:15 > 0:30:19In return, the fish keep the anemone clean of debris.
0:30:24 > 0:30:28As with all clownfish, the family is ruled by a big female.
0:30:30 > 0:30:33Her white face marks her out as the boss.
0:30:33 > 0:30:36CLICKING
0:30:39 > 0:30:42The diminutive male has to prove his worth,
0:30:42 > 0:30:46so he works tirelessly removing debris
0:30:46 > 0:30:49and generally keeping on top of the housework.
0:31:10 > 0:31:13His greatest challenge is to find a safe place
0:31:13 > 0:31:16where the boss can lay her eggs.
0:31:20 > 0:31:23But there's nothing solid here for the female to lay them on.
0:31:30 > 0:31:31A nearby shell could be the solution.
0:31:43 > 0:31:47If only he can move it to the protection of the anemone.
0:31:54 > 0:31:56Too heavy.
0:32:00 > 0:32:02Besides, it has a mind of its own.
0:32:07 > 0:32:08A hermit crab.
0:32:17 > 0:32:20But out here, twice a day,
0:32:20 > 0:32:23the anemone is swept by tidal currents.
0:32:23 > 0:32:25And they bring in new opportunities.
0:32:28 > 0:32:30An old plastic bottle.
0:32:34 > 0:32:36Perhaps this will do.
0:32:44 > 0:32:46Not heavy enough.
0:32:58 > 0:32:59A coconut shell.
0:33:04 > 0:33:07It looks just right.
0:33:07 > 0:33:10But it's a long way from home.
0:33:28 > 0:33:31And he can't move it by himself.
0:33:42 > 0:33:46So the pair now work together.
0:34:28 > 0:34:30A little adjustment to the anemone's tentacles...
0:34:35 > 0:34:37..and the shell is tucked in.
0:34:43 > 0:34:47The female lays a safe nursery at last.
0:34:50 > 0:34:52CLICKING
0:35:03 > 0:35:07He has proved himself worthy to father her young,
0:35:07 > 0:35:09and he fertilises them.
0:35:14 > 0:35:17Now he will meticulously tend the eggs.
0:35:25 > 0:35:30Keeping them clean and healthy until they hatch in 10 days' time.
0:35:44 > 0:35:47Reef creatures go to great lengths
0:35:47 > 0:35:49to give their young a head start in life.
0:35:58 > 0:36:03And nowhere more so than on the remotest reefs in the world.
0:36:06 > 0:36:11French Polynesia - the very heart of the South Pacific.
0:36:17 > 0:36:20Protected by their isolation,
0:36:20 > 0:36:23some of the reefs here are still virtually pristine.
0:36:33 > 0:36:38This marbled grouper has made it in the city and reached adulthood.
0:36:40 > 0:36:42Now it's the time to mate.
0:36:44 > 0:36:46To find a partner, he must head to
0:36:46 > 0:36:51the most dangerous part of this reef - the drop-off.
0:36:57 > 0:37:00Patrolled by grey reef sharks.
0:37:03 > 0:37:05Hundreds of them.
0:37:11 > 0:37:13They seem to be resting.
0:37:16 > 0:37:18For now.
0:37:27 > 0:37:31Thousands of other groupers have gathered on the seabed below.
0:37:48 > 0:37:51The females are almost bursting with eggs.
0:37:57 > 0:37:59But to mate with one,
0:37:59 > 0:38:03he must first get through the crowd of other waiting males.
0:38:05 > 0:38:08And they all have the same thing on their minds.
0:38:15 > 0:38:19They jostle to get as close as possible to a female.
0:38:47 > 0:38:50This male may have secured pole position,
0:38:50 > 0:38:54but the female won't release her eggs for him to fertilise
0:38:54 > 0:38:57until conditions are just right.
0:39:10 > 0:39:13Now the sharks begin to close in,
0:39:13 > 0:39:17sensing that the critical moment is approaching.
0:39:44 > 0:39:46The tide is beginning to turn.
0:39:49 > 0:39:51This could be the moment to spawn.
0:39:59 > 0:40:03Suddenly, the females rush up towards the surface,
0:40:03 > 0:40:06releasing their clouds of eggs as they go.
0:40:08 > 0:40:10The males pursue them,
0:40:10 > 0:40:12simultaneously releasing their sperm.
0:40:28 > 0:40:31It's an act the groupers seem prepared to die for.
0:40:36 > 0:40:38It's now...
0:40:38 > 0:40:39or never.
0:41:16 > 0:41:20Their fertilised eggs will now be swept away...
0:41:24 > 0:41:28..from the many hungry mouths of the reef-dwellers.
0:41:46 > 0:41:51Most of the billions of eggs that cloud the sea will be eaten.
0:41:53 > 0:41:57But a few, a tiny but crucial minority,
0:41:57 > 0:42:01will find another reef and make it their home.
0:42:15 > 0:42:20But today's coral reefs are facing a new threat.
0:42:24 > 0:42:26The seas are warming.
0:42:30 > 0:42:35A rise in temperature of one or two degrees, for just a few weeks,
0:42:35 > 0:42:38can be enough to cause the coral polyps
0:42:38 > 0:42:41to eject their plant-like cells.
0:42:49 > 0:42:53When that happens, the corals lose both their colour
0:42:53 > 0:42:55and their main source of food.
0:43:03 > 0:43:07If the high temperatures are sustained, coral -
0:43:07 > 0:43:11bleached in this way - is likely to die.
0:43:17 > 0:43:21In recent years, it's thought that half the world's coral reefs
0:43:21 > 0:43:24have been affected by bleaching.
0:43:30 > 0:43:36Including, since 2016, around two thirds of the shallow water corals
0:43:36 > 0:43:40on Australia's Great Barrier Reef.
0:43:52 > 0:43:56These once-crowded submarine cities...
0:43:57 > 0:44:01..are reduced to bleak ruins...
0:44:03 > 0:44:07..and many of their inhabitants left homeless.
0:44:21 > 0:44:25Some scientists predict that by the end of the century,
0:44:25 > 0:44:32coral reef cities as we know them could be a thing of the past.
0:44:48 > 0:44:53Is there any future for these most precious of ocean treasures?
0:45:01 > 0:45:06Well, that ultimately depends on how fast they heat up,
0:45:06 > 0:45:09and how warm the seas become.
0:45:12 > 0:45:15And there is a glimmer of hope,
0:45:15 > 0:45:19because of the way that corals reproduce themselves.
0:45:25 > 0:45:30On one special night of the year, the full moon triggers
0:45:30 > 0:45:33an extraordinary event.
0:45:50 > 0:45:53The spawning of the coral.
0:46:02 > 0:46:06With extraordinary synchrony, entire reefs reproduce.
0:46:23 > 0:46:26Billions of fertilised eggs drift away...
0:46:29 > 0:46:31..carried by the ocean currents.
0:46:37 > 0:46:40And it's not just the corals that spawn.
0:46:42 > 0:46:46So do many of the other residents of the reef.
0:46:52 > 0:46:56A whole range of young are swept through the oceans,
0:46:56 > 0:47:01ready to settle on a vacant site and bring back into existence
0:47:01 > 0:47:05the complex community that is a coral reef.
0:47:17 > 0:47:21We may not know what the future holds for our seas...
0:47:24 > 0:47:28..but coral reefs can regenerate.
0:47:28 > 0:47:32As long as some reefs survive...
0:47:32 > 0:47:35some hope can remain.
0:48:03 > 0:48:05French Polynesia.
0:48:11 > 0:48:15Over 4,000 miles from the nearest major landmass.
0:48:17 > 0:48:20Protected from fishing pressures, it's home to perhaps the last
0:48:20 > 0:48:24great gathering of marbled groupers.
0:48:24 > 0:48:26We're finally here.
0:48:26 > 0:48:28It's taken quite a few days.
0:48:28 > 0:48:32We can't actually say where here is, because we've been asked not
0:48:32 > 0:48:36to mention the exact spot, because it's a protected area.
0:48:37 > 0:48:42The team face a huge filming challenge, as the groupers
0:48:42 > 0:48:46normally spawn just once a year and it lasts less than an hour.
0:48:49 > 0:48:53The team's adviser is biologist Yvonne Sadovy, who's been
0:48:53 > 0:48:58studying fish spawning aggregations for over 30 years.
0:48:58 > 0:49:02Here is as close to the situation it has been for thousands of years.
0:49:02 > 0:49:07It's pretty much the only place on the planet that I'm aware of
0:49:07 > 0:49:11where these groupers come together in the numbers that we see here.
0:49:15 > 0:49:17I really hope that they're there,
0:49:17 > 0:49:21because if they're not, then we've come all this way for nothing.
0:49:21 > 0:49:23We've got our timing right, haven't we, Denis?
0:49:23 > 0:49:26- Yes.- Confident?- Yeah.
0:49:33 > 0:49:34..three, four.
0:49:44 > 0:49:47They're not disappointed.
0:49:52 > 0:49:56Denis has filmed groupers gathering here for 15 years,
0:49:56 > 0:49:58and has got to know their behaviour.
0:50:11 > 0:50:14The grouper aren't the only ones to be gathering.
0:50:18 > 0:50:23It's the highest density of grey reef sharks anywhere on the planet.
0:50:28 > 0:50:32This is what a pristine reef should look like.
0:50:32 > 0:50:34Lots and lots of predators.
0:50:36 > 0:50:38With the groupers here,
0:50:38 > 0:50:41now the challenge is predicting exactly when they'll spawn.
0:50:44 > 0:50:49To judge if they're close, Yvonne reviews the day's footage.
0:50:49 > 0:50:52From what we've seen here, the male's courting the female.
0:50:52 > 0:50:55When you can slow these things down, this is where you begin to see
0:50:55 > 0:50:57- some of this detail.- Yeah. - If you were a betting woman,
0:50:57 > 0:50:59would you bet on tomorrow or the second?
0:50:59 > 0:51:01I would say tomorrow or the next day.
0:51:01 > 0:51:03I mean, I think they're pretty close.
0:51:03 > 0:51:08With only six divers, the team can't stay underwater all the time.
0:51:08 > 0:51:12So, based on experience, they decide to dive on the full moon's
0:51:12 > 0:51:17changing tides - thought to help co-ordinate grouper spawning.
0:51:18 > 0:51:22But this year, the strongest tidal change is at night.
0:51:24 > 0:51:27We're just kitting up for the first night dive.
0:51:27 > 0:51:29During the day, we've been seeing sharks around this area,
0:51:29 > 0:51:31but we don't know how they're going to react at night.
0:51:31 > 0:51:33Are you nervous?
0:51:40 > 0:51:45Reef sharks hunt at night, so the divers wear chainmail suits
0:51:45 > 0:51:47and helmets as a precaution.
0:51:50 > 0:51:53One, two, three, four.
0:52:01 > 0:52:03Divers, comms check, comms check.
0:52:03 > 0:52:05RADIO CRACKLES
0:52:09 > 0:52:11Receiving you loud and clear.
0:52:11 > 0:52:13RADIO CRACKLES
0:52:16 > 0:52:19No sign of grouper spawning, but in the dark,
0:52:19 > 0:52:23the shark's attitude has changed.
0:52:23 > 0:52:26They're hunting.
0:52:26 > 0:52:30Using electroreception to hunt, sharks are tuned to even
0:52:30 > 0:52:34the smallest electrical stimulus - including the cameras.
0:52:34 > 0:52:36HE GASPS
0:52:36 > 0:52:37MUFFLED SPEECH
0:52:42 > 0:52:46With the current building, now is the worst possible time to lose
0:52:46 > 0:52:48contact with the surface.
0:52:51 > 0:52:53We're in the middle of nowhere. We have no idea what's
0:52:53 > 0:52:56going on down there. All we can hear is broken communication.
0:52:56 > 0:52:58RADIO CRACKLES
0:53:11 > 0:53:14With no sign of spawning, they end the dive.
0:53:20 > 0:53:22HE SIGHS
0:53:45 > 0:53:49The team decide to save their energy for the next change of the tide.
0:53:58 > 0:54:03At dawn, they prepare for what they hope will be the spawning
0:54:03 > 0:54:05event they've all been waiting for.
0:54:18 > 0:54:21- Bad news, it's...- What?
0:54:21 > 0:54:25But after just 20 minutes, the divers are back on the surface
0:54:25 > 0:54:27with some devastating news for the crew.
0:54:30 > 0:54:33All the groupers have gone?
0:54:33 > 0:54:34- It happened yesterday.- We missed it.
0:54:34 > 0:54:37Maybe they spawned last night. I don't...
0:54:37 > 0:54:39Oh, God, I... I don't know.
0:54:42 > 0:54:44The team were too late.
0:54:49 > 0:54:53So my guess would be that we missed spawning by a few hours and possibly
0:54:53 > 0:54:56it's that they just reached some kind of critical mass
0:54:56 > 0:54:59or critical density and spawned a little bit early.
0:55:09 > 0:55:13The following year, Yvonne returns to French Polynesia,
0:55:13 > 0:55:16hoping to learn more about the grouper.
0:55:22 > 0:55:24This year, Laurent Ballesta
0:55:24 > 0:55:27and his experienced crew take on the challenge.
0:55:30 > 0:55:33With many more divers, they have a new approach,
0:55:33 > 0:55:37and they plan to have Laurent on watch in the water constantly.
0:56:00 > 0:56:04And this year, after hundreds of hours of continual monitoring,
0:56:04 > 0:56:09they have captured the defining moment of this astonishing event.
0:56:10 > 0:56:13The mass spawning of the grouper.
0:56:16 > 0:56:20We have the privilege to be able to watch this process
0:56:20 > 0:56:23actually occurring and that, that's a real thrill.
0:56:23 > 0:56:26They're really important for the biology of the animals.
0:56:42 > 0:56:46Thanks to a combined team effort from both crews,
0:56:46 > 0:56:48and Yvonne's research over many years,
0:56:48 > 0:56:53we could at last tell the marbled grouper's remarkable story.
0:57:00 > 0:57:05Next time, we head into the vastness of the open ocean.
0:57:07 > 0:57:12To survive in this blue wilderness, some are fast,
0:57:12 > 0:57:16others use deadly strategies,
0:57:16 > 0:57:20and a few rely on the closeness of their families.
0:57:23 > 0:57:26To find out more about our oceans
0:57:26 > 0:57:29with this free poster, call...
0:57:30 > 0:57:32..or go to...
0:57:35 > 0:57:39..and follow the links to the Open University.