0:00:05 > 0:00:09Off the east coast of Australia, up to 200 kilometres offshore,
0:00:09 > 0:00:14a line of surf rises out of the open ocean.
0:00:19 > 0:00:22Beneath the surface, creating these breaking waves,
0:00:22 > 0:00:26is the most magical marine environment on Earth.
0:00:26 > 0:00:30I'm Monty Halls and I'm a marine biologist and diver.
0:00:30 > 0:00:33And I've always been fascinated by the sea.
0:00:35 > 0:00:38For me, there's always been one place that is the epitome
0:00:38 > 0:00:43of everything that's wonderful about the marine environment.
0:00:43 > 0:00:46It's a true global icon and is a Mecca for anyone
0:00:46 > 0:00:49who's ever heard the word coral...
0:00:49 > 0:00:51- HE SHOUTS - The Great Barrier Reef!
0:01:00 > 0:01:05Stretching for over 2,000km up the tropical coast of Australia,
0:01:05 > 0:01:10it is quite simply the largest living structure on the planet.
0:01:21 > 0:01:25It's made up of almost 3,000 different reefs,
0:01:25 > 0:01:28each one with a different personality.
0:01:31 > 0:01:36And it's so much more than just coral.
0:01:53 > 0:01:58But the Great Barrier Reef is most famous for its underwater world.
0:01:59 > 0:02:03Here, there is more life than almost anywhere else on Earth.
0:02:06 > 0:02:10Many of the creatures are exquisitely beautiful
0:02:10 > 0:02:14but some seem straight out of science fiction.
0:02:21 > 0:02:24While others can kill in an instant.
0:02:30 > 0:02:34And just when you think you know the reef, it changes.
0:02:34 > 0:02:38Every second, every hour, every day and every year.
0:02:38 > 0:02:42It's a world of continual surprises.
0:02:50 > 0:02:57This is the amazing story of an ever-changing natural miracle,
0:02:57 > 0:02:59the Great Barrier Reef.
0:03:07 > 0:03:11The Great Barrier Reef is so large that it can be seen from space -
0:03:11 > 0:03:17quite an achievement, considering the size of the creatures that built it.
0:03:22 > 0:03:26The vast system that is the Great Barrier Reef
0:03:26 > 0:03:29is created by animals that are tiny.
0:03:29 > 0:03:32Although they might look like plants,
0:03:32 > 0:03:35actually they're animals called polyps.
0:03:36 > 0:03:39Each polyp is like a tiny upside-down jellyfish
0:03:39 > 0:03:43sitting in a stony cup.
0:03:49 > 0:03:53They live together in colonies, like underwater tower blocks.
0:03:59 > 0:04:03They respond to touch, temperature, currents
0:04:03 > 0:04:06and the cycles of the sun and moon,
0:04:06 > 0:04:10and their tiny movements combine to give each colony
0:04:10 > 0:04:11a rhythm of its own.
0:04:45 > 0:04:47The polyps can't build the reef alone.
0:04:47 > 0:04:51For this they need partners that are even smaller.
0:04:54 > 0:04:58Within each polyp's tentacles are millions of tiny brown dots.
0:05:02 > 0:05:04Each is a microscopic plant
0:05:04 > 0:05:08which transforms sunlight into food and energy for the corals.
0:05:12 > 0:05:15It's a miraculous partnership that allows the corals
0:05:15 > 0:05:18to turn minerals in the water into limestone,
0:05:18 > 0:05:21building their stony skeletons.
0:05:23 > 0:05:26In this way, each colony grows.
0:05:33 > 0:05:36If we were able to watch a reef over several years,
0:05:36 > 0:05:41we'd see a continually-growing marine metropolis.
0:05:41 > 0:05:46Its intricate architecture provides homes for thousands of creatures.
0:05:54 > 0:05:58Even though coral reefs cover less than one percent
0:05:58 > 0:06:00of the world's oceans,
0:06:00 > 0:06:03they contain a quarter of all known marine life.
0:06:03 > 0:06:07And the Great Barrier Reef is the biggest of them all.
0:06:09 > 0:06:14I still vividly remember my first dive on a coral reef
0:06:14 > 0:06:16over 20 years ago.
0:06:22 > 0:06:25And the moment I put my head in the water,
0:06:25 > 0:06:28I knew what I wanted to do for the rest of my life.
0:06:34 > 0:06:36HE LAUGHS
0:06:38 > 0:06:41Even if I spent my entire life underwater,
0:06:41 > 0:06:47I still couldn't hope to see all the species that live on this reef.
0:06:47 > 0:06:52400 species of hard coral, 300 of soft coral,
0:06:52 > 0:06:551,600 species of fish,
0:06:55 > 0:07:00134 species of shark and ray,
0:07:00 > 0:07:0330 species of whale and dolphin,
0:07:03 > 0:07:05six species of turtle,
0:07:05 > 0:07:0814 species of sea snake...
0:07:10 > 0:07:12..3,000 species of mollusc,
0:07:12 > 0:07:161,300 species of crustacean...
0:07:16 > 0:07:20The sheer number of creatures that live here makes the reef as hectic
0:07:20 > 0:07:25as any human city, with a rhythm of almost perpetual rush hours.
0:08:04 > 0:08:10Imagine all this activity, this constant ebb and flow of life,
0:08:10 > 0:08:13continuing for over 2,000km.
0:08:24 > 0:08:28It's hard to believe that such a huge solid structure
0:08:28 > 0:08:31hasn't always been here,
0:08:31 > 0:08:37but compared with the rest of Australia this reef is very young.
0:08:37 > 0:08:4120,000 years ago this exact spot where I'm standing,
0:08:41 > 0:08:44and indeed the entire length of the Great Barrier Reef,
0:08:44 > 0:08:46would have been high and dry.
0:08:46 > 0:08:50Back then, in the middle of the last ice age,
0:08:50 > 0:08:55the sea level was 120 metres lower than it is today,
0:08:55 > 0:08:59and the coastline was about 30km in that direction.
0:08:59 > 0:09:05But suppose I could go back those 20,000 years in an instant...
0:09:05 > 0:09:08What would it have looked like?
0:09:08 > 0:09:11Well, for a start, it wasn't even underwater.
0:09:11 > 0:09:15This area would have been covered in hundreds and hundreds of miles
0:09:15 > 0:09:17of eucalyptus and paper-bark forest
0:09:17 > 0:09:20and there wouldn't have been a fish in sight.
0:09:29 > 0:09:33It was a land abounding with the animals of the Australian plains.
0:09:41 > 0:09:44Aboriginal people would've lived here too,
0:09:44 > 0:09:48hunting the wildlife in areas that are now deep under water.
0:09:55 > 0:09:59Then at the end of the last ice age, ice at the poles melted
0:09:59 > 0:10:02and sea levels rose all around the world,
0:10:02 > 0:10:04flooding this low-lying coast.
0:10:06 > 0:10:09This happened only 10,000 years ago.
0:10:11 > 0:10:13In fact, stories of The Great Flood
0:10:13 > 0:10:16are still passed down in aboriginal culture.
0:10:22 > 0:10:24As the sea rose,
0:10:24 > 0:10:28corals began to grow on the rocky fringes of the continental shelf,
0:10:28 > 0:10:31creating the Great Barrier Reef we see today.
0:10:35 > 0:10:39These shallow tropical waters are clear and warm...
0:10:39 > 0:10:42perfect conditions for corals to thrive.
0:10:58 > 0:11:02Sheltered behind this long strip of reef, a lagoon was born.
0:11:02 > 0:11:06An area of protected water larger in size
0:11:06 > 0:11:09than the whole of Great Britain.
0:11:09 > 0:11:14And a new coastline, too, with shallow sandy waters.
0:11:23 > 0:11:26The rising sea also cut off areas of high ground,
0:11:26 > 0:11:30creating the 600 islands that dot the lagoon.
0:11:34 > 0:11:37Some are little more than rocks.
0:11:40 > 0:11:44Others substantial mountains covered in woodland.
0:11:46 > 0:11:50The first British person to see that was Captain Cook,
0:11:50 > 0:11:52who stood here in 1770,
0:11:52 > 0:11:56and for the first time realised the scale of the Great Barrier Reef.
0:11:56 > 0:12:00He also realised the scale of the problem that faced him.
0:12:00 > 0:12:04For Cook and his men, without the benefit of modern charts and sonar,
0:12:04 > 0:12:08it represented nothing more than a deadly labyrinth.
0:12:13 > 0:12:19But it's exactly this complexity and the sheer size of the reef
0:12:19 > 0:12:22that has created so many opportunities for life.
0:12:27 > 0:12:30There are fish of almost every imaginable kind -
0:12:30 > 0:12:33coral eaters, plant eaters, plankton eaters,
0:12:33 > 0:12:36the hunters and the hunted.
0:12:44 > 0:12:49Bluefin trevally -
0:12:49 > 0:12:52powerful predators that hunt in packs.
0:12:54 > 0:12:58One minute apparently minding their own business,
0:12:58 > 0:13:01the next charging their prey with a sudden rush.
0:13:06 > 0:13:09Working together, they create confusion.
0:13:25 > 0:13:29The trevally depart as quickly as they arrived,
0:13:29 > 0:13:33and the colourful reef fish regroup again to feed.
0:13:42 > 0:13:46Beyond them, silver baitfish never let down their guard.
0:13:46 > 0:13:49They shimmer like a thousand tiny mirrors,
0:13:49 > 0:13:53swirling as one amorphous mass to confuse any attackers.
0:14:00 > 0:14:04It is their only defence against this -
0:14:04 > 0:14:06a shark mackerel.
0:14:08 > 0:14:11These are the greyhounds of the ocean.
0:14:11 > 0:14:14They need all their speed and agility against this ghostly school
0:14:14 > 0:14:18that is here one moment and gone the next.
0:14:54 > 0:14:56Despite the dangers,
0:14:56 > 0:15:01small fish have to risk leaving the protection of the reef to feed.
0:15:04 > 0:15:08And perhaps none is bolder than this little wrasse.
0:15:13 > 0:15:17This gigantic grouper is many thousands of times her size
0:15:17 > 0:15:20and an aggressive territorial predator
0:15:20 > 0:15:23with one of the largest mouths on the reef.
0:15:28 > 0:15:33But she's not deterred from approaching.
0:15:39 > 0:15:43What she's about to do seems almost suicidal.
0:16:13 > 0:16:17She's feeding on tiny blood-sucking parasites,
0:16:17 > 0:16:20which is why the grouper allows this.
0:16:25 > 0:16:30Nevertheless, the wrasse has to regularly vibrate her fins
0:16:30 > 0:16:32against the inside of his mouth
0:16:32 > 0:16:34just to remind him not to swallow.
0:16:44 > 0:16:49A single wrasse can eat an incredible 1,200 parasites a day.
0:16:49 > 0:16:53And she's not fussy where she finds them.
0:17:03 > 0:17:06Without this decontamination
0:17:06 > 0:17:09the grouper would quickly become infested.
0:17:12 > 0:17:16This service is so valuable that the grouper is a regular client,
0:17:16 > 0:17:18visiting several times a day.
0:17:20 > 0:17:23But even he has his limits.
0:17:28 > 0:17:30On the reef there seem to be
0:17:30 > 0:17:33an almost infinite variety of ways of feeding.
0:17:33 > 0:17:36Some fish even cultivate their own food,
0:17:36 > 0:17:39so they're very protective of it.
0:17:45 > 0:17:48There's one fish species that's so good at defending
0:17:48 > 0:17:50it's particular patch of reef
0:17:50 > 0:17:55that it's responsible for more attacks on divers than any other.
0:17:55 > 0:17:59And that's this species here.
0:17:59 > 0:18:05This is a white damsel, and it's a constant gardener,
0:18:05 > 0:18:09nurturing and caring for a patch of algae.
0:18:11 > 0:18:14Much of the algae growing on the sandy seabed is edible,
0:18:14 > 0:18:18so the damsel farms it in green patches,
0:18:18 > 0:18:21encouraging some species, nipping down others,
0:18:21 > 0:18:24and gently transforming the landscape around her.
0:18:32 > 0:18:36After all this hard work, she doesn't look kindly on trespassers.
0:18:42 > 0:18:45Anything that that swims too close is chased away.
0:19:04 > 0:19:08But is she brave enough to try to see me off?
0:19:08 > 0:19:14I'm just going to edge onto its patch a little bit more.
0:19:14 > 0:19:16There could be trouble, I sense.
0:19:19 > 0:19:21Oh-ho-ho!
0:19:21 > 0:19:25She can't scare me but she just doesn't give up.
0:19:25 > 0:19:26Just listen to this.
0:19:26 > 0:19:31AGGRESSIVE CLICKING
0:19:33 > 0:19:36This is one feisty little fish.
0:19:36 > 0:19:41Even though I'm so much bigger than this damsel fish,
0:19:41 > 0:19:46it's still quite an intimidating sight.
0:19:46 > 0:19:49Oh! That was close.
0:19:49 > 0:19:55While damsels maintain areas of reef by nurturing, other fish destroy it.
0:19:56 > 0:20:01Bumphead parrotfish, each the size of a small sheep.
0:20:03 > 0:20:05They have a tough beak
0:20:05 > 0:20:09and gigantic jaw muscles that work like bolt cutters.
0:20:21 > 0:20:25It might seem like a lot of hardware for a diet of soft algae
0:20:25 > 0:20:28and coral polyps, but the only way to get at them
0:20:28 > 0:20:31is by sheering off chunks of the coral rock.
0:20:39 > 0:20:42A second set of internal teeth
0:20:42 > 0:20:44then grinds the coral into a fine paste.
0:20:54 > 0:20:58A single bumphead can chew up to five tonnes of coral every year,
0:20:58 > 0:21:02which they excrete back onto the reef as sand.
0:21:05 > 0:21:10As fast as the reef grows, parrotfish break it down again,
0:21:10 > 0:21:14creating a continually-changing environment.
0:21:15 > 0:21:19The constant remodelling of the reef by ravenous parrotfish
0:21:19 > 0:21:22is nothing compared to the effect of the ocean.
0:21:36 > 0:21:39This is the reef crest
0:21:39 > 0:21:44and it's the point where the waves generated by the open ocean
0:21:44 > 0:21:48impact the hard surface of the coral reef.
0:21:52 > 0:21:56It's an unstoppable force hitting an immovable object.
0:21:58 > 0:22:03I'm keenly aware that these are very small waves.
0:22:03 > 0:22:07Believe me, in a big storm, to be here would be suicide.
0:22:07 > 0:22:14But it's a vivid illustration of the power of the sea.
0:22:14 > 0:22:17Oh-ho-ho!
0:22:21 > 0:22:22Tropical storms and cyclones
0:22:22 > 0:22:25regularly whip in from the Pacific Ocean.
0:22:31 > 0:22:36The surf smashes into the reef, pulverising the coral below.
0:22:38 > 0:22:41Waves and currents move the rubble and sand around
0:22:41 > 0:22:46and shape it into beautiful small islands...
0:22:48 > 0:22:51..known as coral cays.
0:23:10 > 0:23:13Some come and go in a matter of days.
0:23:13 > 0:23:16Others build and grow and can last for years,
0:23:16 > 0:23:19as plants take root and protect them from the elements.
0:23:38 > 0:23:41At first glance it might seem that a sand cay
0:23:41 > 0:23:43is a pretty inhospitable environment
0:23:43 > 0:23:47but actually it can support life in abundance.
0:23:50 > 0:23:54Today it's rare to find a cay free from humans
0:23:54 > 0:23:58and mainland predators, but this is one of the best -
0:23:58 > 0:24:00Raine Island.
0:24:15 > 0:24:17It's home to thousands of birds
0:24:17 > 0:24:21and in the centre of the island they jostle for the best available space.
0:24:25 > 0:24:28Nowhere else on the reef are they found in such numbers,
0:24:28 > 0:24:31and such variety.
0:24:31 > 0:24:36Frigate Birds, red-footed boobies and Caspian terns.
0:24:38 > 0:24:41In fact, 84 different species are found here.
0:24:43 > 0:24:49It's also the largest green turtle breeding ground in the world.
0:24:49 > 0:24:51Having mated offshore,
0:24:51 > 0:24:55thousands of females haul themselves up the beach to lay their eggs,
0:24:55 > 0:25:00The record is 26,000 turtles in a single night.
0:25:00 > 0:25:07They turn a remote desert island into a crowded frenzy of nocturnal activity.
0:25:31 > 0:25:34Wind and waves have created an island that,
0:25:34 > 0:25:36for the moment at least,
0:25:36 > 0:25:40has just the right conditions to support this incredible spectacle.
0:25:42 > 0:25:46But the island still changes, every year, every season.
0:25:46 > 0:25:50And the reef itself never stays the same for long.
0:25:51 > 0:25:56Twice a day, life along the length of the Great Barrier Reef
0:25:56 > 0:25:58has to cope with complete upheaval,
0:25:58 > 0:26:03as the rhythm of the moon causes tides to flood and drain the reef.
0:26:41 > 0:26:44During the lowest tides, the water runs off,
0:26:44 > 0:26:46and whole sections of living coral,
0:26:46 > 0:26:49that would normally be underwater, are exposed.
0:27:09 > 0:27:13The level of water on the reef top decreases dramatically,
0:27:13 > 0:27:15getting ever shallower
0:27:15 > 0:27:21and going from a stable environment to an ever changing, hostile one.
0:27:21 > 0:27:24It's almost as though some mighty force has lifted the reef
0:27:24 > 0:27:26right out of the water.
0:27:28 > 0:27:30These are extreme conditions.
0:27:36 > 0:27:39As the exposed corals start to heat up,
0:27:39 > 0:27:43the tiny polyps retract inside their stony skeletons for protection.
0:27:46 > 0:27:49Then they secrete mucous. It acts as a sunscreen,
0:27:49 > 0:27:53and there's no skimping on quantity.
0:27:53 > 0:27:57An area no larger than the size of a coffee table can produce
0:27:57 > 0:27:59five litres of the stuff.
0:28:07 > 0:28:09As the tide retreats to the edge of the reef,
0:28:09 > 0:28:11it cuts off pools of water.
0:28:15 > 0:28:19For most animals, like these sea cucumbers and starfish,
0:28:19 > 0:28:23the best way to survive is by taking refuge in these rock pools.
0:28:36 > 0:28:39With the tropical sun beating down,
0:28:39 > 0:28:43the exposed flat is one of the most hostile environments on the reef.
0:28:43 > 0:28:48For a fish caught out here it should mean certain death.
0:28:52 > 0:28:54But not for this one.
0:28:56 > 0:29:02It's an epaulette shark. It can't breathe out of water.
0:29:02 > 0:29:05But it survives here by shutting down some parts of its brain
0:29:05 > 0:29:08and increasing the blood supply to others.
0:29:12 > 0:29:16Not only that, it can walk on land.
0:29:19 > 0:29:23These abilities make it a specialised reef-top hunter.
0:29:33 > 0:29:36Manoeuvring around the confined space of a rock pool,
0:29:36 > 0:29:40it's master of all it surveys.
0:29:42 > 0:29:46Nothing is safe. Not even animals hiding under the sand.
0:29:51 > 0:29:53It scans for smells,
0:29:53 > 0:29:55and for tiny electric signals given off
0:29:55 > 0:29:59by the bodies of hidden creatures.
0:30:05 > 0:30:07It's detected a crab.
0:30:10 > 0:30:16It doesn't need sharp teeth because it simply sucks up its prey.
0:30:24 > 0:30:28The epaulette isn't the only shark able to adapt
0:30:28 > 0:30:30to a change of the tides.
0:30:40 > 0:30:44When the sea returns, it brings cool oxygenated water back to the reef.
0:30:56 > 0:31:01The relief, though, is short-lived.
0:31:08 > 0:31:10Larger sharks gather at the reef's edge,
0:31:10 > 0:31:14waiting to get at the freshly accessible hunting grounds.
0:31:22 > 0:31:24As the water rises,
0:31:24 > 0:31:28more and more creatures take the opportunity to feed in a new area.
0:31:42 > 0:31:45For the sharks, fish that can normally outrun them
0:31:45 > 0:31:48are temporarily caught in the shallows,
0:31:48 > 0:31:52trapped between sand and surface, with few places to hide.
0:32:05 > 0:32:08Lemon sharks hug the shore, surrounded by schooling bait fish,
0:32:08 > 0:32:10But, for the moment,
0:32:10 > 0:32:13they seem to have little interest in all this food around them.
0:32:18 > 0:32:21But they're simply waiting for help.
0:32:28 > 0:32:30Young trevally.
0:32:30 > 0:32:33They dart for the fish, causing them to panic and break ranks.
0:32:38 > 0:32:40By putting themselves in the midst of the shoal,
0:32:40 > 0:32:45the sharks are in a perfect position to exploit the chaos.
0:33:30 > 0:33:35For a hungry lemon shark, no water seems too shallow.
0:33:39 > 0:33:43The sharks are able to use these special tactics to feed inshore
0:33:43 > 0:33:46because of the rhythm of the tides,
0:33:46 > 0:33:51which change conditions every few hours.
0:33:54 > 0:33:58The cycle of day and night also has a dramatic effect
0:33:58 > 0:34:01on everything that lives on the reef.
0:34:33 > 0:34:37There are many enduring mysteries about the Great Barrier Reef,
0:34:37 > 0:34:41particularly at night, but this is one of the more enchanting ones.
0:34:41 > 0:34:43Watch this.
0:34:50 > 0:34:54This coral is fluorescing under ultraviolet light.
0:34:55 > 0:34:58It's this amazing light-show,
0:34:58 > 0:35:01and it covers pretty much the whole of the reef crest.
0:35:03 > 0:35:07Look at this, Look at this! Vivid, vivid colours!
0:35:07 > 0:35:09Doesn't look real,
0:35:09 > 0:35:16looks like some bizarre galactic broccoli is what it looks like.
0:35:20 > 0:35:22One theory is that these magical colours are
0:35:22 > 0:35:25in some way caused by the coral's natural sunscreen,
0:35:25 > 0:35:29but, despite lots of very clever people looking into it,
0:35:29 > 0:35:31no-one really knows why this happens.
0:35:35 > 0:35:39And the same goes for much of the night-time activity here.
0:36:29 > 0:36:33A coral reef during the day is a very different place
0:36:33 > 0:36:35from a coral reef at night.
0:36:35 > 0:36:38The atmosphere changes completely.
0:36:38 > 0:36:43The day shift has logged off and the night shift has gone to work.
0:36:44 > 0:36:47It's mean and moody and the animals here mean business.
0:36:54 > 0:36:58Just as in human cities, there's a new cast of characters after dark.
0:37:02 > 0:37:06Octopus emerge from their holes to stalk their prey.
0:37:12 > 0:37:16A tiny sole improves his chances of survival
0:37:16 > 0:37:18by mimicking a toxic flatworm.
0:37:20 > 0:37:23Some of the strangest nocturnal creatures are relatives
0:37:23 > 0:37:26of the starfish, like this feather-mouthed sea cucumber,
0:37:26 > 0:37:28sifting food from the sand.
0:37:32 > 0:37:35Another is the basket star.
0:37:35 > 0:37:38It comes alive at night, throwing out thousands of arms
0:37:38 > 0:37:41to filter food from the plankton-rich current.
0:37:49 > 0:37:54These animals can live for a very long time - up to 35 years.
0:37:54 > 0:37:58There's records of these guys being seen night after night,
0:37:58 > 0:38:01in the same place, for over 15 years.
0:38:05 > 0:38:08The day shift is still here, of course, it's just hidden.
0:38:11 > 0:38:14But it's difficult to find somewhere big enough to hide
0:38:14 > 0:38:16if you are the size of turtle.
0:38:19 > 0:38:26This green turtle has found himself a little ledge
0:38:26 > 0:38:29and is tucking in for the evening.
0:38:29 > 0:38:32Hello, big fella.
0:38:41 > 0:38:44He's a bit drowsy, and probably quite grumpy.
0:38:44 > 0:38:47We all get like that occasionally, don't we?
0:38:49 > 0:38:53I'll leave him alone. A lovely sight Sleep well big fella.
0:38:55 > 0:39:01At night many fish sleep, resting in safe nooks and crannies on the reef.
0:39:01 > 0:39:04Without eyelids, they enter a trance-like state,
0:39:04 > 0:39:09barely moving other than to keep water flowing through their gills,
0:39:12 > 0:39:16But they do give off a scent, and that means they can be found.
0:39:25 > 0:39:28This cone snail catches its prey
0:39:28 > 0:39:33in one of the most surprising ways imaginable,
0:39:33 > 0:39:35and specialises in killing fish.
0:39:43 > 0:39:45It sniffs them out whilst they are sleeping.
0:39:51 > 0:39:55The goatfish may be asleep but it's aware of its surroundings.
0:39:58 > 0:40:00First, the snail must sneak close enough
0:40:00 > 0:40:03to bring its secret weapons into play.
0:40:05 > 0:40:09The snail appears to release chemicals that paralyses its victim.
0:40:21 > 0:40:27The goatfish seems powerless to escape, as it's swallowed alive.
0:40:35 > 0:40:36Moments later,
0:40:36 > 0:40:41a venomous barb inside the snail puts the fish out of its misery.
0:40:48 > 0:40:51In a place where even seashells are deadly,
0:40:51 > 0:40:56remaining undetected is essential if you want to survive the night.
0:40:58 > 0:41:01And simply hiding is not enough,
0:41:01 > 0:41:05just a hint of your scent, wafting into the current, can attract trouble.
0:41:08 > 0:41:12The parrotfish has come up with an elegant solution
0:41:12 > 0:41:16in the form of a sleeping bag made out of membrane.
0:41:20 > 0:41:24It stops the smell spreading along the reef and being picked up.
0:41:24 > 0:41:26It's a very good way, if you're a parrotfish,
0:41:26 > 0:41:28of ensuring you get a good night's sleep.
0:41:30 > 0:41:34No-one has ever filmed a parrotfish making its cocoon before.
0:41:36 > 0:41:39It starts by exuding a layer of mucous around its body.
0:41:46 > 0:41:50It then draws water into its mouth and pushes it out of its gills,
0:41:50 > 0:41:53blowing up the mucous bubble around it.
0:42:09 > 0:42:13Within half-an-hour, the fish is completely enveloped
0:42:13 > 0:42:17in a slimy cocoon, which traps its smell.
0:42:21 > 0:42:24But not all the reef hunters are fooled.
0:42:31 > 0:42:35White tipped reef sharks do use smell to find their prey,
0:42:35 > 0:42:38but if that doesn't work they have another sense.
0:42:39 > 0:42:43They can detect an animal's bioelectrical energy,
0:42:43 > 0:42:48the tiny electrical signals given off by even the smallest of its twitching muscles.
0:43:03 > 0:43:07This resting parrotfish risks discovery every time it moves
0:43:07 > 0:43:09a fin, or wafts its gills.
0:43:12 > 0:43:16As the sharks pass, it holds its breath.
0:43:28 > 0:43:32But sharks don't give up that easily.
0:43:39 > 0:43:42White tips are the perfect size
0:43:42 > 0:43:46and shape for hunting in these cramped spaces.
0:44:10 > 0:44:15Their snake-like bodies can even wriggle into crevices,
0:44:19 > 0:44:24and they can fold their dorsal fin flat to squeeze through small holes.
0:44:28 > 0:44:32If discovered, a sleeping parrotfish wouldn't stand a chance.
0:44:51 > 0:44:54Once the first shark has caught its prey,
0:44:54 > 0:44:57the smell and commotion attracts others.
0:45:16 > 0:45:19Every now and then you find yourself in moderately
0:45:19 > 0:45:22strange situations in life, and this is one of them.
0:45:24 > 0:45:29I'm in a cave with FEEDING sharks.
0:45:29 > 0:45:35They are really well designed to be in here and I'm not sure I am, quite frankly.
0:46:24 > 0:46:26Because of their sophisticated senses
0:46:26 > 0:46:31and remarkable physique, white tips are great reef hunters.
0:46:31 > 0:46:34At night they turn it into an art form.
0:46:34 > 0:46:36Better than all other sharks.
0:46:43 > 0:46:46We expect sharks to be hunters,
0:46:46 > 0:46:50but after dark, the reef itself becomes one giant predator.
0:46:52 > 0:46:56By day corals may be passive and plant-like,
0:46:56 > 0:46:59but by night they become active hunters.
0:47:10 > 0:47:13They snatch tiny animals from the current,
0:47:13 > 0:47:16immobilising them with stinging tentacles.
0:47:16 > 0:47:19Just like their relatives the jellyfish.
0:47:32 > 0:47:34And they're highly competitive.
0:47:34 > 0:47:37Here, two individual polyps are fighting for possession
0:47:37 > 0:47:40of a single unfortunate creature.
0:47:59 > 0:48:02At night the corals don't just hunt.
0:48:02 > 0:48:06When territory is at stake, whole colonies go to war with each other.
0:48:11 > 0:48:15Once a coral senses another is too close,
0:48:15 > 0:48:17it launches a barrage of stinging cells,
0:48:17 > 0:48:19and the enemy returns fire.
0:48:44 > 0:48:46It's a war of numbers -
0:48:46 > 0:48:49the coral with the greatest firepower will win.
0:49:00 > 0:49:05Eventually, the coral on the right forces its opponent to retreat.
0:49:12 > 0:49:16Yet another small part of the reef has been changed.
0:49:16 > 0:49:17Every night this goes on,
0:49:17 > 0:49:20from one end of the Great Barrier Reef to the other.
0:49:24 > 0:49:27Day and night, the reef is always changing,
0:49:27 > 0:49:31but that's nothing compared to the impact of the seasons.
0:49:36 > 0:49:39In summer, warmer water means more food,
0:49:39 > 0:49:43making it the best time to bring new life into the world.
0:49:49 > 0:49:53A pair of noddy terns fly close together
0:49:53 > 0:49:54over an island on the reef.
0:49:54 > 0:49:57These are two mates reaffirming their bond.
0:50:07 > 0:50:10Noddies signal to each continually during courtship,
0:50:10 > 0:50:13both in the air and after landing.
0:50:24 > 0:50:26Once in the trees, the performance changes.
0:50:26 > 0:50:29They bow their heads repeatedly.
0:50:31 > 0:50:33It's pretty clear how they got their name.
0:50:56 > 0:51:00Once they've started courting they don't like any interruption.
0:51:02 > 0:51:04BIRD SQUAWKS
0:51:18 > 0:51:20To reinforce their relationship
0:51:20 > 0:51:22the male has to collect nesting materials.
0:51:22 > 0:51:28The females are famously fussy, so he chooses his present carefully.
0:51:43 > 0:51:46A bent twig, it seems, is just the thing.
0:51:46 > 0:51:50A few more of them and they have a nest, if you can call it that,
0:51:50 > 0:51:52ready for their single chick.
0:51:54 > 0:51:56Noddies are devoted partners,
0:51:56 > 0:51:59and often remain together season after season.
0:52:06 > 0:52:10Beneath the waves, the summer warmth triggers fish to court,
0:52:10 > 0:52:14In bicolour parrotfish, the larger male
0:52:14 > 0:52:17leads a tender underwater ballet to woo a partner.
0:52:33 > 0:52:36The urge to breed spreads across the reef.
0:52:39 > 0:52:41These are surgeon fish.
0:52:43 > 0:52:45Single females dart to the surface,
0:52:45 > 0:52:48pursued by groups of competing males.
0:53:02 > 0:53:04As each female releases her eggs,
0:53:04 > 0:53:09the males race to fertilise them, with a puff of white sperm.
0:53:18 > 0:53:21The rise in water temperature also sets
0:53:21 > 0:53:25the conditions for the most remarkable event on the reef.
0:53:30 > 0:53:33On only a few summer nights each year,
0:53:33 > 0:53:38and triggered by a particular phase of the moon, the corals spawn.
0:53:54 > 0:53:59This is the world's largest synchronised breeding event.
0:53:59 > 0:54:01On any one of these nights,
0:54:01 > 0:54:03trillions of eggs and sperm are released,
0:54:03 > 0:54:08flooding the entire 2,000 kilometre length of the reef.
0:54:24 > 0:54:27By spawning at this precise moment
0:54:27 > 0:54:29the corals have caught a lull in the tide,
0:54:29 > 0:54:33allowing sperm and eggs of the same species to meet and fertilise.
0:54:38 > 0:54:42They form baby corals, as the tidal currents pick up,
0:54:42 > 0:54:44they're spread far and wide.
0:54:53 > 0:54:56The young corals joins billions of other baby creatures
0:54:56 > 0:55:00floating in the plankton.
0:55:16 > 0:55:19A baby grouper, only a centimetre in length,
0:55:19 > 0:55:24that will one day grow into a giant, over two meters long.
0:55:29 > 0:55:32Most reef animals have tiny babies like these,
0:55:32 > 0:55:35carried at the mercy of the currents.
0:55:37 > 0:55:40Many have some ability to direct themselves,
0:55:40 > 0:55:43and even the young corals can swim to a degree,
0:55:43 > 0:55:46propelling themselves with microscopic hairs.
0:55:54 > 0:55:58Each one of these could grow into a whole colony
0:55:58 > 0:56:00and start a new coral reef.
0:56:05 > 0:56:09This living soup represents the future of the reef.
0:56:09 > 0:56:12Just one glass full has so much potential.
0:56:13 > 0:56:16It's remarkable to think that such a structure
0:56:16 > 0:56:20as the Great Barrier Reef, large enough to be seen from space,
0:56:20 > 0:56:24and complex enough to support the interwoven lives
0:56:24 > 0:56:27of thousands of species,
0:56:27 > 0:56:30is dependent on minute fragile forms such as these.
0:56:36 > 0:56:40The reef is a dynamic place, in constant flux,
0:56:40 > 0:56:44influenced by the forces of geology,
0:56:44 > 0:56:48weather, wind and waves, the rhythms of the sun and moon.
0:56:53 > 0:56:58Yet its future depends on fragile young life forms
0:56:58 > 0:57:02that must survive in the face of these powerful natural forces,
0:57:02 > 0:57:05and find somewhere safe to settle and grow.
0:57:06 > 0:57:11There can be few tales in the natural world quite so remarkable.
0:57:21 > 0:57:27And this is just the start of what makes the Barrier Reef so great.
0:57:27 > 0:57:32Only 7% of the ecosystem is coral.
0:57:32 > 0:57:36The rest is covered with rainforest and rivers,
0:57:36 > 0:57:38lagoon, swamp, and shore.
0:57:42 > 0:57:46These are home to some of the world's strangest creatures.
0:58:07 > 0:58:13In the next programme we explore the fascinating worlds
0:58:13 > 0:58:15and wildlife beyond the reef,
0:58:15 > 0:58:18and discover their connections to this most magical place.
0:58:35 > 0:58:40Subtitles by Red Bee Media Ltd
0:58:40 > 0:58:47Email subtitling@bbc.co.uk