0:00:23 > 0:00:27The ocean. The largest habitat on Earth,
0:00:27 > 0:00:32and an inhospitable place for those of us who live on land.
0:00:35 > 0:00:40Yet, for a billion years, this was the only place on the planet
0:00:40 > 0:00:41where life existed.
0:00:48 > 0:00:54Today, descendants of those early life forms continue to thrive.
0:00:59 > 0:01:04They share the sea with fish, but outnumber them by ten to one.
0:01:06 > 0:01:08They have no backbones,
0:01:08 > 0:01:11and have evolved into countless different forms.
0:01:14 > 0:01:20Some are huge, large-brained and intelligent.
0:01:26 > 0:01:29Others are minuscule,
0:01:29 > 0:01:33yet build the largest natural structures on the planet.
0:01:45 > 0:01:49They are marine invertebrates, the creatures of the deep.
0:02:04 > 0:02:07They have colonised every corner of the ocean,
0:02:07 > 0:02:10and have a mind-boggling range of solutions
0:02:10 > 0:02:12to the problem of staying alive.
0:02:32 > 0:02:35The ocean is by no means uniform.
0:02:35 > 0:02:39Differences in depth, temperature, sunlight and currents
0:02:39 > 0:02:41pose particular challenges.
0:02:43 > 0:02:48One and a half miles down, these hydrothermal vents
0:02:48 > 0:02:52spew out super-heated water at 450 degrees Centigrade
0:02:52 > 0:02:55from cracks in the Earth's crust.
0:02:58 > 0:03:01Despite the enormous pressure, the total darkness,
0:03:01 > 0:03:03and the scaldingly-high temperatures,
0:03:03 > 0:03:08the ancestors of all life may have evolved in a place just like this.
0:03:11 > 0:03:14Pompeii worms.
0:03:14 > 0:03:18So named for their ability to survive volcanic heat.
0:03:23 > 0:03:28They share the vents with crabs and two-metre-long tube worms.
0:03:29 > 0:03:34They can only survive here because they are able to feed on bacteria
0:03:34 > 0:03:37that thrive around the vents.
0:03:44 > 0:03:47These colonies are extremely rare.
0:03:49 > 0:03:52Not surprisingly, most life thrives
0:03:52 > 0:03:57nearer the surface, where feeding is considerably easier.
0:04:04 > 0:04:09These are krill, tiny shrimp-like crustaceans.
0:04:10 > 0:04:16Swarms can reach astounding numbers - 60,000 per cubic metre.
0:04:19 > 0:04:24During the night they rise towards the surface to feed on plankton.
0:04:27 > 0:04:30Here, in the Sea of Cortez, off Mexico,
0:04:30 > 0:04:33the swarms attract hunters of all kinds,
0:04:33 > 0:04:37from humpback whales to shoals of predatory fish.
0:04:45 > 0:04:48Yet another hunter arrives.
0:04:48 > 0:04:50It's one from the deep.
0:04:53 > 0:04:55A Humboldt squid.
0:04:58 > 0:05:04Two metres long, they have a local reputation as man-eaters.
0:05:07 > 0:05:10Alone, they are formidable enough...
0:05:11 > 0:05:12..but this is a pack...
0:05:13 > 0:05:15..of hundreds.
0:05:20 > 0:05:24They are highly intelligent hunters. Their eyesight is exceptional.
0:05:24 > 0:05:28They have powerful tentacles, suckers ringed with 70,000 hooks
0:05:28 > 0:05:31and a razor-sharp beak for tearing through flesh.
0:05:38 > 0:05:41Now the fish find that they are under attack
0:05:41 > 0:05:44and so group together for safety.
0:05:49 > 0:05:52But the Humboldt squid work as a team,
0:05:52 > 0:05:54herding the fish against the rocks.
0:06:13 > 0:06:18It's thought that the squid flash red and white not only to confuse their prey,
0:06:18 > 0:06:23but also to signal to each other when they are about to attack.
0:07:04 > 0:07:08Humboldt squid numbers are growing rapidly.
0:07:09 > 0:07:12But they remain mysterious.
0:07:13 > 0:07:18As fast as they arrived, they disappear back into the deep.
0:07:29 > 0:07:36800 miles from the South Pole, in the shadow of a smouldering Mount Erebus.
0:07:43 > 0:07:47Winter temperatures are a punishing minus 40.
0:07:51 > 0:07:57This, perhaps, is the last place you might expect to find marine life.
0:08:16 > 0:08:20Now it's the beginning of the polar spring,
0:08:20 > 0:08:22and for the first time in months,
0:08:22 > 0:08:25light reaches the sea beneath the ice.
0:08:34 > 0:08:38It's extremely cold and completely dark for much of the year,
0:08:38 > 0:08:42so conditions are not unlike those of the deep ocean.
0:08:48 > 0:08:52Yet, in McMurdo Sound, life flourishes.
0:09:10 > 0:09:14The creatures here grow extremely slowly.
0:09:14 > 0:09:19But that does mean they can reach a great age and great size,
0:09:19 > 0:09:23and they occur in surprisingly large numbers.
0:09:37 > 0:09:40Three-metre-long, carnivorous nemertean worms.
0:09:40 > 0:09:42Red sea stars
0:09:42 > 0:09:46and urchins carpet the sea floor.
0:09:53 > 0:09:57This monster-worm will eat almost anything
0:09:57 > 0:10:01and is constantly scanning the sea floor for food.
0:10:13 > 0:10:18Animals are swarming here in such numbers because of this...
0:10:19 > 0:10:21..a dead seal pup.
0:10:25 > 0:10:27Such a great quantity of food
0:10:27 > 0:10:30may only arrive once in ten years.
0:10:30 > 0:10:33But a seal's body won't be easy to eat.
0:10:40 > 0:10:43Nemerteans have a snout like a harpoon,
0:10:43 > 0:10:46that enables them to puncture the skin of the corpse.
0:10:56 > 0:10:59It's harder work for the sea stars.
0:10:59 > 0:11:03They feed by pushing out their stomachs through their mouths.
0:11:06 > 0:11:11As this sea star presses its stomach against the seal's skin,
0:11:11 > 0:11:16it secretes digestive juices that dissolve the seal's tissue.
0:11:16 > 0:11:18But that takes time.
0:11:25 > 0:11:29These scavengers will feed here throughout the summer,
0:11:29 > 0:11:34until all that remains of the seal will be a skeleton, stripped bare.
0:11:41 > 0:11:46The shallows are only a tiny part of the marine world.
0:11:46 > 0:11:5090% of it is open water.
0:11:53 > 0:11:57Its currents carry life for thousands of miles.
0:12:01 > 0:12:06The masters of this nomadic existence are jellyfish.
0:12:08 > 0:12:12A life spent drifting in the empty ocean could be a lonely one...
0:12:12 > 0:12:15but not for this jellyfish.
0:12:18 > 0:12:19Aurelia.
0:12:22 > 0:12:25Swarms like this are not accidental.
0:12:25 > 0:12:27These individuals all hatched together
0:12:27 > 0:12:31when the temperature and currents were just right.
0:12:34 > 0:12:37Their timing has ensured that they can make the most
0:12:37 > 0:12:41of feeding on a late-summer plankton bloom.
0:13:04 > 0:13:08Jellyfish have no brain and no blood.
0:13:08 > 0:13:15But they do have eyespots that enable them to tell the difference between light and dark.
0:13:15 > 0:13:17And they can move independently of the current
0:13:17 > 0:13:20by a simple form of jet propulsion.
0:13:29 > 0:13:33All jellyfish have stinging tentacles with which to catch their food.
0:13:33 > 0:13:37As they pulse their bodies, the tentacles trap plankton,
0:13:37 > 0:13:41which is then passed towards their mouth in the centre.
0:14:02 > 0:14:06A swarm of 100,000 stinging jellyfish
0:14:06 > 0:14:10might seem a daunting prospect for a predator...
0:14:12 > 0:14:15..but not for this one.
0:14:15 > 0:14:18A huge fried egg jellyfish.
0:14:19 > 0:14:22It is a killer.
0:14:25 > 0:14:30Its weapons are harpoon-like cells that cover its tentacles.
0:14:32 > 0:14:37When they come into contact with a victim, they spear it.
0:14:51 > 0:14:55The fried egg then hauls in its prey.
0:15:02 > 0:15:07A few lucky Aurelia do manage to pull themselves free.
0:15:13 > 0:15:17But for the majority, there is only one outcome...
0:15:18 > 0:15:20Death.
0:15:34 > 0:15:37Jellyfish, like a great many marine invertebrates,
0:15:37 > 0:15:40have soft, vulnerable bodies.
0:15:44 > 0:15:48They are protected by stinging cells.
0:15:53 > 0:15:56But there are other kinds of defence.
0:16:00 > 0:16:02Armour plating.
0:16:18 > 0:16:20These are spider crabs.
0:16:20 > 0:16:24They spend most of their lives in deep water.
0:16:24 > 0:16:28But once a year, off the coast of Southern Australia,
0:16:28 > 0:16:34a quarter of a million crabs set off on a long journey to the shallows.
0:16:41 > 0:16:44They are here because they all share a problem.
0:16:44 > 0:16:49Each crab has been wearing the same suit of armour for a year now.
0:16:53 > 0:16:57And it's getting uncomfortably tight.
0:17:01 > 0:17:08So each crab eventually has to shed its shell and produce a bigger one.
0:17:08 > 0:17:14Replacing an old shell is understandably a tricky process.
0:17:17 > 0:17:22First the crab grows an entirely new skin within the old shell.
0:17:25 > 0:17:31It then flexes its body to force its shell to split along the back...
0:17:34 > 0:17:38..before gingerly backing itself out.
0:17:45 > 0:17:51The spider crabs are not only here to moult, but to mate.
0:17:52 > 0:17:57And they grab the opportunity with considerable enthusiasm.
0:18:05 > 0:18:09Coming together in such numbers does however have a drawback.
0:18:12 > 0:18:15It attracts predators.
0:18:19 > 0:18:21A stingray.
0:18:35 > 0:18:38Despite there being so many potential prey,
0:18:38 > 0:18:41the stingray seems to ignore them.
0:18:41 > 0:18:43Perhaps there's safety in numbers...
0:18:46 > 0:18:48..although not all the crabs
0:18:48 > 0:18:50are prepared to rely on it.
0:18:57 > 0:19:00Alarm spreads amongst the crabs.
0:19:09 > 0:19:14But in fact, most of them are safe. This ray is being very choosy.
0:19:14 > 0:19:17It's only interested in the softest shelled,
0:19:17 > 0:19:20most recently moulted crabs.
0:19:32 > 0:19:37Once a target is singled out, there is no escape.
0:20:06 > 0:20:09When their new armour has hardened,
0:20:09 > 0:20:13the crabs return to the comparative safety of the deep,
0:20:13 > 0:20:17leaving behind only their old, empty shells.
0:20:20 > 0:20:28Like spider crabs, the behaviour of most marine creatures is controlled by a very simple nervous system.
0:20:28 > 0:20:31But there are exceptions.
0:20:36 > 0:20:42This cuttlefish is one of the cleverest animals in the ocean.
0:20:42 > 0:20:44She has a very large brain.
0:20:44 > 0:20:50In fact, it's larger for her size than that of most fish or reptiles.
0:20:52 > 0:20:56Her life is both complex and full of intrigue.
0:20:58 > 0:21:01Giant Australian cuttlefish usually live alone,
0:21:01 > 0:21:06but once in their short lives they must come together to mate.
0:21:16 > 0:21:20As she approaches the traditional mating grounds,
0:21:20 > 0:21:25one of the largest males starts to show interest in her.
0:21:32 > 0:21:36She moves him away from his rivals to a quieter spot -
0:21:36 > 0:21:40a place where she will be able to lay her eggs in safety.
0:21:51 > 0:21:56The male takes the female in his arms and turns her to face him,
0:21:56 > 0:22:00before using one of his arms to pass sacks of sperm
0:22:00 > 0:22:02to an opening near her mouth.
0:22:38 > 0:22:41Once mated, he hovers over her,
0:22:41 > 0:22:45standing guard until she's laid her eggs.
0:22:51 > 0:22:53But he's got a problem.
0:22:53 > 0:22:56Males outnumber females four to one.
0:22:56 > 0:23:01So keeping her to himself is a constant battle.
0:23:23 > 0:23:28This larger rival is more difficult to intimidate.
0:23:30 > 0:23:34Cuttlefish can make very dramatic changes to their skin pattern
0:23:34 > 0:23:36in order to signal their moods.
0:23:41 > 0:23:46Flushes of bright colour and stripes that pulse along his side,
0:23:46 > 0:23:48tell the rival to keep off.
0:23:59 > 0:24:04Most rivals back down at this stage, but not this one.
0:24:05 > 0:24:10Although the male's flashing signals get more and more emphatic,
0:24:10 > 0:24:14in the end, he has no choice but to fight.
0:24:50 > 0:24:55Victory! And the male can return to guard his female.
0:24:58 > 0:25:03Cuttlefish are great communicators, but there is a flip side -
0:25:03 > 0:25:06they can also be masters of deception.
0:25:15 > 0:25:21This male is too small to fight for a mate, but he has another plan -
0:25:21 > 0:25:23and it's sneaky.
0:25:24 > 0:25:27He approaches the couple cautiously,
0:25:27 > 0:25:30holding his tentacles tucked up at the front,
0:25:30 > 0:25:33mimicking a female that wants to mate.
0:25:34 > 0:25:35To complete his disguise,
0:25:35 > 0:25:40he changes colour to appear even more like a female.
0:25:41 > 0:25:46The guarding male seems convinced, maybe he thinks his luck is in -
0:25:46 > 0:25:50another female to add to his conquests.
0:26:07 > 0:26:09The sly, cross-dressing male
0:26:09 > 0:26:13edges closer and closer to the female, holding his nerve.
0:26:13 > 0:26:19As long as he avoids being grabbed in a mating embrace, the sneak is safe.
0:26:28 > 0:26:33At what point the female guesses his true identity is unclear,
0:26:33 > 0:26:37but she isn't choosy, and surreptitiously mates with him
0:26:37 > 0:26:40right under the larger male's tentacles.
0:27:05 > 0:27:08It's time for the female to lay her eggs.
0:27:08 > 0:27:14Using the sperm from both males, she fertilises her eggs one by one
0:27:14 > 0:27:18and glues them to a rock in a hidden crevice.
0:27:21 > 0:27:24With luck, she will now have a mix of offspring.
0:27:24 > 0:27:29Some may become masterful males, and others, little sneaks.
0:27:29 > 0:27:32She'll have all the bases covered.
0:27:37 > 0:27:40The coastal waters of British Columbia.
0:27:45 > 0:27:51Home to this four-metre-long Pacific Giant Octopus.
0:27:57 > 0:28:03She is a formidable predator, but at the moment hunting isn't on her mind.
0:28:10 > 0:28:14She has just mated for the first time.
0:28:15 > 0:28:19And now she's searching for a safe refuge.
0:28:32 > 0:28:35She makes her choice carefully.
0:28:35 > 0:28:39This is going to be her home for many months to come.
0:28:52 > 0:28:55It's her nursery den.
0:28:58 > 0:29:01100,000 eggs hang from its ceiling,
0:29:01 > 0:29:04and she's guarding them with her life.
0:29:08 > 0:29:12Without her to protect them, they would be eaten by predators,
0:29:12 > 0:29:14or become diseased.
0:29:16 > 0:29:18She caresses them with her tentacles,
0:29:18 > 0:29:24ensuring that algae don't grow on them, and that fish don't eat them.
0:29:25 > 0:29:29She constantly keeps the water moving around them,
0:29:29 > 0:29:31so they're well-supplied with oxygen.
0:29:42 > 0:29:45She cares for them for six months,
0:29:45 > 0:29:49and during all this time, she doesn't eat a thing.
0:29:57 > 0:30:00And now, as they are hatching,
0:30:00 > 0:30:02she is dying.
0:30:13 > 0:30:16One night, as the baby octopus emerge,
0:30:16 > 0:30:22she jets water over them for the last time, helping them on their way.
0:30:27 > 0:30:29This will be her final act.
0:30:35 > 0:30:39This is the only time she will reproduce,
0:30:39 > 0:30:46and to give her young their best chance, she sacrifices her life.
0:30:55 > 0:31:01Out of the depths comes one of the largest and most aggressive starfish in the ocean.
0:31:01 > 0:31:03Pycnopodia,
0:31:03 > 0:31:08a giant sunstar the size of a dustbin lid.
0:31:14 > 0:31:19It's a hunter. Each arm is covered by super-sensitive tube feet,
0:31:19 > 0:31:22that can detect prey by touch and smell.
0:31:29 > 0:31:33But the sunstar is also partial to carrion,
0:31:33 > 0:31:37and it detects the carcass of the giant octopus mother.
0:31:45 > 0:31:49The miniature suckers on its feet clamp on to the corpse,
0:31:49 > 0:31:52and drag it out of the cave.
0:32:02 > 0:32:05Other scavengers rush to join the feast.
0:32:20 > 0:32:23Although it's a fearsome predator,
0:32:23 > 0:32:26Pycnopodia doesn't have it all its own way.
0:32:28 > 0:32:31These sea urchins aren't speedy enough to escape,
0:32:31 > 0:32:34but they do have a formidable defence.
0:32:34 > 0:32:37The sharp spines are hard to get past,
0:32:37 > 0:32:42and, what's more, the urchins can move each spine independently,
0:32:42 > 0:32:45pinching the starfish's probing arms.
0:32:57 > 0:32:59Trapped by an army of urchins,
0:32:59 > 0:33:04Pycnopodia is spotted by an enormous king crab.
0:33:11 > 0:33:13Pycnopodia has more than met its match,
0:33:13 > 0:33:17and within seconds, the crab rips off one of its arms.
0:33:27 > 0:33:30But that is just a temporary inconvenience.
0:33:30 > 0:33:35Starfish are able to quickly regrow a lost limb.
0:33:41 > 0:33:45The most impressive invertebrates may seem to be the giants,
0:33:45 > 0:33:50but in fact it's some of the smallest that can make the biggest impact.
0:33:53 > 0:33:56Every square inch of this island has been created
0:33:56 > 0:33:59by an ever-growing, living superstructure,
0:33:59 > 0:34:00a coral reef.
0:34:02 > 0:34:05It's taken thousands of years to reach this size,
0:34:05 > 0:34:10and it all began with creatures smaller than a pinhead.
0:34:31 > 0:34:34A reef can't be built just anywhere,
0:34:34 > 0:34:37it needs something to give it a firm footing.
0:34:42 > 0:34:46A wreck like this provides an excellent foundation.
0:34:59 > 0:35:01As soon as it settles on the sea bed,
0:35:01 > 0:35:05the wreck comes under attack from invaders -
0:35:05 > 0:35:08plankton, carried here by ocean currents.
0:35:12 > 0:35:17These are the microscopic larvae of barnacles, sponges,
0:35:17 > 0:35:20and, most importantly, corals.
0:35:29 > 0:35:33The larvae must attach themselves to the wreck.
0:35:33 > 0:35:37Once there, they can develop into young corals called polyps.
0:35:40 > 0:35:43But the polyps are very slow-growing,
0:35:43 > 0:35:47and there is lots of competition from other invaders.
0:35:55 > 0:35:57Algae quickly cover the wreck,
0:35:57 > 0:36:00and that's a problem for the young coral.
0:36:14 > 0:36:16Algae attract grazers.
0:36:23 > 0:36:29The polyps are in danger of being eaten before they've even got a proper foothold.
0:36:31 > 0:36:33If conditions are right,
0:36:33 > 0:36:36the survivors can go on to build a reef.
0:36:39 > 0:36:41Position is critical.
0:36:42 > 0:36:47Too deep, and not enough light will reach the corals for them to grow.
0:36:47 > 0:36:53Too shallow, and they risk being exposed to the air at low tide.
0:36:55 > 0:36:57For the reef to really flourish,
0:36:57 > 0:37:01it also needs to be in the path of currents carrying food.
0:37:11 > 0:37:14Fast-forward half a dozen years or so,
0:37:14 > 0:37:17and the wreck will begin to show the first signs
0:37:17 > 0:37:19of corals visible to the naked eye.
0:37:27 > 0:37:30A decade later, and the wreck will be transformed.
0:37:36 > 0:37:42Thousands of polyps will form coral heads that encrust its surface.
0:37:45 > 0:37:50This ship was sunk during the Second World War,
0:37:50 > 0:37:54and there has been enough time for a substantial reef to develop.
0:37:58 > 0:38:00After decades of growth,
0:38:00 > 0:38:05different species of corals dominate particular areas of the wreck.
0:38:11 > 0:38:17The fastest-growing types grow best on the edges and overhangs,
0:38:17 > 0:38:20reaching far out into the water, and up to the light.
0:38:22 > 0:38:25They need only a small area to establish themselves,
0:38:25 > 0:38:29yet they can rapidly grow dozens of plates, or branches,
0:38:29 > 0:38:33crammed with polyps, to gather as much light as possible.
0:38:51 > 0:38:56Slower-growing, much more robust corals like these brain corals,
0:38:56 > 0:38:59are better suited to the heart of the developing reef.
0:39:01 > 0:39:06It's these that give the reef their structure and permanence.
0:39:10 > 0:39:13There could be nearly 500 different species here,
0:39:13 > 0:39:17each striving to win a foothold on the rusting hulk.
0:39:20 > 0:39:23The pace of life for corals may seem to be so slow,
0:39:23 > 0:39:28that it's hard to imagine that there is any conflict here.
0:39:32 > 0:39:36But, as night falls, the mood on the reef changes.
0:39:39 > 0:39:43Corals are, in fact, extremely aggressive,
0:39:43 > 0:39:47and will fight to the death to expand their territory.
0:39:48 > 0:39:50There can be no honourable retreat,
0:39:50 > 0:39:55a winner will, literally, eat its enemy alive.
0:39:57 > 0:40:03Along the battle front, the polyps of both combatants extrude their guts,
0:40:03 > 0:40:06long thread-like filaments, over their opponents.
0:40:08 > 0:40:13At the fringe, all that remains of the destroyed polyps
0:40:13 > 0:40:14are their skeletons.
0:40:16 > 0:40:20The coral that can digest fastest, wins.
0:40:25 > 0:40:30Corals constantly grow over the skeletons of their dead comrades,
0:40:30 > 0:40:32building a bigger and bigger reef.
0:40:35 > 0:40:37Then, just once a year,
0:40:37 > 0:40:41a few days after the November full moon, the corals take part
0:40:41 > 0:40:44in a mass spawning event.
0:40:50 > 0:40:53Millions of eggs and sperm are released into the water,
0:40:53 > 0:40:59and join, to develop into larvae that drift in search of a place to settle.
0:41:37 > 0:41:42Eventually, every inch of the wreck's surface will be colonised.
0:41:42 > 0:41:46The steel will rust away, and the reef will be on its own.
0:41:56 > 0:42:01Most reefs grow without the help of a wreck to start them off.
0:42:01 > 0:42:05But, given time, they can create something as huge as this,
0:42:05 > 0:42:10the Great Barrier Reef, the largest living structure on Earth.
0:42:26 > 0:42:32A coral reef rivals even a rainforest for its diversity of life.
0:42:32 > 0:42:36Yet corals like this are found in waters where food is very scarce.
0:42:40 > 0:42:44All the creatures here have had to adopt a different
0:42:44 > 0:42:49and highly specialised way to gather every nourishing scrap.
0:42:53 > 0:42:59Christmas tree worms bore into the coral's skeleton for protection,
0:42:59 > 0:43:03swirling out and grabbing food particles with their feathery gills.
0:43:05 > 0:43:08Coral barnacles are, in fact, related to lobsters.
0:43:08 > 0:43:10They lie on their backs,
0:43:10 > 0:43:14waving their feet to gather any food floating past.
0:43:21 > 0:43:25Crabs have evolved many different ways of gathering food.
0:43:27 > 0:43:33This porcelain crab has a fan of filaments on his front legs.
0:43:37 > 0:43:41A boxer crab attaches a tiny sea anemone to each fist.
0:43:41 > 0:43:45As well as for defence, he uses their sticky tentacles
0:43:45 > 0:43:47to gather passing plankton.
0:43:51 > 0:43:55This orangutan crab's whole body is coated with sticky hairs,
0:43:55 > 0:43:58in this case, perhaps just a bit too sticky!
0:44:24 > 0:44:28This strange creature is a sea cucumber.
0:44:28 > 0:44:32It uses its tentacles to grab food from the sediment.
0:44:34 > 0:44:36There are hunters here too,
0:44:36 > 0:44:39like these nudibranchs, or sea slugs.
0:44:40 > 0:44:43Their vibrant colours are a warning that they're toxic.
0:44:43 > 0:44:49There are over 3,000 species, many hunt just one specific prey.
0:44:49 > 0:44:53Some hunt each other.
0:44:57 > 0:45:02This emperor shrimp makes the most of the poisonous nature of its host.
0:45:02 > 0:45:05But, it's a rather one-sided affair.
0:45:05 > 0:45:09As it feeds, the shrimp gets protection and a free ride.
0:45:15 > 0:45:19Other shrimps have developed a more balanced relationship.
0:45:19 > 0:45:22Some even solicit for partners.
0:45:24 > 0:45:27These dance for their dinner.
0:45:35 > 0:45:40And these advertise to passers-by
0:45:40 > 0:45:42that they are open for business.
0:45:44 > 0:45:47They are a parasite removal team,
0:45:47 > 0:45:50providing a service for countless fish on the reef,
0:45:50 > 0:45:54including those that could happily eat them.
0:46:02 > 0:46:06In return for their bravery, they get a meal that comes to them.
0:46:26 > 0:46:30Coral reefs, built by the tiniest of creatures,
0:46:30 > 0:46:34occupy less than half of one per cent of the ocean's floor.
0:46:36 > 0:46:41Yet they support a quarter of all marine species.
0:47:15 > 0:47:18Marine creatures, all without backbones,
0:47:18 > 0:47:21from corals, to cuttlefish, to crabs,
0:47:21 > 0:47:26make up the majority of life in the oceans.
0:47:26 > 0:47:30But they have also had a surprisingly important impact
0:47:30 > 0:47:32beyond the marine world.
0:47:33 > 0:47:36Their fossilised bodies, shells and skeletons
0:47:36 > 0:47:40form the limestone and chalk that now covers huge tracts
0:47:40 > 0:47:43of Asia, Europe, and the Americas.
0:47:43 > 0:47:48They may be small, but over their two billion-year history,
0:47:48 > 0:47:53they have, literally, changed the world.
0:48:01 > 0:48:04To capture some of the sequences in this episode,
0:48:04 > 0:48:09the Life team had to take underwater filming into uncharted territory.
0:48:10 > 0:48:15One shoot meant spending weeks diving under two metres of ice,
0:48:15 > 0:48:19another involved laying the foundation for a new coral reef
0:48:19 > 0:48:20in the tropics.
0:48:26 > 0:48:31A sunken ship can make an ideal location for corals to grow.
0:48:31 > 0:48:32So, with this in mind,
0:48:32 > 0:48:36the Life team set themselves a challenge...
0:48:36 > 0:48:38to make their very own shipwreck.
0:48:41 > 0:48:43After months of searching,
0:48:43 > 0:48:46they find a boat in the Bahamas that might be suitable.
0:48:46 > 0:48:49But there's a lot to do before it can be sunk to the sea bed.
0:48:49 > 0:48:52Apparently it's been here for eight years on the jetty.
0:48:52 > 0:48:57All this fibreglass insulation has all got to be removed
0:48:57 > 0:48:59and it's all hands to the pump, Ian.
0:48:59 > 0:49:01We've got to get a pair of overalls.
0:49:01 > 0:49:05The team have to put away their cameras and get their hands dirty.
0:49:06 > 0:49:08I've got a 50-tonne boat,
0:49:08 > 0:49:10and I'm trying to clean it with a paint scraper!
0:49:10 > 0:49:12Thoroughly cleaning the boat
0:49:12 > 0:49:15increases the chance that coral will grow on it,
0:49:15 > 0:49:17and ensures it won't pollute the sea.
0:49:17 > 0:49:20It's like Changing Rooms, isn't it?
0:49:20 > 0:49:22After eight years rusting on the jetty,
0:49:22 > 0:49:25there's no guarantee that she'll even float.
0:49:25 > 0:49:29Well, we actually... we do have some holes.
0:49:29 > 0:49:31All up!
0:49:31 > 0:49:35To get this ship ready in time for the inspectors is a massive task,
0:49:35 > 0:49:38and they've been doing a brilliant job.
0:49:38 > 0:49:41We're cutting panels through to let the water rush through
0:49:41 > 0:49:42when she starts sinking.
0:49:43 > 0:49:49That's the last bit, OK, guys. Vamos!
0:49:49 > 0:49:52Shipshape at last.
0:49:52 > 0:49:56Well, she's clean, she's been inspected, and she's ready to be sunk.
0:49:56 > 0:49:59What can go wrong now?
0:49:59 > 0:50:03- TELEVISION:- 'Here's the very latest with Dean...'
0:50:03 > 0:50:06As the tail end of Hurricane Dean sweeps through,
0:50:06 > 0:50:09the team are forced to put their plans on hold.
0:50:09 > 0:50:12- TELEVISION:- '..winds of 150 miles an hour...'
0:50:13 > 0:50:16Luck doesn't seem to be on their side.
0:50:23 > 0:50:26But, two days later, good weather returns.
0:50:26 > 0:50:28Here comes a crane, finally!
0:50:31 > 0:50:35Fingers crossed, arms crossed, legs crossed, everything's crossed!
0:50:35 > 0:50:38They urgently need to get the boat into the water,
0:50:38 > 0:50:42or they won't reach the chosen wreck site before nightfall.
0:50:45 > 0:50:47She's safely in the water, so far, so good.
0:50:47 > 0:50:49It floats!
0:50:51 > 0:50:54Her final voyage...
0:50:54 > 0:50:57Cameras are mounted around the deck,
0:50:57 > 0:51:00to film her sinking below the surface.
0:51:00 > 0:51:05We've spent a week preparing for this, and finally the afternoon's arrived.
0:51:05 > 0:51:07But the light's going, so we have to move.
0:51:09 > 0:51:14A last few holes are cut. It's time to pump in water,
0:51:14 > 0:51:16and to abandon ship.
0:51:20 > 0:51:22The crew dive in,
0:51:22 > 0:51:27ready to film her descent, while the support boat moves away to a safe distance.
0:51:28 > 0:51:30There's nothing more to do, but wait.
0:51:30 > 0:51:33Here we go, look at this!
0:52:01 > 0:52:07She gently comes to rest, and the right way up, just as they'd hoped.
0:52:17 > 0:52:20Now, it's time to let nature take its course.
0:52:20 > 0:52:24The crew will be back over the next two years to see how life takes hold.
0:52:26 > 0:52:29At this site, the wreck has a very good chance
0:52:29 > 0:52:31that it'll be colonised by coral.
0:52:31 > 0:52:36With luck and time, it'll eventually become a full-scale reef.
0:52:38 > 0:52:41Meanwhile, at the other end of the world,
0:52:41 > 0:52:44the Life team's challenge is very different.
0:52:48 > 0:52:52Here in Antarctica, just getting underwater will be tough.
0:52:57 > 0:52:59In order to work in such a demanding location,
0:52:59 > 0:53:03the team needed the help of the National Science Foundation,
0:53:03 > 0:53:06at the McMurdo polar research station.
0:53:11 > 0:53:15Everyone here has to be able to cope out on the ice
0:53:15 > 0:53:16if there's an emergency.
0:53:16 > 0:53:21So the Life team joins research scientists for survival training,
0:53:21 > 0:53:24to prepare them for any situation.
0:53:25 > 0:53:28Even one like this.
0:53:37 > 0:53:40A colleague lost in a whiteout is very serious.
0:53:40 > 0:53:44This training might mean the difference between rescue, or not.
0:53:44 > 0:53:48This is supposed to simulate a whiteout,
0:53:48 > 0:53:51we've been looking for someone who's been lost, and you wear
0:53:51 > 0:53:55the bucket so you cannot see at all, and it does actually work.
0:53:55 > 0:53:59It's just... You can't see anything in this.
0:54:02 > 0:54:06With the training over, it's time to travel over the ice to the dive site,
0:54:06 > 0:54:10ready to go beneath the frozen surface of the Ross Sea.
0:54:13 > 0:54:17But to get through this ice, thick enough to land a jet on,
0:54:17 > 0:54:20needs the help of McMurdo's specialist drilling team.
0:54:44 > 0:54:47Once the hole is drilled,
0:54:47 > 0:54:51a specially designed hut is slid into position.
0:54:56 > 0:55:00This will be the team's base for the next four weeks of diving.
0:55:02 > 0:55:06Look, this is all we need, all we need to go in here...
0:55:06 > 0:55:09the door to another world.
0:55:09 > 0:55:12All the equipment the team need for the shots
0:55:12 > 0:55:14must come in and out of this hole.
0:55:14 > 0:55:17This is as far south as it's possible to dive.
0:55:17 > 0:55:22The ice, eight feet thick, it's here for 11½ months of the year,
0:55:22 > 0:55:25and we're goin go down underneath and have a look,
0:55:25 > 0:55:28to see just what it's like underneath there.
0:55:28 > 0:55:31Doug and the team venture below the ice.
0:56:02 > 0:56:07The under-ice landscape is both surprising and spectacular,
0:56:07 > 0:56:10but it's the animals they've come to film.
0:56:13 > 0:56:18So, specialist time-lapse cameras are moved into position.
0:56:24 > 0:56:28The animals move very slowly at these freezing temperatures.
0:56:28 > 0:56:33But by using time-lapse to speed up the action 500 times,
0:56:33 > 0:56:39the team hopes to reveal the behaviour of these creatures for the first time.
0:56:50 > 0:56:54With air and warmth running out for this dive,
0:56:54 > 0:56:57Doug makes his way back to the lifeline,
0:56:57 > 0:56:58and the surface.
0:57:19 > 0:57:24Oh! That is amazing. It is so beautiful down there.
0:57:24 > 0:57:29All sorts of colours, beautiful stalactites made of ice crystals
0:57:29 > 0:57:31hanging down from the top of the ice.
0:57:31 > 0:57:34All sorts of things on the bottom, starfish, urchins,
0:57:34 > 0:57:36just an amazing profusion of life.
0:57:36 > 0:57:39Really lovely, totally unique, unlike anywhere else.
0:57:39 > 0:57:44However, after an hour underwater it is getting pretty cold,
0:57:44 > 0:57:47so can you give me a hand up, please? Thanks.
0:57:49 > 0:57:54There would be another month here, and over 100 dives,
0:57:54 > 0:57:57before the sequence was eventually completed.
0:58:17 > 0:58:20Subtitles by Red Bee Media Ltd
0:58:20 > 0:58:23Email subtitling@bbc.co.uk