The Largest Coral Reef in the WORLD! | Pacific 102

The Pacific, our biggest ocean. 150 million square kilometers of water. Stretching all the way from the USA in the east to Australia in the west, it covers almost half our world. And it's bursting with life. From the biggest animal alive, the gigantic blue whale, to one of the most fearsome, the great white shark. Extraordinary sea otters, regal king penguins, and tiny green turtles all call this great ocean their home. This time, we dip our toes into the shallow waters of the Pacific, discover a remarkable dolphin display, and the largest coral reef in the world. We reveal some surprising shark behavior and join turtle moms-to-be in one of the world's biggest animal gatherings. This is life in the shallows. A quarter of all the marine life in the Pacific can be found on coral reefs. And some of the richest, most diverse, and most luxuriant reefs, not just in this vast ocean, but the whole world, are found between Japan, Indonesia, Australia, and Fiji in the western corner of the Pacific. These shallow waters are so full of life, it's known as the Coral Triangle. And it's a magical place to be. Diving on a coral reef in the Pacific is amazing. And you feel that it's incredibly busy. Every bit of space is occupied. And there's animals of different types inhabiting almost every corner. That is my absolute favorite place to be. It's absolutely just gives you a feeling of complete joy. But all is not always as it first appears on these reefs. At a few special places, fish seem to behave as though they have a death wish, swimming willingly into the mouth of a shark. But this isn't as risky as it may seem, because this shark is here for a bit of me time. I don't know about you, but I like going to the spa. I generally feel better after brushing my teeth. And washing your skin and brushing your teeth is very important for our health. The same is true for animals. He's come to the reef to get his teeth cleaned. He swims slowly, signaling he's not here to eat the fish, known as cleaner rats. They run a cleaning station, the fish equivalent of a spa. Most fish, when they come into cleaning stations, will exhibit very specific behaviors. They may change their swimming pattern. They may swim vertically in the water. They may change their coloration. And these are all signals to the fish that, hey, I'm not going to eat you. I really just want to be cleaned. Reassured that the shark's no threat, they nip in and out, removing food from between his teeth, leaving him with clean, pearly whites. Cleaner rats are the toothbrushes and the scrubs of the marine world. And they typically get together in what we call cleaning stations. All shapes and sizes of fish will form orderly queues at cleaning stations, waiting for their appointments with the very well-turned-out beauticians. These cleaner fish are often a special color, advertising the fact that they shouldn't be eaten. But they're there to provide a service. The dark stripes of this cleaner fish show that he's open for business. He nibbles off dead skin, dandruff, and parasites. He works for just four hours a day. But in that time, he'll see 2,000 clients. And all that frantic pampering has a surprisingly positive impact on the health of the entire reef. Reefs that have large numbers of cleaner fish tend to be healthier reefs. There's greater diversity on those reefs. Where there aren't many of them, the fish have too many parasites. So the presence of those cleaner fish help the health of the reef. One of the Pacific's most ancient animals, the green turtle, is also heading into the reef for a quick spa visit. In her case, nature's equivalent of a pedicure. She's about 35 years old. And she spent her whole life cruising the waters of the Pacific. Along the way, she's gained some unwanted passengers. Barnacles and algae have hitched a ride on her shell. They're weighing her down and making swimming hard work. To remove them, she's checking in with the reef's beauticians. And like fish and sharks, she's got a very specific way of showing she's ready to be cleaned. They'll sort of dip their head forward, and their fins will point down. And that indicates to the cleaner, I'm here for a clean. Her shell is covered in nerve endings. And she can feel the tickles as the fish work. Once the cleaners have removed her unwanted passengers, she's more streamlined and aquadynamic. She's looking her best, and just in time, because she's got a rather special appointment to keep at Rain Island, near the far northeast tip of Australia, where every year, one of the largest gatherings of turtles on the planet takes place. We get up to 60,000 turtles all using the same beaches, crawling up out of the sea to make their nests, lay their eggs, and then return to the water. And it's an absolutely phenomenal sight. For the next three months, this beach is the biggest turtle maternity ward in the world. The mums-to-be were hatched here themselves. They have now navigated their way back over thousands of kilometres of the Pacific. It's completely amazing, because turtles will manage to find their way back to the very beach where they themselves hatched. Turtles are amazing navigators, and they go off to sea for years, and then they manage to come back to the same beach where they were actually born, through reading the moon, the sun, the stars. And once they're close to shore, they know they're homing in on the right beach, because each one has a unique smell. And this beach smells like home. Now she'll make a nest of her own. She digs a deep hole and buries her 100 or so eggs for protection. And then she leaves. She'll be back in two years' time to repeat the process over again. The eggs and the baby turtles developing inside must take their chances on their own. It'll be 60 days before they're ready to hatch. And strangely, whether they're boy or girl turtles depends on the weather. Like a lot of reptiles, the sex is not just determined on genetics, but also depends on the temperature. So, for example, if it's warmer, you tend to have more females. If it's cooler, you tend to have more males. Unfortunately, due to global warming, it now turns out that many more females are born than male turtles. And this may be a problem in the future, as it appears we won't have enough male hatchlings to foster the next generation of sea turtles. The hatchlings are only five centimetres long. And in the first moments of their lives, without the help of mum, they're faced with a huge beach-based assault course. A desert of sand they must cross to reach the ocean. Little flippers are better suited to water than sand. There's a lot of traffic, which makes the going even slower. Finally, they make it to the water. Even though they're just a few hours old, they're already strong swimmers and ready to embark on an epic journey across the ocean. Drifting on the Pacific's currents, over the next 35 years, they'll travel thousands of kilometres and will go as far as Indonesia, Papua New Guinea and Vanuatu before returning here to lay their own eggs. It's an awfully big adventure for such small animals. One particular reef in the Pacific leaves all others in the world in the shade. It sits off the north-eastern coast of Australia and stretches along 2,000 kilometres of the shallows. The Great Barrier Reef is truly amazing. It's one of the only structures that's built by living organisms that you can see from outer space. It's the largest living structure that we know on Earth. And all 350,000 square kilometres of it is alive, built as a home by some of the smallest animals of all. Coral reefs are actually made up of billions of tiny animals. You can think of them as jellyfish, tiny little jellyfish that are upside down with their tentacles out. Some can measure only a couple of millimetres across. These are embedded in a stony skeleton that they build themselves. It's fascinating that these very, very tiny animals can build simply enormous structures. The reef buzzes with such activity that it's like an underwater city, a thriving metropolis during rush hour. And just like any big city, different residents set up home in different neighbourhoods. Corals form these three-dimensional structures, like in a rainforest, like trees, and they provide homes for lots of other species. Shrimp and crabs hide out among the white balls of bubble coral, while exotic fish take cover in staghorn coral, which looks a bit like a deer's antlers. And a very special lodger, the 2.4-centimetre-long pygmy seahorse, is beautifully camouflaged to blend in with just one particular type known as a gorgonian coral. But to many fish, the small animals that make up the coral are themselves tasty. So, to avoid becoming fish food, coral have got one or two tricks up their sleeves to see off attackers. They can either retreat inside the hardened calcified skeleton, or when they spread out their tentacles, they can sting, just like a jellyfish or sea anemone. While these defences keep most fish at bay, in the evolutionary arms race of the reef, many fish have found ways to get around them. Butterflyfish have long snouts which work a bit like vacuum cleaners, allowing them to suck corals out of their defensive casings. The pouty lips of this fish, a tube-lip wrasse, are covered in a thin layer of mucus, like lip balm. When the coral stings them, they simply can't feel it. They suck the coral out in a short kissing motion. One coral specialist has less polite table manners. Parrotfish love coral, and they work their way through the reef like a wrecking ball. The parrotfish has a specialised hardened beak, just like a parrot, and it uses this mouth to crunch away at the coral. They bite off whole chunks of coral, gobbling down the soft animals and their hard skeletons in each crunchy mouthful. It digests away the softened animal and passes the sandy, crunched-up skeleton as parrotfish poo. This fish poo should look familiar, as this pure, ground-down coral is what we like to call sand. I don't think a lot of people know that it comes mostly from parrotfish poo. I think most people see these beautiful white sandy beaches and enjoy them for what they are, without any understanding that the sand has originated inside a fish. Not all fish are so destructive. Most forage for tasty scraps amongst the coral itself, while others graze the surrounding seabed. It's a dangerous strategy. Predators like sharks patrol the edge of the reef and they're not always here to be cleaned. So small fish only venture out to a distance from which they know they can safely return. The fish act like underwater lawnmowers and come out from the refuge of the coral to graze the algae and seagrass immediately surrounding the coral and then return to the reef for safety. Over time, these grazers gradually strip bare narrow patches of sand around the coral. These white bands are known as halos and they can reveal a lot about the condition of the reef. The halos that you find around coral heads are actually really important. It was thought initially that this might be an indication of poor health. In fact, it's quite the opposite. The halos indicate that there's a large population of herbivorous fish which are actually grazing the algae and seagrass surrounding the coral heads. Halos around a reef are a sign of good health. It means that that reef has got lots of fish that can come out and crop the grass and algae around it. During the day, corals take energy from sunlight like plants. In fact, they make a home for tiny plants, algae within their bodies. These algae feed the coral in exchange for a place to live. The sunlight's actually used to produce energy, striking the algal cells that live within the corals themselves, allowing them to produce sugars and other nutrients that the corals need to feed on. Coral polyps rely on these so-called symbiotic algae for much of their energy. It's also what makes them so vibrant. The colours of coral reefs actually come from algae, little plants that live within the polyps themselves. But these algae are sensitive things, and if sea temperatures are too high, they'll vacate their polyp homes, leaving the coral bleached white. Starved of the energy their algal lodges provide, these white reefs can quickly die. Although coral reefs are a huge structure globally, unfortunately, due to climate change, they're dwindling fast. In fact, we probably have less than 50% of the reefs that were there 30 years ago. Reefs may get food from algae during the day, but it's at night when these reefs really come alive. As the coral themselves venture out to feed, they extend their tentacles to pluck small morsels of food out of the water, like shrimp, worms, or even baby squid. Once snagged, they're reeled into their mouths, and the coral retreat back inside their limestone homes. But tonight is a special night for the coral. For six nights after the full moon in October, these shallow reefs are transformed into the marine equivalent of a Christmas snow globe, as the Pacific fills with eggs. This is coral reef sex. Once a year, on a particular state of the moon, all coral will actually release their eggs and sperm at the same time. Most corals have both male and female reproductive organs, and they have these tiny little bundles that contain both sperm and egg. The eggs are huge. They can be all sorts of colours. They can be pink, orange, white, and they're like these flakes just in the water column. It's quite amazing to see. It's a bit like swimming through a snow globe. That coral spawn forms these snow-like clouds is essential to their survival. It's a good strategy, because if you've got lots of them together in the water at the same time, the chances of fertilisation is much greater. The sperm and eggs come together to form baby coral, known as larvae. And unlike coral on the reef, these coral larvae can swim. So if you imagine frogs have tadpoles, it's like the tadpole stage of a coral. So it's adapted to swim in the water, and these larvae can travel in the water for days, weeks, even months in some cases. The coral larvae now form part of what's known as the zooplankton, and they can be a huge attraction for those that like to eat them, including one very large fish indeed, whale sharks. Every year around now, whale sharks congregate off Ningaloo Reef in Western Australia in large numbers to feed on little more than coral larvae. Witnessing a whale shark while you're diving is an incredible experience. It's this gentle giant the size of a small school bus. These animals can be 10 metres or more in length. I mean, they're huge. And to share the water with something that big is quite an experience. So important is this abundance of food. This whale shark may have travelled over 19,000km across the whole Pacific Ocean to get here at just the right time. Because of the vastness of the Pacific, it means that some animals have to travel quite far distances to be able to find food. Despite being the biggest fish in the water, they eat the tiniest organisms in the oceans, which is plankton. And to do this, they swim to the surface of the ocean and gulp in huge amounts of water. Per hour, it's about 600 cubic metres of water. That is equivalent to 8,000 bathtubs. And what they then do is they sieve all this water through their gills and keep all the delicious plankton. They're not alone. Manta rays join the feast, filtering out as many baby coral as they can fit into their huge mouths. Fortunately, there are plenty to go round. And many of the baby coral do survive, find a place to settle down and start a new reef of their own. And that new reef may be quite far from here. They can travel in the water currents for considerable distances across the whole of the Pacific Ocean and maybe start life on a new reef quite a long way from where they started. And that ability of coral to regenerate many miles from their parents offers a glimmer of hope for those reefs in the Pacific that have experienced the worst that we have thrown at them. Bikini Atoll, one of the most remote places on Earth. Roughly 4,000 kilometres from Hawaii, Japan and the Philippines. It's literally in the middle of nowhere. And that remote location made it the perfect place for America to test out their most destructive weapons of war. Nuclear bombs. It is here that the military and scientific personnel will conduct the tests with the atomic bomb. After the local people of Bikini were evacuated, an entire fleet of ships, submarines and aircraft carriers was brought in. Weight in almost agonised tenseness. Now but five seconds to go. And... Between 1946 and 1958, nuclear weapons totalling 7,000 times the power of the ones that destroyed Hiroshima and Nagasaki at the end of the Second World War were exploded here. A nuclear bomb must be one of the most destructive forces you can imagine. The largest explosion, detonated 30 metres underwater, blasted a crater through more than two kilometres of the coral reef. What happened at Bikini Atoll is arguably the most devastating impact any reef can suffer. It was blown apart. It was blown away. It was completely removed. But time is a great healer. And thanks to baby coral drifting in from all across the Pacific, life has begun to bounce back. It's taken 70 years for the radiation to sink to levels which are safe for humans to go back there. When we did go back, we found coral reefs that were in amazing condition. Some coral has even grown on the ships, tanks and guns sunk here during the nuclear tests. And with the coral, the fish are returning too. Over the last 70 years, the reefs of Bikini Atoll have recovered to an amazing extent. This is fascinating because it shows that while the reefs are incredibly fragile, they do have the capacity to recover and they can be quite resilient. Even the most degraded habitats will bounce back. As the famous quote from Jurassic Park, life will find a way. Maybe not the same as it was before, but certainly life will bounce back. A mysterious and magnificent light show eliminates the shallow waters of the Pacific. From May onwards, the coastline from California all the way south to Mexico glows like a Christmas decoration. And if you're close enough, like these San Diego surfers, you may also be lucky enough to be dazzled by it first hand. This is bioluminescence, light produced by life. Bioluminescence is where organisms make their own light and they do this through chemical reactions inside their bodies. It's the most amazing sight in the world when you swish your hands in the water at night and it's like a galaxy of stars exploding around you. As strange as it may seem, these incredible light displays are made by the tiniest of living things. Zooplankton. But they're not the only ones illuminating the ocean. Toyama Bay, Japan, on the other side of the Pacific, is filled with the sparkles of stars. But this isn't zooplankton. This natural fireworks display is made by a very special type of squid. Firefly squid actually, when they're trying to attract a mate, have these tentacles with very, very bright blue lights and they will wave these around and it's absolutely phenomenal. Firefly squid usually live in deep waters nearly a kilometre down and come to the surface in their millions just once a year to breed. Unusually for squid, fireflies only have one partner and having mated and laid their eggs, they die. The lights extinguished forever. They're not the only ones. This ocean is full of squid. In fact, there may be more squid in the Pacific than fish. Including these opalescent squid, also known as leligo or the Californian market squid, because they're the ones you're most likely familiar with as a calamari starter. They're also one of the most common squid in the whole of the Pacific. Found in the shallows from Alaska to Mexico, they're in their largest numbers here off Baja, California. They're here because these waters are perfect for mating. Like all squid, they live for less than a year, so they only get one chance to give their babies the best start in life. When leligo squid mate, the male will actually grasp the female and you'll see his tentacles go red. That provides a warning to other males not to approach. Each squid mum may lay 20 capsules, each one containing up to 300 eggs. By anchoring them to the seabed, the squid mums hope they won't float away. Side by side, they create a carpet on the ocean floor. After mating and laying their eggs, both the male and the females die. Their dead bodies litter the sea floor, but new life is about to emerge. After three to eight weeks, depending on how warm the waters are, the squid eggs hatch. The timing is crucial because if their parents have got it right, these squidlets should emerge when plankton numbers peak, which is perfect because that's what they need to eat and to grow. The squidlets grow incredibly fast and can reach adulthood in just four months. And a year from now, they'll return to these same shallow waters to repeat the process. It's not just squid who know how rich these shallow seas are. Spinner dolphins. Spinners can travel up to 400 kilometres in a single day as they search for food. Although dolphins usually travel in small groups, they're incredibly social and can sometimes come together in crowds of 10,000 or more. They put on astonishing demonstrations of synchronised swimming and leaping. Sheer muscle drives their spectacular aerial displays. It's thought the jumps help them communicate, while also dislodging parasites attached to their skin. It's probably also just fun. Like the squid, they've timed their arrival here perfectly because as plankton numbers have bloomed, so have the numbers of an animal that feeds on plankton, a fish that's also the spinner's favourite food, sardines. These sardines have swum all the way from Alaska to get here, knowing that on arrival they can hoover up plenty of plankton. But they haven't got long because their mass migration, the largest migration of any fish on the planet, hasn't gone unnoticed. It's not just dolphins who like to eat them. Striped marlin do too. The striped marlin is truly the Ferrari of the ocean. It can swim at speeds up to 50 miles an hour. Sea lions join the action too. Sea lions are incredible hunters underwater. They are perfectly streamlined, meaning that they can torpedo through the water and are able to catch most fish that they're aiming for. For sardines, the only form of defence is to clump together. There's safety in their numbers. Not only does it reduce the chance of any one individual being picked off, the blurring mass of fish make it increasingly difficult for predators to single out individual prey. But that doesn't stop the marlin. They try to break the school of sardines into ever smaller groups. And they have a secret weapon when it comes to attack. So they have that long bill which will stun or injure those bait fish and that allows them to then go in and start eating the wounded fish. Sea lions attack the sardines from the sides, trying to pick off any stragglers chased out by the marlin. Because you have predators attacking from multiple different angles, some from below, some from the side, it forces the bait fish to form this tight ball close to the surface. But close to the surface is exactly where another group of sardine eaters want them to be. Frigate birds spend days on end scouring the shallow waters for schools of food like this. Unlike most seabirds, they don't have waterproof feathers, but they do like to eat fish. So they have to pluck them right out of the water. Other seabirds are able to dive down and go underwater to catch their prey. Frigate birds can't do that. They have to go and pick things up from the surface. The sardines are being attacked from above and the sides. There's only one place left, below. But that's not safe either. Leviathan, a bride's whale, the smaller relative of the mighty blue whale, scoops up the whole shoal in its massive mouth. Game over. Until another year's time. Sun-dappled forests sway beneath the surface of the shallow waters of the Pacific. But these are no ordinary trees. These are forests of kelp. Most people think of forests on land, but we also do have actual forests in our seas as well, and they're kelp forests. They're very similar to forests on land. You have effectively trees in the form of large kelp, and sometimes these can be 30 metres tall. So they're enormous. Some of the best-known and most well-studied kelp forests of all are found in the shallow waters off California. And they contain just as rich a mix of life as any forests on land. Kelp provides this amazing three-dimensional forest-type habitat, a bit like the tree trunks in a woodland that animals can forage and hide between. So you get a very, very large diversity of life supported within a kelp forest. They're home to everything, from weightless jellyfish to harbour seals and leopard sharks who hunt between the leaves, and some of the most adorable of animals, sea otters. Anyone who has seen sea otters knows that they are the cutest and cuddliest of the marine mammals. They sometimes form rafts, and that's how they tend to sleep. They're all just, like, hugging up on the surface. Every day, the otters go hunting, and this one's hunting for two. She has a hungry suckling pup to feed. Before heading out, she has to make sure she'll be safe and won't drift away. To not get pulled away by the currents, they intertwine in kelp, and they use this as an anchor. Once her pup is secure, she turns to the job at hand, diving after an otter delicacy, sea urchins. She hunts for them around the base of the kelp. Dinner is served. To keep warm on such deep-water dives, otters rely on their thick, hairy coats. They amazingly have the highest hair density of any animal, with 100,000 hairs per square centimetre, and that makes them almost, like, waterproof, basically, cos the hair is so dense. But this dense hair nearly led to the extinction of sea otters in the Pacific, because over the last century, we like to wear their coats too. Because sea otters have this truly soft and phenomenal coat, just over a century ago, they were nearly hunted to extinction. Slaughtered so their pelts could become fashion accessories, their lifeless fur was made into everything from gloves and hats to overcoats, scarves and wrist-warming cuffs. It led to the population declining from 300,000 to almost less than 1,000 otters. As otter numbers fell, the kelp forests and everything else that lived there also changed dramatically. Sea otters eat urchins and urchins eat kelp. When sea otters were nearly hunted to extinction, sea urchin populations completely boomed. Without sea otters to eat them and keep their numbers down, the sea urchin population grew out of control, with a devastating consequence for these underwater forests. The sea urchins will nibble away at the kelp's anchor and essentially dislodge them, and the kelp will die. And because of those booming numbers, the kelp forests were essentially transformed into these sort of barren wastelands. But when hunting was stopped, sea otter numbers gradually recovered, which brought the urchins under control and allowed the kelp to grow once more. And today the otters, the kelp forests and all the other life that make this place their home, are doing well. Life is good in these shallows once more.