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These are the user uploaded subtitles that are being translated: 1 00:00:01,280 --> 00:00:03,520 (THEME MUSIC PLAYING) 2 00:00:28,480 --> 00:00:30,840 (BIRDS CHIRPING) 3 00:00:37,080 --> 00:00:39,640 DAVID ATTENBOROUGH: In a far corner of South-East Asia, 4 00:00:39,920 --> 00:00:42,440 lies the Coral Triangle. 5 00:00:45,880 --> 00:00:49,600 A cluster of the richest coral reefs, in the world. 6 00:00:55,280 --> 00:00:59,040 Undersea cities crammed full of life. 7 00:01:05,720 --> 00:01:08,160 As in any crowded metropolis, 8 00:01:08,600 --> 00:01:11,360 there is fierce rivalry for space, 9 00:01:15,720 --> 00:01:17,000 for food 10 00:01:21,840 --> 00:01:23,160 and for a partner. 11 00:01:25,240 --> 00:01:29,560 But the reef is also a place full of opportunity. 12 00:01:42,760 --> 00:01:43,880 A cuttlefish. 13 00:02:00,440 --> 00:02:04,480 It specialises in hunting crabs. 14 00:02:12,960 --> 00:02:16,280 But a large crab is a dangerous quarry. 15 00:02:24,240 --> 00:02:26,240 It has powerful claws. 16 00:02:30,200 --> 00:02:33,640 The cuttlefish, however, has a remarkable talent. 17 00:02:39,480 --> 00:02:42,840 Its skin contains millions of pigment cells 18 00:02:43,040 --> 00:02:45,520 with which it can create ever-changing colours 19 00:02:45,600 --> 00:02:46,880 and patterns. 20 00:02:56,280 --> 00:03:00,160 And that apparently hypnotises the crab. 21 00:03:22,440 --> 00:03:24,040 A cuttlefish may be clever, 22 00:03:24,600 --> 00:03:26,160 but a shark is bigger. 23 00:03:28,920 --> 00:03:30,880 And it eats cuttlefish. 24 00:03:33,720 --> 00:03:35,200 Time to disappear. 25 00:03:51,400 --> 00:03:52,840 Back to the hunt. 26 00:03:55,920 --> 00:03:59,840 A new target, but the same mesmerising technique. 27 00:04:15,840 --> 00:04:20,240 For those that manage to establish themselves in these bustling 28 00:04:20,360 --> 00:04:21,720 undersea cities, 29 00:04:21,960 --> 00:04:24,520 there can be great rewards. 30 00:04:44,640 --> 00:04:47,920 Corals build themselves homes of limestone, 31 00:04:48,080 --> 00:04:51,280 in the warm, clear, shallow seas of the Tropics. 32 00:04:55,360 --> 00:04:57,920 Their reefs occupy less than one tenth 33 00:04:58,040 --> 00:05:01,000 of one percent of the ocean floor. 34 00:05:04,840 --> 00:05:07,240 Yet, they're home to a quarter 35 00:05:07,400 --> 00:05:09,720 of all known marine species. 36 00:05:14,280 --> 00:05:15,760 They are complex, 37 00:05:16,160 --> 00:05:18,480 infinitely variant structures, 38 00:05:19,400 --> 00:05:22,920 providing all kinds of homes for their many residents, 39 00:05:26,040 --> 00:05:27,520 from penthouse suites, 40 00:05:29,400 --> 00:05:31,200 to backstreet dens. 41 00:05:37,200 --> 00:05:39,920 Here, on Australia's Great Barrier Reef, 42 00:05:40,560 --> 00:05:44,280 a coral grouper lives by hunting for small fish. 43 00:05:50,040 --> 00:05:51,800 But how do you get a meal here, 44 00:05:53,000 --> 00:05:55,720 when you're too big to squeeze into crevices? 45 00:06:04,560 --> 00:06:07,800 And the grouper also has a rival, 46 00:06:12,200 --> 00:06:14,360 one which is highly intelligent, 47 00:06:19,160 --> 00:06:21,280 and seeks the same kind of prey. 48 00:06:24,200 --> 00:06:25,320 An octopus. 49 00:06:46,560 --> 00:06:50,400 It can reach into really narrow cracks. 50 00:06:58,800 --> 00:07:00,080 Even so, 51 00:07:00,560 --> 00:07:02,240 its prey often escapes. 52 00:07:14,160 --> 00:07:16,160 What if they could work together? 53 00:07:32,360 --> 00:07:34,480 The grouper turns pale, 54 00:07:38,720 --> 00:07:41,920 and tries to attract the octopus's attention. 55 00:07:47,400 --> 00:07:49,080 It performs a head stand. 56 00:07:55,960 --> 00:07:59,000 Not only is the grouper signalling to the octopus, 57 00:07:59,360 --> 00:08:02,200 it's indicating where the prey is hiding. 58 00:08:15,480 --> 00:08:17,720 The octopus reaches inside. 59 00:08:21,440 --> 00:08:22,960 The fish take fright. 60 00:08:26,680 --> 00:08:30,760 And swim straight into the grouper's jaws. 61 00:08:38,440 --> 00:08:41,000 Sometimes, the octopus gets the reward, 62 00:08:42,480 --> 00:08:45,080 sometimes, the grouper does. 63 00:08:50,640 --> 00:08:52,720 Two very different species 64 00:08:52,920 --> 00:08:56,600 have discovered that teamwork can bring success, 65 00:08:56,800 --> 00:08:58,800 in reef city. 66 00:09:15,240 --> 00:09:18,760 Teamwork , in fact, is the very foundation of life 67 00:09:18,920 --> 00:09:20,040 on the reef. 68 00:09:27,880 --> 00:09:31,840 The corals themselves also rely on a partnership. 69 00:09:33,240 --> 00:09:36,480 But one of a much more intimate kind. 70 00:09:45,040 --> 00:09:48,520 Corals are colonies of anemone-like animals, 71 00:09:48,880 --> 00:09:50,000 polyps. 72 00:09:50,880 --> 00:09:53,760 Some as small as grains of sand. 73 00:09:56,600 --> 00:09:59,320 Living inside the tissues of each polyp, 74 00:09:59,800 --> 00:10:04,640 are minute plant-like cells, invisible to the naked eye. 75 00:10:10,160 --> 00:10:13,040 By day flourishing in tropical sunshine, 76 00:10:13,400 --> 00:10:15,960 the plant-like cells provide the polyps 77 00:10:16,080 --> 00:10:18,880 with up to 90% of their food. 78 00:10:31,760 --> 00:10:35,200 And when it's dark, the polyps continue to feed 79 00:10:35,520 --> 00:10:40,880 by using their tentacles to grab edible particles drifting by. 80 00:10:48,760 --> 00:10:52,480 The polyps also extract calcium carbonate from the sea water, 81 00:10:52,720 --> 00:10:56,520 and use it to build a stony housing for themselves. 82 00:10:59,600 --> 00:11:03,120 Coral colonies can continue to grow for centuries. 83 00:11:03,840 --> 00:11:05,320 Possibly millennia. 84 00:11:09,040 --> 00:11:13,560 And they can build structures that can reach the size of a house. 85 00:11:28,680 --> 00:11:32,040 The biggest of their cities, is the Great Barrier Reef. 86 00:11:33,520 --> 00:11:39,320 It runs for over 1400 miles along the coast of North East Australia. 87 00:11:50,720 --> 00:11:55,080 Activity in coral reefs, wherever they are, never ceases. 88 00:11:57,360 --> 00:11:59,800 At dawn, the day shift begins. 89 00:12:00,800 --> 00:12:05,200 Accompanied by a chorus of submarine song, 90 00:12:05,600 --> 00:12:09,840 created by fish, shrimps, and other inhabitants of the reef. 91 00:12:09,920 --> 00:12:13,760 (ETHEREAL SEA SOUNDS) 92 00:12:23,920 --> 00:12:26,960 Every resident in this city, has its role. 93 00:12:27,920 --> 00:12:30,160 Scavengers, like the sea cucumber, 94 00:12:30,360 --> 00:12:32,280 recycle the waste of others. 95 00:12:34,960 --> 00:12:37,640 These parrot fish, bite off chunks of coral 96 00:12:37,720 --> 00:12:41,080 and crunch it to extract the contents. 97 00:12:53,440 --> 00:12:56,240 And then, excrete the rest as sand. 98 00:13:12,120 --> 00:13:14,760 Green turtles, here in Borneo, 99 00:13:14,960 --> 00:13:18,840 pay regular visits to a particular patch of coral. 100 00:13:24,240 --> 00:13:28,960 This little female is up early and one step ahead of the others. 101 00:13:48,360 --> 00:13:52,720 Turtles travel long distances along the reef in order to get here. 102 00:13:56,400 --> 00:13:58,560 This is their destination. 103 00:13:59,080 --> 00:14:00,520 Turtle Rock. 104 00:14:07,120 --> 00:14:11,560 Generations of visiting turtles have worn a hollow in its top. 105 00:14:25,000 --> 00:14:27,680 This is home to blennies 106 00:14:28,760 --> 00:14:30,360 and surgeon fish. 107 00:14:31,840 --> 00:14:36,800 They clean the visitors picking off any algae, parasites and dead skin, 108 00:14:36,920 --> 00:14:38,240 that they can find. 109 00:14:42,880 --> 00:14:45,680 Other client are close behind. 110 00:14:53,520 --> 00:14:55,480 A queue is forming. 111 00:15:12,000 --> 00:15:14,560 The big males barge their way in. 112 00:15:30,160 --> 00:15:33,640 And the smaller female is forced out. 113 00:15:42,360 --> 00:15:45,000 She takes a quick turn around the block, 114 00:15:50,320 --> 00:15:51,800 while the others are squabbling. 115 00:16:05,320 --> 00:16:07,640 And then, she sneaks back in. 116 00:16:19,440 --> 00:16:22,240 So the cleaners get a nutritious meal, 117 00:16:22,880 --> 00:16:27,280 and their customers are freed of their parasites and other encumbrances. 118 00:16:32,600 --> 00:16:37,280 And it's now thought, that a spot of pampering at a cleaning station 119 00:16:37,400 --> 00:16:39,360 may even reduce stress. 120 00:16:55,360 --> 00:16:58,760 The lands of the Middle East are so hot and dry, 121 00:16:58,920 --> 00:17:01,800 that much of their surface is almost lifeless. 122 00:17:03,840 --> 00:17:05,840 But here in the Red Sea, 123 00:17:06,360 --> 00:17:08,600 Coral Reefs flourish wonderfully. 124 00:17:11,840 --> 00:17:13,280 The waters offshore, 125 00:17:13,440 --> 00:17:16,520 are almost as rich in life as a rainforest. 126 00:17:19,640 --> 00:17:21,760 And some animals come to the reefs 127 00:17:22,000 --> 00:17:25,080 simply for rest and relaxation. 128 00:17:44,800 --> 00:17:47,240 The family of bottlenose dolphins 129 00:17:47,320 --> 00:17:48,720 are resting on the reef 130 00:17:49,040 --> 00:17:51,520 after a night's feeding offshore. 131 00:17:54,840 --> 00:17:56,320 (WHISTLING) 132 00:18:11,680 --> 00:18:14,400 Whilst the adult and their babies sleep, 133 00:18:16,120 --> 00:18:19,240 the adolescents set off to explore... 134 00:18:21,200 --> 00:18:22,840 (WHISTLING) 135 00:18:23,160 --> 00:18:25,680 ...and they appear to make up games. 136 00:18:30,720 --> 00:18:33,720 (CLICKING) 137 00:18:35,000 --> 00:18:37,840 You might call this one, "Catch the coral." 138 00:18:38,280 --> 00:18:42,320 (WHISTLING AND CLICKING) 139 00:18:43,920 --> 00:18:46,120 The rules aren't entirely clear. 140 00:18:50,000 --> 00:18:53,480 (WHISTLING AND CLICKING) 141 00:19:03,040 --> 00:19:05,680 They pick up different bits of broken coral, 142 00:19:08,640 --> 00:19:09,880 and drop them. 143 00:19:12,120 --> 00:19:13,760 Some fall fast. 144 00:19:17,680 --> 00:19:19,600 Others sink more slowly. 145 00:19:37,160 --> 00:19:39,520 And some seem to be descending in a way 146 00:19:39,640 --> 00:19:41,080 that pleases everybody. 147 00:19:41,560 --> 00:19:44,920 (CLICKING) 148 00:19:50,080 --> 00:19:54,000 (WHISTLING) 149 00:19:56,840 --> 00:20:02,360 Only the most successful city residents can afford the luxury of playing games. 150 00:20:04,680 --> 00:20:07,320 But, such games do that their value. 151 00:20:07,680 --> 00:20:11,440 They may help the youngsters develop the coordination and the agility 152 00:20:11,560 --> 00:20:15,400 that they will need when they start hunting in the open sea. 153 00:20:27,200 --> 00:20:30,480 Every reef has a sharply defined boundary. 154 00:20:33,160 --> 00:20:34,960 Its city walls. 155 00:20:40,200 --> 00:20:43,240 On the outer side is the drop off. 156 00:20:44,280 --> 00:20:47,840 These ramparts protect the city from the ocean waves. 157 00:20:52,080 --> 00:20:55,800 But twice a day, the walls are covered by the incoming tide. 158 00:21:05,760 --> 00:21:10,640 In the Bahamas, the rush of the water creates a truly strange phenomenon. 159 00:21:22,240 --> 00:21:26,040 Seamen once told tales of a giant sea monster, 160 00:21:26,200 --> 00:21:28,600 lurking here deep inside the reef, 161 00:21:29,360 --> 00:21:32,080 that would drag sailors to their doom. 162 00:21:50,320 --> 00:21:55,880 Today, we know it is in fact a whirlpool created by the incoming tide 163 00:21:56,000 --> 00:21:58,440 rushing over deep coral caves. 164 00:22:04,240 --> 00:22:08,720 These currents bring in fresh supplies of microscopic food to the reef 165 00:22:08,880 --> 00:22:10,280 from the open ocean. 166 00:22:13,720 --> 00:22:17,000 And in the Maldives, on the biggest tides, 167 00:22:17,320 --> 00:22:19,640 one particular coral lagoon 168 00:22:19,840 --> 00:22:22,240 become so flooded with plankton, 169 00:22:24,040 --> 00:22:27,560 that it attracts hundreds of ocean giants. 170 00:22:30,280 --> 00:22:32,040 Manta Rays. 171 00:22:33,880 --> 00:22:36,120 With three meter wing spans. 172 00:22:45,640 --> 00:22:48,560 With their huge sloth like mouths wide open 173 00:22:50,400 --> 00:22:52,440 they filter out the plankton. 174 00:23:01,720 --> 00:23:05,160 And the mantas create a vortex of their own 175 00:23:05,600 --> 00:23:07,880 that further concentrates their food. 176 00:23:50,720 --> 00:23:55,280 This behaviour has been called the manta cyclone. 177 00:24:05,960 --> 00:24:08,080 Then the tide changes, 178 00:24:08,920 --> 00:24:10,920 and the supply of food is cut of. 179 00:24:14,640 --> 00:24:17,080 The mantas leave the lagoon. 180 00:24:26,640 --> 00:24:31,280 Life on the sheltered side of the reef is tranquil and peaceful. 181 00:24:36,360 --> 00:24:39,600 In these suburbs, any creature wishing to escape 182 00:24:39,720 --> 00:24:42,240 from the bustling crowds of reef city, 183 00:24:43,760 --> 00:24:45,360 can find plenty of space. 184 00:24:49,080 --> 00:24:50,200 On the other hand, 185 00:24:50,360 --> 00:24:52,320 there is nowhere to hide. 186 00:25:00,280 --> 00:25:01,480 That, at night, 187 00:25:01,960 --> 00:25:03,920 makes it a dangerous place. 188 00:25:10,640 --> 00:25:14,000 Nocturnal predators, such as this lion fish, 189 00:25:14,640 --> 00:25:16,600 patrol the reef edge. 190 00:25:47,560 --> 00:25:50,600 The hunter has become the hunted. 191 00:26:09,720 --> 00:26:10,920 A bobbit, 192 00:26:13,600 --> 00:26:15,440 a giant carnivorous worm 193 00:26:16,080 --> 00:26:18,240 with jaws as sharp as daggers. 194 00:26:22,440 --> 00:26:27,120 It has an ancestry that stretches back more than four hundred million years. 195 00:26:35,000 --> 00:26:36,960 It's a meter long. 196 00:26:57,240 --> 00:27:00,560 It tastes for scent. 197 00:27:07,040 --> 00:27:09,480 And feels for movement. 198 00:27:54,200 --> 00:27:59,000 Dawn, and with light, the reef becomes a safer place. 199 00:28:02,480 --> 00:28:04,880 Little bream return to foraging. 200 00:28:14,120 --> 00:28:16,200 The bobbit might still be hungry. 201 00:28:19,000 --> 00:28:22,560 But now in daylight the odds have shifted. 202 00:28:25,720 --> 00:28:27,960 The bream can see the bobbit. 203 00:28:32,800 --> 00:28:34,560 Instead of retreating, 204 00:28:35,200 --> 00:28:39,560 they join together to blow away the sand covering the worm. 205 00:28:45,360 --> 00:28:49,480 So, taking away its advantage of surprise. 206 00:28:55,040 --> 00:28:57,480 By revealing the bobbit's hiding place, 207 00:28:57,720 --> 00:29:00,160 they will all be able to feed more safely. 208 00:29:04,200 --> 00:29:06,200 But it pays to remember 209 00:29:15,720 --> 00:29:17,440 there is a bobbit about. 210 00:29:30,000 --> 00:29:32,080 Out here, on the sand flats, 211 00:29:32,320 --> 00:29:35,160 there is safe accommodation for some. 212 00:29:44,400 --> 00:29:49,760 The family of saddleback clownfish have found an excellent home. 213 00:29:52,480 --> 00:29:56,040 The tentacles of this carpet anemone can kill. 214 00:29:57,040 --> 00:29:58,640 (CLICKING) 215 00:29:58,800 --> 00:30:01,400 But the clownfish are immune to the poison, 216 00:30:07,760 --> 00:30:09,760 so they can shelter from danger. 217 00:30:14,720 --> 00:30:19,120 In return, the fish keep the anemone clean of debris. 218 00:30:23,960 --> 00:30:25,560 As with all clownfish, 219 00:30:25,640 --> 00:30:28,840 the family is ruled by a big female. 220 00:30:30,800 --> 00:30:34,360 Her white face marks her out as the boss. 221 00:30:39,560 --> 00:30:42,240 The diminutive male has to prove his worth, 222 00:30:42,880 --> 00:30:44,920 so he works tirelessly, 223 00:30:45,320 --> 00:30:49,040 removing debris and generally keeping on top of the housework. 224 00:31:10,400 --> 00:31:14,120 His greatest challenge is to find a safe place 225 00:31:14,280 --> 00:31:16,760 where the boss can lay her eggs. 226 00:31:20,080 --> 00:31:23,480 But there's nothing solid here for the female to lay them on. 227 00:31:29,680 --> 00:31:31,840 A nearby shell could be the solution. 228 00:31:42,840 --> 00:31:46,760 If only he can move it to the protection of the anemone. 229 00:31:54,760 --> 00:31:55,880 Too heavy. 230 00:32:00,120 --> 00:32:02,600 Besides, it has a mind of its own. 231 00:32:06,720 --> 00:32:07,880 The hermit crab. 232 00:32:17,000 --> 00:32:19,200 But out here, twice a day, 233 00:32:20,120 --> 00:32:23,080 the anemone is swept by tidal currents, 234 00:32:23,280 --> 00:32:25,440 and they bring in new opportunities. 235 00:32:28,520 --> 00:32:30,360 An old plastic bottle. 236 00:32:34,880 --> 00:32:36,240 Perhaps, this will do. 237 00:32:44,200 --> 00:32:45,520 Not heavy enough. 238 00:32:58,000 --> 00:32:59,280 A coconut shell. 239 00:33:04,800 --> 00:33:06,640 It looks just right. 240 00:33:07,480 --> 00:33:09,840 But it's a long way from home. 241 00:33:28,520 --> 00:33:31,160 And he can't move it by himself. 242 00:33:43,360 --> 00:33:46,080 So the pair now work together. 243 00:34:27,560 --> 00:34:30,480 A little adjustment to the anemone's tentacles, 244 00:34:30,560 --> 00:34:32,280 in order to clear a space for it. 245 00:34:35,440 --> 00:34:37,520 And the shell is tucked in. 246 00:34:43,200 --> 00:34:44,640 The female lays. 247 00:34:45,320 --> 00:34:47,720 A safe nursery at last. 248 00:35:03,760 --> 00:35:07,360 He has proved himself worthy to father her young. 249 00:35:07,920 --> 00:35:09,400 And he fertilizes them. 250 00:35:14,000 --> 00:35:17,280 Now he will meticulously tend the eggs, 251 00:35:25,000 --> 00:35:27,840 keeping them clean and healthy until they hatch 252 00:35:29,200 --> 00:35:30,560 in 10 days time. 253 00:35:44,600 --> 00:35:49,840 Reef creatures go to great lengths to give their young a head start in life. 254 00:35:58,840 --> 00:36:00,520 And nowhere more so 255 00:36:00,760 --> 00:36:04,040 than on the remotest reefs in the world. 256 00:36:06,480 --> 00:36:08,120 French Polynesia, 257 00:36:08,520 --> 00:36:11,320 the very heart of the South Pacific. 258 00:36:18,040 --> 00:36:20,040 Protected by their isolation, 259 00:36:20,120 --> 00:36:23,720 some of the reefs here, are still virtually pristine. 260 00:36:33,560 --> 00:36:38,640 This marbled grouper has made it in the city and reached adulthood. 261 00:36:39,920 --> 00:36:42,440 Now, it's the time to mate. 262 00:36:44,360 --> 00:36:49,880 To find a partner, he must head to the most dangerous part of this reef: 263 00:36:51,080 --> 00:36:52,120 The drop off. 264 00:36:57,400 --> 00:37:00,480 Patrolled by grey reef sharks, 265 00:37:03,600 --> 00:37:04,800 hundreds of them. 266 00:37:11,800 --> 00:37:13,280 They seem to be resting, 267 00:37:16,280 --> 00:37:17,400 for now. 268 00:37:27,880 --> 00:37:32,120 Thousands of other groupers have gathered on the seabed below. 269 00:37:48,680 --> 00:37:51,920 The females are almost bursting with eggs. 270 00:37:56,800 --> 00:37:58,120 But to mate with one, 271 00:37:59,440 --> 00:38:03,040 he must first get through the crowd of other waiting males. 272 00:38:05,160 --> 00:38:08,640 And they all have the same thing on their minds. 273 00:38:15,880 --> 00:38:19,600 They jostle to get as close as possible to a female. 274 00:38:47,280 --> 00:38:50,160 This male may have secured pole position, 275 00:38:51,200 --> 00:38:54,560 but the female won't release her eggs for him to fertilize, 276 00:38:55,160 --> 00:38:57,520 until conditions are just right. 277 00:39:10,000 --> 00:39:12,840 Now the sharks begin to close in, 278 00:39:13,600 --> 00:39:17,280 sensing that the critical moment is approaching. 279 00:39:44,040 --> 00:39:46,600 The tide is beginning to turn. 280 00:39:49,560 --> 00:39:52,120 This could be the moment to spawn. 281 00:39:59,920 --> 00:40:03,760 Suddenly, the females rush up towards the surface 282 00:40:03,880 --> 00:40:06,480 releasing their clouds of eggs as they go. 283 00:40:08,520 --> 00:40:12,480 The males pursue them, simultaneously releasing their sperm. 284 00:40:28,520 --> 00:40:31,920 It's an act the groupers seem prepared to die for. 285 00:40:36,240 --> 00:40:39,280 It's now or never. 286 00:41:27,000 --> 00:41:28,480 The fertilized eggs, 287 00:41:28,840 --> 00:41:30,360 will now be swept away 288 00:41:31,160 --> 00:41:34,920 from the many hungry mouths of the reef dwellers. 289 00:41:46,640 --> 00:41:49,920 Most of the billions of eggs that cloud the sea, 290 00:41:50,160 --> 00:41:51,440 will be eaten. 291 00:41:53,520 --> 00:41:54,720 But a few, 292 00:41:54,840 --> 00:41:57,000 a tiny but crucial minority, 293 00:41:57,800 --> 00:41:59,640 will find another reef 294 00:42:00,280 --> 00:42:01,880 and make it their home. 295 00:42:15,760 --> 00:42:19,720 But today's coral reefs are facing a new threat. 296 00:42:24,720 --> 00:42:26,800 The seas are warming. 297 00:42:30,480 --> 00:42:33,240 A rise in temperature of just one for two degrees 298 00:42:33,720 --> 00:42:35,200 for just a few weeks, 299 00:42:35,840 --> 00:42:41,480 can be enough to cause the coral polyps to eject their plant-like cells. 300 00:42:49,320 --> 00:42:53,280 When that happens, the corals lose both their colour, 301 00:42:53,800 --> 00:42:56,080 and their main source of food. 302 00:43:03,440 --> 00:43:05,920 If the high temperatures are sustained, 303 00:43:06,720 --> 00:43:09,040 coral, bleached in this way, 304 00:43:09,440 --> 00:43:11,040 is likely to die. 305 00:43:17,440 --> 00:43:21,760 In recent years, it's thought that half the world's coral reefs, 306 00:43:22,040 --> 00:43:23,880 have been affected by bleaching. 307 00:43:31,080 --> 00:43:33,920 Including, since 2016, 308 00:43:34,160 --> 00:43:37,440 around two thirds of the shallow water corals, 309 00:43:37,600 --> 00:43:40,040 on Australia's Great Barrier Reef. 310 00:43:52,760 --> 00:43:55,840 These once crowded submarine cities, 311 00:43:57,800 --> 00:43:58,960 are reduced 312 00:43:59,520 --> 00:44:01,000 to bleak ruins. 313 00:44:03,400 --> 00:44:06,760 and many of their inhabitants, left homeless. 314 00:44:21,640 --> 00:44:25,560 Some scientists predict that by the end of the century, 315 00:44:26,920 --> 00:44:29,040 coral reef cities as we know them, 316 00:44:29,960 --> 00:44:32,080 could be a thing of the past. 317 00:44:48,560 --> 00:44:53,440 Is there any future for these most precious of ocean treasures? 318 00:45:02,000 --> 00:45:06,560 Well, that ultimately depends on how fast they heat up, 319 00:45:06,840 --> 00:45:09,080 and how warm the seas become. 320 00:45:13,160 --> 00:45:14,920 And there is a glimmer of hope, 321 00:45:15,320 --> 00:45:19,120 because of the way the corals reproduce themselves. 322 00:45:25,800 --> 00:45:28,040 On one special night of the year, 323 00:45:28,800 --> 00:45:32,680 the full moon triggers an extraordinary event: 324 00:45:50,680 --> 00:45:53,240 The spawning of the coral. 325 00:46:02,600 --> 00:46:07,080 With extraordinary synchrony, entire reefs reproduce. 326 00:46:23,280 --> 00:46:26,200 Billions of fertilized eggs drift away 327 00:46:29,240 --> 00:46:31,480 carried by the ocean currents. 328 00:46:37,960 --> 00:46:41,240 And it's not just the corals that spawn, 329 00:46:43,160 --> 00:46:46,360 so do many other residents of the reef. 330 00:46:51,920 --> 00:46:53,960 A whole range of young 331 00:46:54,240 --> 00:46:56,200 are swept through the oceans, 332 00:46:56,960 --> 00:46:59,640 ready to settle on a vacant site, 333 00:46:59,960 --> 00:47:03,560 and bring back into existence the complex community, 334 00:47:04,000 --> 00:47:06,040 that is a coral reef. 335 00:47:17,800 --> 00:47:21,720 We may not know what the future hold for our seas, 336 00:47:24,880 --> 00:47:27,560 but coral reefs can regenerate. 337 00:47:28,280 --> 00:47:30,880 As long as some reefs survive, 338 00:47:32,440 --> 00:47:35,160 some hope can remain. 339 00:48:03,840 --> 00:48:05,320 French Polynesia. 340 00:48:11,240 --> 00:48:15,080 Over 4,000 miles from the nearest major land mass. 341 00:48:17,320 --> 00:48:19,200 Protected from fishing pressures, 342 00:48:19,280 --> 00:48:23,480 it's home to perhaps the last great gathering of marbled groupers. 343 00:48:24,280 --> 00:48:25,440 We're finally here. 344 00:48:26,560 --> 00:48:28,120 It's taken quite a few days. 345 00:48:28,560 --> 00:48:30,440 We can't actually say where "here" is, 346 00:48:30,680 --> 00:48:34,280 because uh, we've been asked not to mention the exact spot, 347 00:48:34,400 --> 00:48:36,400 because it's a protected area. 348 00:48:37,760 --> 00:48:40,360 The team face a huge filming challenge 349 00:48:41,400 --> 00:48:44,200 as the groupers normally spawn just once a year, 350 00:48:44,360 --> 00:48:46,560 and it lasts less than an hour. 351 00:48:49,600 --> 00:48:53,400 The team's advisor is biologist, Yvonne Sadovy who's been 352 00:48:53,520 --> 00:48:57,040 studying fish spawning aggregations for over 30 years. 353 00:48:58,480 --> 00:49:01,720 YVONNE: Here, it is close to the situation it has been 354 00:49:01,840 --> 00:49:03,040 for thousands of years. 355 00:49:03,200 --> 00:49:07,040 It's pretty much the only place on the planet that I'm aware of 356 00:49:07,360 --> 00:49:11,320 where these groupers come together in the numbers that we see here. 357 00:49:15,680 --> 00:49:17,320 I really hope that they're there 358 00:49:17,400 --> 00:49:20,040 because if they're not, then we've come all this way for nothing. 359 00:49:21,440 --> 00:49:22,760 We've got our timing right, haven't we Denis? 360 00:49:22,840 --> 00:49:23,840 Yes. 361 00:49:24,280 --> 00:49:25,720 -Confident? -Yeah. 362 00:49:44,720 --> 00:49:46,320 They're not disappointed. 363 00:49:52,120 --> 00:49:55,760 Denis has filmed groupers gathering here for 15 years 364 00:49:55,880 --> 00:49:57,760 and has got to know their behaviour. 365 00:49:58,800 --> 00:50:01,200 (DENIS SPEAKING) 366 00:50:12,080 --> 00:50:14,640 The grouper aren't the only ones to be gathering. 367 00:50:19,120 --> 00:50:21,720 It's the highest density of grey reef sharks 368 00:50:21,800 --> 00:50:23,240 anywhere on the planet. 369 00:50:28,440 --> 00:50:32,240 This is what a pristine reef should look like. 370 00:50:32,840 --> 00:50:34,200 Lots and lots of predators. 371 00:50:36,760 --> 00:50:39,920 With the groupers here, now the challenge is predicting 372 00:50:40,000 --> 00:50:42,000 exactly when they'll spawn. 373 00:50:45,000 --> 00:50:48,880 To judge if they're close, Yvonne reviews the day's footage. 374 00:50:49,760 --> 00:50:52,120 From what we've seen here, the male's courting the female. 375 00:50:52,440 --> 00:50:54,120 When you can slow these things down, 376 00:50:54,240 --> 00:50:56,600 this is when you begin to see some of this detail. 377 00:50:56,880 --> 00:50:58,840 If you were a betting woman would you bet on tomorrow? 378 00:50:58,960 --> 00:51:01,040 I would say tomorrow or the next day. 379 00:51:01,240 --> 00:51:02,760 I mean I think they're pretty close. 380 00:51:04,000 --> 00:51:08,280 With only six divers, the team can't stay underwater all the time. 381 00:51:08,680 --> 00:51:10,520 So, based on experience, 382 00:51:10,600 --> 00:51:14,040 they decide to dive on the full moon's changing tides, 383 00:51:14,200 --> 00:51:17,000 thought to help coordinate grouper spawning. 384 00:51:19,040 --> 00:51:22,480 But this year, the strongest tidal change is at night. 385 00:51:24,200 --> 00:51:27,040 RACHEL: We're just kitting up for the first night dive. 386 00:51:27,400 --> 00:51:30,440 During the day, we've been seeing sharks around this area, but we don't know 387 00:51:30,520 --> 00:51:31,960 how they're going to react at night. 388 00:51:32,080 --> 00:51:33,200 Are you nervous? 389 00:51:33,280 --> 00:51:35,240 (DENIS SPEAKING) 390 00:51:40,760 --> 00:51:42,760 Reef sharks hunt at night, 391 00:51:42,920 --> 00:51:47,160 so the divers wear chain-mail suits and helmets as a precaution. 392 00:51:51,200 --> 00:51:54,280 DENIS: One, two, three, four. 393 00:52:01,880 --> 00:52:03,400 Divers comms check, comms check. 394 00:52:04,200 --> 00:52:05,520 (VOICE OVER RADIO TRANSMITTER) 395 00:52:10,040 --> 00:52:11,480 Receiving you loud and clear. 396 00:52:16,600 --> 00:52:19,680 No sign of groupers spawning, but in the dark, 397 00:52:19,760 --> 00:52:21,880 the shark's attitude has changed. 398 00:52:23,480 --> 00:52:24,560 They're hunting. 399 00:52:26,480 --> 00:52:28,760 Using electro-reception to hunt, 400 00:52:28,840 --> 00:52:34,200 sharks are tuned to even the smallest electro stimulus including the cameras. 401 00:52:35,040 --> 00:52:37,440 (MUMBLING) 402 00:52:42,240 --> 00:52:43,640 With the current building, 403 00:52:43,960 --> 00:52:48,200 now is the worst possible time to lose contact with the surface. 404 00:52:50,920 --> 00:52:53,480 We're in the middle of nowhere. We have no idea what's going on 405 00:52:53,560 --> 00:52:56,160 down there. All we can hear is broken communication. 406 00:52:56,280 --> 00:52:57,280 (STATIC OVER TRANSMITTER) 407 00:53:11,360 --> 00:53:14,680 With no sign of spawning, they end the dive. 408 00:53:21,240 --> 00:53:22,400 Phew! 409 00:53:45,640 --> 00:53:47,800 The team decide to save their energy 410 00:53:47,880 --> 00:53:49,880 for the next change of the tide. 411 00:53:59,120 --> 00:54:02,160 At dawn, they prepare for what they hope will be 412 00:54:02,240 --> 00:54:04,840 the spawning event they've all been waiting for. 413 00:54:18,760 --> 00:54:20,720 DENIS: Bad news, we still are... 414 00:54:21,320 --> 00:54:24,960 But after just 20 minutes, the divers are back on the surface 415 00:54:25,200 --> 00:54:27,480 with some devastating news for their crew. 416 00:54:28,880 --> 00:54:31,280 (INDISTINCT CONVERSATION) 417 00:54:32,720 --> 00:54:33,880 RACHEL: We missed it. 418 00:54:34,240 --> 00:54:36,120 Maybe they spawned last night. 419 00:54:36,560 --> 00:54:38,880 I don't... Oh, God, I don't know. 420 00:54:42,440 --> 00:54:43,880 The team were too late. 421 00:54:49,640 --> 00:54:53,320 YVONNE: So my guess would be that we've missed spawning by a few hours. 422 00:54:53,640 --> 00:54:56,160 And possibly, it's that they just reached some 423 00:54:56,280 --> 00:54:58,080 kind of critical mass or critical density 424 00:54:58,160 --> 00:54:59,760 and spawned a little bit early. 425 00:55:10,000 --> 00:55:13,520 The following year, Yvonne returns to French Polynesia, 426 00:55:13,800 --> 00:55:16,240 hoping to learn more about the grouper. 427 00:55:22,960 --> 00:55:26,320 This year, Laurent Ballesta and his experienced crew, 428 00:55:26,400 --> 00:55:27,880 take on the challenge. 429 00:55:30,440 --> 00:55:33,000 With many more divers, they have a new approach 430 00:55:33,200 --> 00:55:37,400 and they plan to have Laurent on watch in the water constantly. 431 00:55:38,640 --> 00:55:41,040 (SPEAKING FRENCH) 432 00:56:00,360 --> 00:56:04,720 And this year, after hundreds of hours of continual monitoring, 433 00:56:05,040 --> 00:56:09,320 they have captured the defining moment of this astonishing event - 434 00:56:10,960 --> 00:56:13,240 the mass spawning of the grouper. 435 00:56:16,040 --> 00:56:18,960 YVONNE: We have the privilege to be able to watch this process 436 00:56:19,840 --> 00:56:23,240 actually occurring. Uh, and that's a real thrill. 437 00:56:23,720 --> 00:56:26,480 They're really important for the biology of the animals. 438 00:56:27,800 --> 00:56:29,920 (SPEAKING FRENCH) 439 00:56:43,040 --> 00:56:46,320 Thanks to a combined team effort from both crews 440 00:56:46,440 --> 00:56:48,680 and Yvonne's research over many years, 441 00:56:48,880 --> 00:56:52,840 we could at last, tell the marbled groupers' incredible story. 442 00:57:00,040 --> 00:57:01,120 (DOLPHINS WHISTLING) 443 00:57:01,240 --> 00:57:05,080 Next time, we head into the vastness of the open ocean. 444 00:57:08,000 --> 00:57:11,920 To survive in this blue wilderness, some are fast, 445 00:57:12,760 --> 00:57:15,000 others use deadly strategies, 446 00:57:16,320 --> 00:57:20,200 and a few rely on the closeness of their families. 447 00:57:21,080 --> 00:57:22,320 (CLICKING) 448 00:57:23,680 --> 00:57:25,800 (THEME MUSIC PLAYING) 32645

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